The NEC reserves Art. 100 to cover the essential definitions required to properly apply its provisions. Not included are general terms that are commonly defined, or technical terms that are used in the same way as in related codes and standards. In addition, if a term is only used in one article, it will be defined within that article and not in Art. 100. Part I of the article applies throughout the NEC; Part II covers definitions that only apply to installations operating over 600 V, nominal. Consult Art. 100 if you are unclear as to how a specialized electrical term is defined that appears in the NEC.
Accessible (as Applied to Wiring Methods):
Accessible (as Applied to Equipment):
Accessible, Readily (Readily Accessible):
The best way to look at these definitions is to consider all three at the same time because although they are necessarily related, there are important differences. Each of the three terms involves the concept of unimpeded approach. That is, accessible items, whether wiring methods, equipment, or either of these, if readily accessible, must be capable of unimpeded approach as required, but that is about the extent of what these terms have in common.
Wiring methods are accessible if they can be removed or exposed without damaging the building finish or structure. Wiring methods are any of the NE Code-recognized techniques for running circuits between equipment, as covered in the articles in Chap. 3 of the Code. Wiring methods are also accessible if they are not permanently closed in by the building structure or finish. Any surface wiring method would obviously qualify if in plain view, but what about above a suspended ceiling? The definition uses the word “exposed” which is also defined in Art. 100 as being on or attached to the surface, or behind panels designed to allow access. Since suspended ceiling panels are clearly designed for that purpose, wiring such as that shown in Fig. 100-1 above a suspended ceiling is exposed, and since it is exposed, it is also accessible.
Fig. 100-1.
The same word used to describe equipment does not mean quite the same thing. Equipment covers all the products that are connected or hooked up by a recognized wiring method, together with the other components of the wiring system. Equipment is accessible if it allows close approach. It is not accessible if it is guarded by a locked door or by height or other barrier that effectively precludes approach by personnel. The word guarded is also defined in Art. 100, and it means protected by any of various means to remove the likelihood of “approach or contact by persons or objects to a point of danger.”
Consider the busbars in a panelboard located chest high in a corridor, and then think about the panelboard itself, including its enclosing cabinet. Are the busbars themselves “accessible”? No, because they are guarded by the dead-front. Is the panel accessible? Yes, the deadfront makes it safe to approach, and nothing about its location precludes approach. What if the panelboard is for a tenancy, and is located in another tenancy for which access to the supplied tenancy is forbidden? Such a panel would still be accessible, but not to those for whom access is required by the NEC.
This brings us to the final concept, readily accessible. This term also applies to equipment, and requires access without climbing over or removing obstacles, or arranging for a ladder or lift to reach the equipment, as covered in Fig. 100-2. Equipment in the open and reachable only by ladder is probably accessible, but could never be considered “readily accessible.” Overcurrent (OC) devices are usually required to be readily accessible, but what about a fused switch on an air-conditioning compressor high in the air? This is the reason for the special allowance in 240.24(A)(4). It is understood that such equipment is not readily accessible, and a special allowance permits it to be so. Figure 100-3 shows other examples of these special allowances.
Fig. 100-2.
Fig. 100-3.
There is one other provision in the ready access definition that neatly ties some of the key concepts together. Readily accessible equipment must be reachable quickly by those for whom ready access is requisite. This pointedly does not mean everyone. A locked electrical room is a very well-understood concept, and perfectly acceptable as long as those who belong in the room have a key.
Ampacity:
Ampacity is the maximum amount of current in amperes that a conductor may carry continuously under specific conditions of use without exceeding the temperature rating of its insulation. Refer to the discussion on NEC 310.10 and 310.15 in Chap. 3 of this book, together with coverage at the end of this book on Annex D, Example D3(a), for a detailed analysis of ampacity calculations. The calculation of conductor ampacities is one of the most important skills to be learned in the electrical trade, and unfortunately it is also one of the most complicated. There are two key points to raise here, however, in terms of the actual content of the definition.
First, ampacity applies to electrical conductors. Other parts of an electrical system may have current ratings, such as switches, circuit breakers, motor contactors, etc., but only electrical conductors have an ampacity. Second, ampacity in its true sense cannot be defined by a table in a code book, or even a hundred tables. Every condition of use defines a different ampacity. And every time a condition changes, such as when the ambient temperature changes, the applicable ampacity changes. For example, 12 AWG THHN has an allowable ampacity of 30 A at 30°C with three (or fewer) current-carrying conductors in a raceway. Raise the number of current-carrying conductors in the raceway, or raise the ambient temperature, or both, and the ampacity will decrease by varying degrees, all based on the conditions of actual use.
Approved:
Identified:
Listed:
These three definitions are covered together in one location, because they cover the three methods of product acceptance recognized by the NEC. They are crucial to the proper application of the Code. Code-making panels (CMPs) have robust discussions every code cycle about which one to apply in a given situation.
The word approved means acceptable to the inspectional authority [technically, the authority having jurisdiction (AHJ)], and nothing more or less. It does not mean “identified” unless the inspector chooses to use compliance with the definition of “identified” as the basis for his or her decision. Similarly it does not mean “listed” unless the inspector chooses that standard as the basis for his or her decision. For this reason, any statements in product literature (and they are common) that something is “approved” by some testing laboratory is necessarily fallacious. A product may be listed by a testing laboratory, but never approved.
The word identified is routinely confused with the normal usage in the English language of the word marked. It does not mean marked. It means what Art. 100 says it means. It means generally recognizable as suitable for the specific application called out in the NEC requirement. This often comes from product literature generated by manufacturers. This use of the term also correlates with the fine print note (FPN) in 100.3(A)(1), where suitability is explained first in terms of “a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application.” The note goes on to indicate that suitability may also be evidenced by listing or labeling, an additional possibility.
For an example of correct usage of this term in a Code rule, the NEC requires two-winding transformers reconnected in the field as autotransformers to be identified for use at elevated voltage in 450.4(B). These transformers are frequently listed, but as two-winding transformers. They could not be listed as autotransformers because they do not leave the factory this way, and they have wide application as two-winding transformers. A listing would be excessive because the transformer manufacturers would have to run two production lines with two different labels for the same product. The installer needs to rely on product literature from the manufacturer to verify suitability for reconnection, and fortunately, these manufacturers all provide specific information on how to make the reconnections so the transformers will buck or boost the voltage as desired.
The word listed covers the most specific method of product acceptance, because it means that a qualified testing laboratory, usually with testing facilities that an inspector could not possibly duplicate, has performed exhaustive tests to judge the performance of the product under the conditions contemplated in a specific Code rule. The Code note that follows the definition needs some explanation as well. Although the note is written in a general and explanatory manner, in fact, all qualified testing laboratories operating under the current North American electrical safety system do require a label as evidence of the listing. It follows, then, that if a label falls off, the product no longer has the status of being listed. Further, the only way a label can be reapplied is in the presence of an employee of the testing laboratory. Sending labels through the mail is not an option and will result in disciplinary action against the manufacturer by the testing laboratory. The testing laboratories will all send personnel into the field to witness the reapplication of labels.
Be aware that OSHA rules governing workplaces generally require a “listed,” “labeled,” or otherwise “certified” product to be used in preference to the same “kind” of product that is not recognized by a national testing lab (Fig. 100-4).
Authority Having Jurisdiction:
This definition clarifies the meaning of this term, which is used repeatedly throughout the Code. As indicated by the FPN, the authority having jurisdiction (AHJ) is not necessarily the electrical inspector. In some instances it may be the head of a fire department or an insurance company representative.
Fig. 100-4.
Most jurisdictions have procedures in place that allow for taking an appeal from an adverse decision of an inspectional authority. However, there are inevitable trade-offs in terms of time lost in such a proceeding, so usually only the most compelling instances end up in appellate hearings.
Bathroom:
As defined here and used in the rule of 210.8, a bathroom is “an area” (which means it could be a room or a room plus another area) that contains first a “basin” (usually called a sink) and then at least one more plumbing fixture—a toilet, a tub, and/or a shower. A small room with only a “basin” (a “washroom”) is not a “bathroom.” Neither is a room that contains only a toilet and/or a tub or shower (Fig. 100-5). Figure 100-6 shows application in hotel and motel bathrooms.
Bonded (Bonding):
This definition has been simplified and now simply covers the connection of parts in an electrical system to provide continuity and conductivity. This is one of the many definitions and other rules that were impacted by a special task group on grounding and bonding. The definitions have been simplified and the requirements placed in Art. 250, with only special exceptions remaining in other parts of the NEC. The performance criteria for a bonded connection are covered in 250.4.
Bonding Jumper:
This is the means of connection between noncurrent-carrying metallic components of the electrical system that are provided to ensure continuity. Examples of bonding jumpers are given in Figs. 100-7 and 100-8. They may be bare, covered, or insulated conductors, or it may be a mechanical device, such as the 10-32 screws often provided to connect a neutral terminal bar to a service enclosure.
Fig. 100-5.
Fig. 100-6.
Fig. 100-7.
Fig. 100-8.
Bonding Jumper, Equipment:
These are bonding connections made between two portions of the equipment grounding system. For example, bonding jumpers are routinely used to ensure an electrically conductive connection between a metal switchboard enclosure and metal conduits entering the open bottom from a concrete floor. If Fig. 100-9 depicted a feeder and not a service, the jumpers from each conduit to the enclosure frame would be equipment bonding jumpers.
Fig. 100-9.
Bonding Jumper, Main:
A main bonding jumper provides the Code-required connection between the grounded system conductor and the equipment ground bus at the service equipment for a building or structure. The connection between equipment ground and the grounding electrode system in ungrounded services is a “bonding jumper,” but not a “main bonding jumper.” The connection between the equipment ground bus and the neutral bus in the drawing shown in Fig. 100-9 is an example of a main bonding jumper. The NEC maintains a distinction between a main bonding jumper and the same conductor performing the identical function at a separately derived system, which is a system bonding jumper. Since this term is only used in Art. 250, the definition is now found there.
Branch Circuit:
A branch circuit is that part of a wiring system that (1) extends beyond the final Code-required automatic overcurrent protective device (i.e., fuse or breaker) which qualifies for use as branch-circuit protection, and (2) ends at an outlet, which is another defined term in Art. 100. Thermal cutouts or motor overload devices are not branch-circuit protection. Neither are fuses in luminaires nor in plug connections, which are used for ballast protection or individual fixture protection. Such supplementary overcurrent protection is on the load side of outlet and is not required by the Code, nor a substitute for the Code-required branch-circuit protection and does not establish the point of origin of a branch circuit. The extent of a branch circuit is illustrated in Fig. 100-10.
Fig. 100-10.
Branch Circuit, Appliance:
The point of differentiation between “appliance” branch-circuits and “general” branch-circuits is related to what is actually connected. For a circuit to be considered an “appliance” branch circuit, it may not supply any lighting, unless that lighting is part of an appliance. Refer to Fig. 100-11.
Fig. 100-11.
Branch Circuit, General Purpose:
Such circuits are identified by the fact that they supply two or more outlets for receptacles, lighting, or appliances. Refer to Fig. 100-12.
Fig. 100-12.
Branch Circuit, Individual:
As indicated by the term itself, such a branch circuit supplies a single, or “individual” piece of equipment. Refer to Fig. 100-13. A circuit supplying both halves of a duplex receptacle is not an individual branch circuit in most cases, because each half of the duplex is classified as a separate device.
Fig. 100-13.
Branch Circuit, Multiwire:
A multiwire branch circuit must be made up of a neutral or grounded conductor—as in corner-grounded delta systems—and at least two ungrounded or “hot” conductors. The most common multiwire circuits are shown in Fig. 100-14.
Fig. 100-14.
A 3-wire, 3-phase circuit (without a neutral or grounded conductor) ungrounded delta system is not a “multiwire branch circuit,” even though it does consist of “multi” wires, simply because there is no “neutral” or other grounded conductor. Remember, such a circuit must, by definition, also contain a “grounded” conductor, which may be a neutral, as in the typical 3-phase, 4-wire systems, or a grounded phase conductor, such as in a “corner-grounded” delta system (Fig. 100-15).
Fig. 100-15.
Branch Circuit Overcurrent Device:
These are devices capable of providing protection over the full range of over-currents between the device rating and its interrupting rating, but never less than 5000 A. They are far more robust than the supplementary overcurrent protective devices that offer limited protection for certain applications such as limiting the amount of energy that could enter a luminaire.
Building:
Most areas have building codes to establish the requirements for buildings, and such codes should be used as a basis for deciding the use of the definition given in the National Electrical Code. The use of the term fire walls in this definition has resulted in differences of opinion among electrical inspectors and others. Since the definition of a fire wall may differ in each jurisdiction, the processing of an interpretation of a “fire wall” has been studiously avoided in the National Electrical Code because this is a function of building codes and not a responsibility of the National Electrical Code.
In most cases a “building” is easily recognized by its stand-alone nature. However, one or more “fire walls” also establishes two (or more) buildings in one structure. It is frequently crucial to distinguish between a “fire-separation wall” (or however the local building code describes it) and a “fire wall.” As discussed here, a “fire wall” is made of concrete and masonry and will still be standing after a conflagration on one side proceeds to complete destruction. A “fire-separation wall” may consist of several layers of drywall and will have a rating in hours, designed to assure time for the occupants to exit. They are fundamentally different, in kind and not just degree.
Many, many code rules depend on whether a structure comprises multiple buildings, such as whether multiple services will be permitted, which grounding rules will apply at which locations, and whether residential occupancies separated by such construction will be classified as single-family or multiple-family housing. Where in doubt, check with your local electrical inspector for guidance. If the electrical inspector doesn’t know, or doesn’t have jurisdiction over this particular decision, then the electrical inspector should be able to direct you to the proper authority for a determination. This is a good example of where the AHJ may be the local building commissioner and not the electrical inspector.
Cabinet/Cutout Box:
There are two distinguishing characteristics that differentiate a “cabinet” from a “cutout box.” The first is the physical construction. The door of a cabinet is (or could be) hinged to a trim covering wiring space, or gutter. The door of a cutout box is hinged (or screwed) directly to the side of the box. The other distinction is mounting. Cabinets may be surface- or flush-mounted, while cutout boxes may only be surface-mounted. In terms of use, cabinets usually contain panel-boards; cutout boxes contain cutouts, switches, or miscellaneous apparatus.
Concealed:
Any electrical equipment or conductors that are closed in by structural surfaces are considered to be “concealed,” as shown in Fig. 100-16.
Fig. 100-16.
Circuits run in an unfinished basement or an accessible attic are not “rendered inaccessible by the structure or finish of the building,” and are therefore considered as exposed work rather than a concealed type of wiring. Equipment and wiring in hung-ceiling space behind lift-out panels and underneath raised floors beneath removal tiles are also considered “exposed.”
Conduit Body:
The last sentence notes that FS and FD boxes—as well as larger cast or sheet metal boxes—are not considered to be “conduit bodies,” as far as the NE Code is concerned. Although some manufacturers’ literature refers to FS and FD boxes as conduit fittings, care must be used to distinguish between “conduit bodies” and “boxes” in specific Code rules. For instance, the first sentence of 314.16(C)(2) limits splicing and use of devices to conduit bodies that are “durably and legibly” marked with their cubic inch capacity by the manufacturer. However, FS and FD boxes are not conduit bodies and may contain splices and/or house devices. Table 314.16(A) lists FS and FD boxes as “boxes.” See Fig. 100-17.
Fig. 100-17.
Continuous Load:
Any condition in which the maximum load current in a circuit flows without interruption for a period of not less than 3 h. Although somewhat arbitrary, the 3-h period establishes whether a given load is continuous. If, for example, a load were energized for 2 h, 59 min, 59 s, then switched off and immediately reenergized, it would technically be a “noncontinuous” load. This is an extreme example, but that is the Code-prescribed evaluation for this important definition.
Coordination (Selective):
This term refers to the design concept whereby an individual fault will be cleared by the OC protective closest to the faulted circuit or equipment. This design goal is achieved by studying the time-current trip curves of the selected devices and ensuring that the operating characteristics of all selected OC devices are such that the fuse or breaker closest to a fault will blow or open before OC devices upstream (toward the service) operate. This has become mandatory for the main overcurrent protective devices for elevators (620.62), and for protective devices generally for applications covered by 700.27, 701.18, and 708.54.
Demand Factor:
The following discussion provides a distinction between two very closely related, but different concepts. For the purposes of NEC application, any design or application of “demand factors” that results in a feeder or service smaller than would be permitted by the applicable rules of the NEC, such as Art. 220, is a violation. From a practical standpoint in new construction, this generally should not be a problem because NEC requirements are essentially bare minimums and provide absolutely no additional capacity. That precludes system expansion and supply of additional loads in the future, which, of course is poor design. Because design goals should, and typically do, include consideration of potential future needs, actual ratings and sizes of selected equipment and conductors should be larger than the Code-required minimum. BUT, if a designer calculates a load that is less than would be permitted by the Code, the larger, Code-mandated load shall be accommodated by selection of equipment and conductors that are adequate to supply the Code-complying load.
Two terms constantly used in electrical design are “demand factor” and “diversity factor.” Because there is a very fine difference between the meanings for the words, the terms are often confused.
Demand factor is the ratio of the maximum demand of a system, or part of a system, to the total connected load on the system, or part of the system, under consideration. This factor is always less than unity.
Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. This factor generally varies between 1.00 and 2.00.
Demand factors and diversity factors are used in design. For instance, the sum of the connected loads supplied by a feeder is multiplied by the demand factor to determine the load for which the feed must be sized. This load is termed the maximum demand of the feeder. The sum of the maximum demand loads for a number of subfeeders divided by the diversity factor for the sub-feeders will give the maximum demand load to be supplied by the feeder from which the subfeeders are derived.
It is a common and preferred practice in modern design to take unity as the diversity factor in main feeders to loadcenter substations to provide a measure of spare capacity. Main secondary feeders are also commonly sized on the full value of the sum of the demand loads of the subfeeders supplied.
From power distribution practice, however, basic diversity factors have been developed. These provide a general indication of the way in which main feeders can be reduced in capacity below the sum of the demands of the subfeeders they supply. On a radial feeder system, diversity of demands made by a number of transformers reduces the maximum load that the feeder must supply to some value less than the sum of the transformer loads. Typical application of demand and diversity factors for main feeders is shown in Fig. 100-18.
Device:
Switches, fuses, circuit breakers, controllers, receptacles, and lampholders are examples of “devices” that “carry or control” electricity as their principal function. The fact that they may use incidental quantities of power in the process does not affect their principal function.
Fig. 100-18.
Dwelling:
Dwelling unit. Because so many Code rules involve the words “dwelling” and “residential,” there have been problems applying Code rules to the various types of “dwellings”—one-family houses, two-family houses, apartment houses, condominium units, dormitories, hotels, motels, etc. The NE Code includes terminology to eliminate such problems and uses definitions of “dwelling” coordinated with the words used in specific Code rules.
A dwelling unit is defined as a single unit that provides “complete and independent living facilities for one or more persons.” It must have “permanent provisions for living, sleeping, cooking, and sanitation.” A one-family house is a “dwelling unit.” So is an apartment in an apartment house or a condominium unit. And a guest room in a hotel or motel or a dormitory room or unit is a “dwelling unit” if it contains permanent or cord-connected provisions for “cooking.”
Any “dwelling unit” must include all the required elements shown in Fig. 100-19.
Fig. 100-19.
Exposed (as Applied to Wiring Methods):
Wiring methods and equipment that are not permanently closed in by building surfaces or finishes are considered to be “exposed.” See Fig. 100-20.
Feeder:
A feeder is a set of conductors which carry electric power from the service equipment (or from a transformer secondary, a battery bank, or a generator switchboard where power is generated on the premises) to the overcurrent protective devices for branch circuits supplying the various loads. Basically stated, any conductors between the service, separately derived system, or other source of supply and the branch-circuit protective devices are “feeders.”
A feeder may originate at a main distribution center and feed one or more subdistribution centers, one or more branch-circuit distribution centers, one or more branch circuits (as in the case of plug-in busway or motor circuit taps to a feeder), or a combination of these. It may be a primary or secondary voltage circuit, but its function is always to deliver a block of power from one point to another point at which the power capacity is apportioned among a number of other circuits. In some systems, feeders may be carried from a main distribution switchboard to subdistribution switchboards or panelboards from which subfeeders originate to feed branch-circuit panels or motor branch circuits. In still other systems, either or both of the two foregoing feeder layouts may be incorporated with transformer substations to step the distribution voltage to utilization levels. In any of these described scenarios, the conductors would be considered to be feeders because they interconnect the service and branch-circuit.
Fig. 100-20.
Ground:
In another example of the major reevaluation of definitions involving grounding concepts, the ground is now simply the planet earth. There is no longer any reference to a conductive body that serves in its place. For example, a little portable generator is no longer classified as being connected to ground just because a connection may have been made to the generator frame. Since the definition no longer refers to connections to the earth, it is no longer correct to refer to insulation failures and the like as grounds; instead, they should be described as the ground faults they really are.
Here again, the concept of a conductive body serving in place of the earth has been discontinued. The definition now only applies to connections to the planet earth, either directly or through a conductive body that extends the ground connection. Although the concept of conductive entities serving in place of the earth still survives in such areas as motor vehicles and railroad rolling stock, these areas are generally beyond the scope of the NEC. Recreational vehicles (RVs) are covered, but even there most of the equipment and systems affected by this change are those connected to premises wiring in RV parks, for which a connection to the earth is routine.
Grounded Conductor:
Here the Code distinguishes between a “grounding” conductor and a “grounded” conductor. A ground ed conductor is the conductor of an electrical system that is intentionally connected to earth via a grounding electrode conductor and a grounding electrode at the service of a premises, at a transformer secondary, or at a generator or other source of electric power. See Fig. 100-21. It is most commonly a neutral conductor of a single-phase, 3-wire system or 3-phase, 4-wire system but may be one of the phase legs—as in the case of a corner-grounded delta system.
Fig. 100-21.
Grounding one of the wires of the electrical system is done to limit the voltage upon the circuit that might otherwise occur through exposure to lightning or other voltages higher than that for which the circuit is designed. Another purpose in grounding one of the wires of the system is to limit the maximum voltage to ground under normal operating conditions. Also, a system that operates with one of its conductors intentionally grounded will provide for automatic opening of the circuit if an accidental or fault ground occurs on one of its ungrounded conductors.
Selection of the wiring system conductor to be grounded depends upon the type of system. In 3-wire, single-phase systems, the midpoint of the transformer winding—the point from which the system neutral is derived—is grounded. For grounded 3-phase, 4-wire wiring systems, the neutral point of the wye-connected transformer(s) or generator is usually the point connected to ground. In delta-connected transformer hookups, grounding of the system can be effected by grounding one of the three phase legs, by grounding a center-tap point on one of the transformer windings (as in the 3-phase, 4-wire “red-leg” delta system), or by using a special grounding transformer which establishes a neutral point of a wye connection which is grounded.
Grounding Conductor, Equipment:
The phrase “equipment grounding conductor” is used to describe any of the electrically conductive paths that tie together the noncurrent-carrying metal enclosures of electrical equipment in an electrical system. The term equipment grounding conductor includes bare or insulated conductors, metal raceways [rigid metal conduit, intermediate metal conduit, electrical metallic tubing (EMT)], and metal cable jackets where the Code permits such metal raceways and cable enclosures to be used for equipment grounding—which is a basic Code-required concept as follows:
Equipment grounding is the intentional electrical interconnection of all metal enclosures that contain electrical wires or equipment with the grounding electrode conductor (all systems) and with the grounded conductor of the system (grounded systems only). When an insulation failure occurs in such enclosures on ungrounded systems, the result is the system simply becomes corner or otherwise system grounded at the fault, and no hazardous voltage will be present on the enclosures. However, it is still important to correct the insulation failure promptly and the NEC now requires ground detectors on all such systems for this reason. If a second insulation failure happens to occur on a second phase before the first one is fixed, the result will be a line-to-line short circuit flowing through a potentially very long equipment grounding run, perhaps between opposite ends of the factory. A single loose locknut or forgotten setscrew could easily generate a sustained arc in such a case before overcurrent devices operate, with severe consequences and a dangerous voltage on the intervening enclosures while the failure is in progress.
When the insulation failure occurs on a grounded system, equipment grounding serves to ensure adequate current flow to cause the affected circuit’s overcurrent protective device to “open,” usually in the instantaneous portion of the overcurrent device tripping curve. This prevents the enclosures from remaining energized, which would otherwise constitute a shock or fire hazard. Simply stated, proper connections of all metallic enclosures of electric wires and equipment to each other and to the system grounded conductor are shown in Fig. 100-22 prevents any potential-above-ground on the enclosures.
Fig. 100-22.
Workmanship and attention to detail are crucial to the proper implementation of these concepts; a single poor connection can easily reduce the current flowing in a ground fault so it falls into the overload portion of the overcurrent device trip curve. In effect, the fuse or circuit breaker acts as though the arcing fault is a motor trying to start, and by the time the device finally trips a fire is in progress and the damage to the electrical system can easily involve an outage lasting many weeks.
Grounding Electrode:
The grounding electrode is any one of the building or structural elements recognized in 250.52 that is in actual physical contact with the earth.
Grounding Electrode Conductor:
Basically stated, this is the connection between either the grounded conductor of a grounded electrical system (typically the neutral) and the grounding electrode system, or the connection between the equipment ground bus and the grounding electrode system for ungrounded systems. The conductor that runs from the bonded neutral block or busbar or ground bus at service equipment, separately derived systems, or main building disconnects to the system grounding electrode is clearly and specifically identified as the “grounding electrode conductor.” See Fig. 100-23. It should be noted that “main building disconnects” referred to here are those that would be required where one building receives its supply from another as covered in part II of Art. 225.
Fig. 100-23.
Ground-Fault Circuit-Interrupter (GFCI):
This revised definition makes clear that the device described is a GFCI (breaker or receptacle) of the type listed by Underwriters Laboratories Inc. (UL) and intended to eliminate shock hazards to people. “Class A” devices must operate within a definite time from initiation of ground-fault current above the specified trip level (4 to 6 mA, as specified by UL). See Fig. 100-24. It should be noted that this is not the protective device called for by the rule of 210.12. That sections calls for the use of a device called an arc-fault circuit interrupter, or AFCI, which is required for protection specifically against high-resistance arcing-ground-faults in circuits supplying residential bedroom outlets. (See 210.12.)
Fig. 100-24. GFCI protection required for temporary power applications, as covered in 590.6, should be listed for temporary power use. Refer to the caption for Fig. 590-13 at the end of Chapter 5 for more information on this topic.
There are essentially two types of Class A GFCIs: those intended to be permanently installed and those intended for temporary power use. It is important that only those listed as “temporary power” GFCIs be used to satisfy the rules of 590.6 and 525.23(A). That caution is based on the fact that GFCIs listed for temporary power are tested differently than those intended for permanent installation and, as a result, only those listed for temporary power applications may be used for temporary power. There are also “Class B” GFCIs with 20 mA trips; these are only for use with underwater swimming pool luminaires installed before local adoption of the 1965 NEC and they are seldom applied today. For all other Code rules requiring GFCIs, those Class A devices listed for permanent installation may be used.
Ground-Fault Protection of Equipment:
Although any type of ground-fault protection is aimed at protecting personnel using an electrical system, the so-called ground-fault protection required by 215.10, 230.95, and 240.13 for 480Y/277-V disconnects rated 1000 A or more, for example, is identified in 230.95 as “ground-fault protection of equipment (GFPE)”. This is essential because a 480/277-V system has an instantaneous peak voltage to ground of 277 V × 
= 392 V. This voltage is frequently enough to constantly reignite an arc powered by a failed phase leg. The result is an arcing burndown that is extremely destructive. The so-called ground-fault circuit interrupter (GFCI), as described in the previous definition and required by 210.8 for residential receptacles and by other NEC rules, is essentially a “people protector” and is identified in 210.8 as “ground-fault protection for personnel.” Because there are Code rules addressing these distinct functions—people protection versus equipment protection—this definition distinguishes between the two types of protection.
Note that there are other protective devices that provide equipment protection and not personnel protection, but that typically operate in the 30 mA range. For example, pipe tracing circuits covered in 427.22 require this protection because a ground fault in a pipe tracing cable can sputter for a very long time without tripping an overcurrent device, given the inherent resistance of this equipment. A GFPE device will de-energize the failed cable promptly.
Guest Room:
The only difference between a dwelling unit and a guest room hinges on whether or not provisions for cooking—either permanently installed or cord-and-plug connected—are present. Where microwaves or other types of cooking equipment are not present, then the location is a guest room. If such items are present, then the occupancy is a dwelling unit if the cooking equipment is permanently installed. A loose cord-and-plug-connected microwave oven will not trigger a reclassification, unless it is permanently installed into or below a cabinet.
This definition describes any one of a number of small to medium-sized in-ground pull and junction boxes for use in underground distribution, covered in detail in 314.30.
Identified:
This term is covered together with “Approved” (and also “Listed”) as part of the discussion of “Approved” and related standards for product acceptance near the beginning of this chapter.
In Sight From:
The phrase “in sight from” or “within sight from” or “within sight” means visible and not more than 50 ft away. These phrases are used in many Code rules to establish installation location of one piece of equipment with respect to another. A typical example is the rule requiring that a motor-circuit disconnect means must be in sight from the controller for the motor [430.102(A)]. This definition in Art. 100 gives a single meaning to the idea expressed by the phrases—not only that any piece of equipment that must be “in sight from” another piece of equipment must be visible, but also that the distance between the two pieces of equipment must not be over 50 ft. If, for example, a motor disconnect is 51 ft away from the motor controller of the same circuit, it is not “within sight from” the controller even though it is actually and readily visible from the controller. In the interests of safety, it is arbitrarily defined that separation of more than 50 ft diminishes visibility to an unacceptable level.
There are places in the NEC where the wording of rules takes these limitations into account. For example, 610.32(2) allows certain crane disconnects to be “within view” (and not “within sight”) of certain equipment. This is because on large cranes it may be impossible to meet the 50 ft limitation, and yet the disconnect can still be seen and will be capable of being locked in the open position.
Interrupting Rating:
This definition covers both “interrupting ratings” for overcurrent devices (fuses and circuit breakers) and “interrupting ratings” for control devices (switches, relays, contactors, motor starters, etc.).
Labeled:
The label of a nationally recognized testing laboratory on a piece of electrical equipment is a sure and ready way to be assured that the equipment is properly made and will function safely when used in accordance with the application data and limitations established by the testing organization. Each label used on an electrical product gives the exact name of the type of equipment as it appears in the listing book of the testing organization.
Typical labels are shown in Fig. 100-25(a).
Fig. 100-25(a)
Underwriters Laboratories Inc., the largest nationally recognized testing laboratory covering the electrical field, describes its “Identification of Listed Products,” as shown in Fig. 100-25(b).
It should be noted that the definitions for “labeled” and “listed” in the NEC do not require that the testing laboratory be “nationally recognized.” But OSHA rules do require such “labeling” or “listing” to be provided by a “nationally recognized” testing lab. Therefore, even though those NEC definitions acknowledge that a local inspector may accept the label or listing of a product by a testing organization that is qualified and capable even though it operates in a small area or section of the country and is not “nationally recognized,” OSHA requirements may only be satisfied when “labeling” or “listing” is provided by a “nationally recognized” testing facility.
By universal test lab policies, the label is the field evidence of the listing. If the label falls off, the product is no longer presumed to be listed and it can only be relabeled by or in the presence of a test lab employee; labels cannot simply be sent through the mail. The test labs will send personnel to field locations to witness the application of a label.
Listed:
This term is covered together with “Approved” (and also “Identified”) as part of the discussion of “Approved” and related standards for product acceptance near the beginning of this chapter.
Fig. 100-25(b)
As a result of broader, more intensive and vigorous enforcement of third-party certification of electrical system equipment and components, OSHA and the NE Code have made it necessary that all electrical construction people be fully aware of and informed about testing laboratories. The following organizations are widely known and recognized by governmental agencies for their independent product testing and certification activities. Each should be contacted directly for full information on available product listings and other data on standards and testing that are recognized by OSHA.
Underwriters Laboratories Inc.
Chicago Corporate Headquarters
333 Pfingsten Road
Northbrook, IL 60062-2096 USA
Telephone: 847-272-8800
Fax: 847-272-8129
E-mail: northbrook@us.ul.com
California Laboratory and Testing Facility
1655 Scott Boulevard
Santa Clara, CA 95050-4169
Telephone: 408-985-2400
Fax: 408-296-3256
E-mail: santaclara@ul.com
New York Laboratory and Testing Facility
1285 Walt Whitman Road
Melville, NY 11747-3081 USA
Telephone: 631-271-6200
Fax: 631-271-8259
E-mail: melville@us.ul.com
North Carolina Laboratory and Testing Facility
12 Laboratory Drive
P.O. Box 13995
Research Triangle Park, NC 27709-3995
Telephone: 919-549-1400
Fax: 919-547-6000
E-mail: rtp@us.ul.com
MET (Maryland Electrical Testing)
914 W. Patapsco Ave.
Baltimore, MD 21230 USA
Telephone: 410-354-3300
Fax: 410-354-3313
Factory Mutual Engineering Corp.
500 River Ridge Drive
Norwood, MA 02062 USA
Telephone: (781) 440-8000
Fax: (781) 440-8742
Jeffrey Newman, Test Center Manager
Telephone: 401-568-6240
Fax: 401-568-6241
E-mail: Jeff.Newman@fmglobal.com
Intertek Testing Services (formerly Electrical Testing Laboratories, Inc.)
Americas
Intertek Testing Services
70 Codman Hill Road
Boxborough, MA 01719 USA
Telephone: 1-800-967-5352
International +1-607-758-6439
Fax: 1-800-813-9442
Electronic mail General Information: info@ETLSEMKO
General Information: info@ETLSEMKOASIA
ASIA
Intertek Testing Services
2/F Garment Centre, 576 Castle Peak Road
Kowloon, HONG KONG
Telephone: +852 2173 8888
Fax: +852 27 855 487
General Information: info@SEMKO
EUROPE
Intertek Testing Services/SEMKO AB
Torshamnsgatan 43
Box 1103
SE-16422 Kista
SWEDEN
Telephone: +46 8 750 00 00
Fax: +46 8 750 60 30
Canadian Standards Association (CSA)
Etobicoke (Toronto)
178 Rexdale Boulevard
Etobicoke (Toronto), ON
M9W 1R3
Telephone: (416) 747-4000
Fax: (416) 747-4149
E-mail: certinfo2@csa-international.org
E-mail: sales@csa-international.org
In United States and Canada, call toll-free 1-800-463-6727
Publications of nationally recognized testing laboratories may be obtained by contacting the various test labs.
Live Parts:
This definition indicates what is meant by that term as it is used throughout the Code. An insulated conductor contains a live part at any time by definition if it is energized (the conductor itself), even if the live part is insulated. For example, 312.2 requires that wiring entries to cabinets, cutout boxes, and meter sockets in wet locations use fittings listed for wet locations if the entry point is above the level of uninsulated live parts. The focus of this rule is not insulated conductors that are wet, but only the impact of moisture on uninsulated meter jaws and lugs, etc.
Luminaire:
This definition indicates all elements that are covered by the term luminaire. This term was adopted to correlate the NEC with other international standards and replace the term “fixture” used in the NEC prior to the 2002 Code. There was no intent on the part of the Code-making panels involved to require any change in application; this is simply an editorial revision. In this cycle the definition has been additionally revised to refer to a “light source such as” (but not necessarily) “a lamp or lamps.” This allows for light-emitting diode (LED) and other sources that do not involve lamps as technology continues to move ahead.
Motor Control Center:
This definition indicates the necessary elements that would identify a piece of equipment as a motor control center. Such equipment would be subject to all rules aimed at motor control centers (MCCs) such as 110.26(E), covering “headroom” in front of certain types of equipment, including “motor control centers.”
Neutral Conductor:
Neutral Point:
At long last the NEC has actually defined the term neutral. It does so by first defining a “neutral point” in a way that is sensible and not controversial, but the definition of “neutral conductor” is more problematic. Refer back to Fig. 100-21. The top two drawings show the most common neutral points, namely the star point of a wye and the center point of a single-phase system. No one would argue that those are neutral points. Since such star or center points must be grounded by rules in Art. 250, any conductor connected to such a point must be a grounded circuit conductor, and must be identified in accordance with 200.6. Therefore, any white wire run in conjunction with a grounded system is now a neutral, whether or not it is neutral between two (or more) associated ungrounded conductors. A two-wire branch circuit that includes a white wire connected on a neutral bus is now an official neutral all the way to the outlet.
Although this certainly legitimizes common trade slang, it may lead people to believe all white (or gray) wires are neutrals. Not so. Look now at the bottom drawing in Fig. 100-21. That corner-grounded delta system has a white phase conductor, which is not and cannot ever be a neutral because the delta system shown has no neutral point. It remains to be seen whether this effort will add or reduce confusion.
Nonlinear Load:
Those loads that cause distortion of the current waveform are defined as nonlinear loads. A typical nonlinear load current and voltage waveform are shown in Fig. 100-26. As can be seen, while the voltage waveform (Fig. 100-26[b]) is a sinusoidal, 60-Hz wave, the current waveform (Fig. 100-26[a]) is a series of pulses, with rapid rise and fall times, and does not follow the voltage waveform.
Fig. 100-26(a)
Fig. 100-26(b)
The FPN following this definition is not intended to be a complete list, but rather, just a few examples. There are many more such loads. The substantiation for inclusion of this FPN stated in part:
It has been known within the entertainment industry for some time that due to the independent single-phase phase-control techniques applied to three-phase, four-wire feeder, solid-state dimming can cause neutral currents in excess of the phase currents. This is in addition to the harmonics generated. This situation is dealt with in theaters in 520.27 and 520.51, etc. Dimming is also used in non-theatrical applications such as hotel lobbies, ballrooms, conference centers, etc. This effect must be taken into account wherever solid-state dimming is employed.
Outlet:
This term refers to a point on a wiring system where current is taken to supply utilization equipment. This is a critical definition because the term is frequently misapplied. For example, a hard-wired fluorescent luminaire set in a suspended ceiling in an office is an outlet and the branch circuit ends at the ballast channel. Article 400 covering flexible cord appears in Chap. 4 (equipment for general use) and not Chap. 3 (wiring methods and materials) because (with limited exceptions) flexible cord is not supposed to substitute for Chap. 3 wiring methods. The terminal housing on a motor, even a motor operating on a 4160-V branch circuit, is the outlet at the end of that medium-voltage branch circuit. Receptacles are outlets, but only a small fraction of the category.
Overcurrent:
Overload:
This is a very important concept. Overcurrent considers current in excess of rated current or ampacity in three different ways. A short circuit is a direct line-to-line connection between two circuit conductors, and if it occurs, it can be extremely destructive because of the enormous amounts of energy that will be released unless it is cleared immediately. A ground fault is a connection from an ungrounded conductor and an equipment grounding conductor. Although the available energy is somewhat lower, it may be just low enough so that overcurrent devices do not respond immediately. This type of arcing burn-down is extremely destructive if not cleared immediately. The third variety of overcurrent is an overload. These are sustained currents that are above an equipment full load rating or the ampacity of a conductor, but low enough that it will only cause a problem if it persists for an extended period of time, the period being inversely proportional to the degree of overload.
Plenum:
This definition is intended to clarify use of this word, which is referred to in Sec. 300.22(B) and other sections. A plenum is a compartment or chamber to which one or more air ducts are connected and which forms part of an air distribution system. This definition replaces the fine print note that was in Sec. 300.22(B) of the 1987 NEC. As now noted in the text of Sec. 300.22(B), a plenum is an enclosure “specifically fabricated to transport environmental air.” The definition further clarifies that an air-handling space above a suspended ceiling or under a raised floor (such as in a computer room) is not a plenum, but is “other spaces” used for environmental air, as covered by Sec. 300.22(C). These areas are frequently referred to as plenum cavity ceilings, but they are not plenums.
Premises Wiring (System):
Published discussions of the Code panel’s meaning of this phrase make clear the panel’s intent that premises wiring includes all electrical wiring and equipment on the load side of the “service point,” including any electrical work fed from a “source of power”—such as a transformer, generator, computer power distribution center, an uninterruptible power supply (UPS), or a battery bank. Premises wiring includes all electrical work installed on a premises. Specifically, it includes all circuits and equipment fed by the service or fed by a separately derived electrical source (transformer, generator, etc.). This makes clear that all circuiting on the load side of a so-called computer power center or computer distribution center (enclosed assembly of an isolating transformer and panel-board[s]) must satisfy all NEC rules on hookup and grounding, unless the power source in question is listed as “Information Technology Equipment,” in which case the rules of Art. 645 would apply. When a “computer power center” is specifically “listed” and supplied with or without factory-wired branch-circuit “whips” (lengths of flexible metal conduit or liquidtight flex—with installed conductors), such equipment may be grounded as indicated by the manufacturer as given in the rules of Art. 645, Information Technology Equipment.
Other sources, such as solar photovoltaic systems or storage batteries, also constitute “separately derived systems.” All NEC rules applicable to premises wiring also pertain to the load side wiring of batteries and solar power systems.
Here the Code spells out the necessary elements that designate someone as a “qualified person.” This rule is used in many sections of the Code and typically compliance with any such rule hinges on the personnel involved being a “qualified person.” Notice that it is not simply enough to be knowledgeable about the equipment and/or application, but also, such persons must have “received safety training.” Presumably that means attending formal or informal training, or even on-the-job-training, all of which, presumably, must be documented and maintained in a personnel file on “Qualifications” or “Training” or the like. A new FPN directs the reader to another NFPA standard (70E) for additional guidance with regard to training requirements.
Raceway:
Whenever this term is used in the Code, it applies to the various enclosed channels that are designed specifically for running conductors between cabinets and housings of electrical distribution components, including busbars, as covered in the relevant wiring method articles in Chap. 3. The clear implication presented by the choice of wiring methods listed is that raceways are for extended lengths of run, and that more limited enclosed channels such as those within equipment are not to be so classified. This interpretation has been thoroughly tested. If any such enclosed channel were classified as a raceway, then surely an auxiliary gutter would be so classified. In the 1993 NEC cycle, the panel initially accepted a proposal to place “auxiliary gutters” into the list, and then unanimously reversed course in the face of negative comments from the current author, NEMA, and others. The issues of auxiliary gutters and panel-board gutter spaces is particularly pressing because 230.7 forbids the sharing of raceways between service conductors and other conductors. If such enclosures are deemed to be raceways, then service panel wiring, as we know it, would be contrary to the NEC.
There are other wiring methods omitted from the list as well, and for good reason. A cable tray is a “support system” and not a “raceway.” When the Code refers to “conduit,” it means only those raceways containing the word “conduit” in their title. But “EMT” is not conduit. Table 1 of Chap. 9 in the back of the Code book refers to “Conduit and Tubing.” The Code, thus, distinguishes between the two. “EMT” is tubing. Notice that cable trays and cablebuses are not identified as “raceways.” The consequence of their omission is that general rules applying to “raceway” do not apply to cable trays or cablebuses.
Receptacle:
Each place where a plug cap may be inserted is a “receptacle,” as shown in Fig. 100-27. Multiple receptacles on one strap are just that, multiple receptacles.
Only a single receptacle can be served by an individual branch circuit. See 210.21(B) and 555.19(A)(3).
Fig. 100-27.
Receptacle Outlet:
The outlet is the outlet box. But this definition must be carefully related to 220.12(I) for calculating receptacle loads in other than dwelling occupancies. For purposes of calculating load, 220.12(I) requires receptacle outlets to be calculated at not less than 180 for each single or for each multiple receptacle on one yoke. Because a single, duplex, or triplex receptacle is a device on a single mounting strap, the rule requires that 180 VA must be counted for each strap, whether it supports one, two, or three receptacles. On the other hand, the new multi-outlets that feature four or more receptacles permanently molded into a single piece of equipment mounted to an outlet box must be calculated at 90 VA per receptacle.
Remote-Control Circuit:
The circuit that supplies energy to the operating coil of a relay, a magnetic contactor, or a magnetic motor starter is a remote-control circuit because that circuit controls the circuit that feeds through the contacts of the relay, contactor, or starter, as shown in Fig. 100-28.
A control circuit, as shown, is any circuit that has as its load device the operating coil of a magnetic motor starter, a magnetic contactor, or a relay. Strictly speaking, it is a circuit that exercises control over one or more other circuits. And these other circuits controlled by the control circuit may themselves be control circuits, or they may be “load” circuits—carrying utilization current to a lighting, heating, power, or signal device. Figure 100-28 clarifies the distinction between control circuits and load circuits.
The elements of a control circuit include all the equipment and devices concerned with the function of the circuit: conductors, raceways, contactor operating coils, source of energy supply to the circuit, overcurrent protective devices, and all switching devices which govern energization of the operating coil.
Fig. 100-28.
The NE Code covers application of remote-control circuits in Art. 725 and in 430.71 through 430.75.
Separately Derived Systems:
This applies to all separate sources of power and includes transformers, generators, battery systems, fuel cells, solar panels, etc., that have no electrical connection—including a grounded (neutral) conductor connection to another system. Virtually all power transformers are separately derived systems, while a backup generator, for example, may or may not be depending on whether the neutral from the generator is also switched with the phase conductors. Where the grounded (neutral) conductor is switched—such as where a four-pole transfer switch is used on a three-phase, four-wire generator output—then the generator is a separately derived system and must comply with the rules of 250.30.
Service:
The word service includes all the materials and equipment involved with the transfer of electric power from the utility distribution line to the electrical wiring system of the premises being supplied. Only a utility can supply a service, so if a facility generates its own power, it will have no service, only one or more feeders and building disconnects. The purpose of special rules for actual services is to address the necessary transitional rules that will assure a safe transition from utility work governed by the National Electrical Safety Code (NESC) and premises wiring governed by the NE Code. Similarly, if a building is supplied by premises wiring in any form, then the disconnect for the entrance of that wiring will be a building disconnect and not a service disconnect.
Although service layouts vary widely, depending upon the voltage and amp rating, the type of premises being served, and the type of equipment selected to do the job, every service generally consists of “service-drop” conductors (for overhead service from a utility pole line) or “service-lateral” conductors (for an underground service from either an overhead or underground utility system)—plus metering equipment, some type of switch or circuit-breaker control, overcurrent protection, and related enclosures and hardware. A typical layout of “service” for a one-family house breaks down as in Fig. 100-29.
Fig. 100-29.
The NEC does not govern where in a service layout the NEC begins and the NESC ends. This is determined by the local public authority that governs public utility activities. Although we hear a lot about deregulated utilities, this concept only applies to the generation of electric energy, not its distribution down public streets. Since only one set of line wire can run on any given street, the distribution of electric energy is what economists call a natural monopoly; competition is effectively impossible. In such cases there will be regulation by public authorities. This is the case here. Part of the regulatory process will be determining where the service points are allowed to be. If the service point is at the pole, then the NEC applies to the service drop as installed by the electrical contractor, with only the final connections at the street being made by utility personnel. If the service point is at the splices at the bottom of the drip loops, then a utility line crew will install the drop in accordance with the NESC and the NEC does not apply.
That part of the electrical system which directly connects to the utility-supply line is referred to as the service entrance. Depending upon the type of utility line serving the house, there are two basic types of service entrances—an overhead and an underground service.
The overhead service has been the most commonly used type of service. In a typical example of this type, the utility supply line is run on wood poles along the street property line or back-lot line of the building, and a cable connection is made high overhead from the utility line to a bracket installed somewhere high up on the building. This wood pole line also carries the telephone lines, and the poles are often called telephone poles.
The underground service is one in which the conductors that run from the utility line to the building are carried underground. Such an underground run to a building may be tapped from either an overhead utility pole line or an underground utility distribution system. Although underground utility services tapped from a pole line at the property line have been used for many years to eliminate the unsightliness of overhead wires coming to a building, the use of underground service tapped from an underground utility system has only started to gain widespread usage in residential areas over recent years. This latter technique is called URD—which stands for underground residential distribution.
Service Conductors:
This is a general term that covers all the conductors on the load side of the service point used to connect the utility-supply circuit or transformer to the service equipment of the premises served. This term includes “service-drop” conductors, “service-lateral” (underground service) conductors, and “service-entrance” conductors, although the NEC may not always govern, depending on the service point location. Although Fig. 100-29 covers an ordinary one-family house, the NEC necessarily applies to major industrial occupancies taking power at 69 kV or even higher, and every conceivable size and type of occupancy in between. See also Figs. 100-31 and 32.
Where the supply is from an underground distribution system, the service conductors may begin at the point of connection to the underground street mains, or at the property line, or at the terminals of the meter socket, or at the terminals of a pad-mounted transformer, again all as governed by state and local rule making around service point locations.
In every case the service conductors terminate at the service equipment, including the service disconnecting means.
Service Drop:
As the name implies, these are the conductors that “drop” from the overhead utility line and connect to the service-entrance conductors at their upper end on the building or structure supplied. See Fig. 100-30.
Fig. 100-30.
This is the equipment connected to the load end of service entrance conductors for the purpose of providing the principal means to control and disconnect the premises wiring from the source of utility supply. A meter socket is not service equipment in and of itself, but would be part of such equipment in the case of a combination meter-disconnect with the service disconnecting means located at the meter, all in one piece of equipment. The meter and meter socket in Fig. 100-29 is not part of the service equipment. The service disconnecting means will consist of some form of fused switch or circuit breaker because 230.91 requires the overcurrent protective device to be an integral part of the disconnecting means, or located immediately adjacent thereto. Note that any service “overcurrent device” only provides overload protection for service conductors. It cannot possibly respond to an arcing failure in progress on a service conductor located on its line side; such faults must usually burn clear, and this is why the NEC severely limits the exposure of any building to unprotected service conductors.
Service Lateral:
This is the name given to a set of underground service conductors. A service lateral serves a function similar to that of a service drop, as shown in Fig. 100-31.
Service Point:
Service point is the “point of connection between the facilities of the serving utility and the premises’ wiring.” All equipment on the load side of that point is subject to NE Code rules. Any equipment on the line side is the concern of the power company and is not regulated by the Code. This definition of “service point” must be construed as establishing that “service conductors” originate at that point. The whole matter of identifying the “service conductors” is covered by this definition.
The definition of “service point” does tell where the NE Code becomes applicable, and does pinpoint the origin of service conductors. And that is a critical task, because a corollary of that determination is identification of that equipment which is, technically, “service equipment” subject to all applicable NE Code rules on such equipment. Any conductors between the “service point” of a particular installation and the service disconnect are identified as service conductors and subject to NE Code rules on service conductors (Fig. 100-32).
Solar Photovoltaic System:
This refers to the equipment involved in a particular application of solar energy conversion to electric power. This definition correlates to NEC Art. 690 covering design and installation of electrical systems for direct conversion of the sun’s light into electric power. While the proliferation of such installations accelerates, remember that any and all installations of solar-photovoltaic equipment at premises covered by the NEC must be performed in accordance with all general requirements given in Chaps. 1 through 4 and the specific requirements given in Art. 690.
Fig. 100-31.
Special Permission:
It must be carefully noted that any Code reference to special permission as a basis for accepting any electrical design or installation technique requires that such “permission” be in written form. Whenever the inspection authority gives “special permission” for an electrical condition that is at variance with Code rules or not covered fully by the rules, the authorization must be “written” and not simply verbal permission. This rule corresponds to the wording used in 90.4, which requires any inspector-authorized deviation from standard Code-prescribed application to be in writing.
Fig. 100-32. NE Code rules apply on load side of “service point”—not from property line. (230.200.)
Switches:
Bypass isolation switch This is “a manually operated device” for bypassing the load current around a transfer switch to permit isolating the transfer switch for maintenance or repair without shutting down the system. The second paragraph of 700.6(B) permits a “means” be provided to bypass and isolate transfer equipment. This definition ties into that rule.
Transfer switch This is a switch for transferring load-conductor connections from one power source to another. Note that such a switch may be automatic or nonautomatic.
For a grounded electrical system, voltage to ground is the voltage that exists from any ungrounded circuit conductor to either the grounded circuit conductor (if one is used) or the grounded metal enclosures (conduit, boxes, panel-board cabinets, etc.) or other grounded metal, such as building steel. Examples are given in Fig. 100-33.
Fig. 100-33.
For an ungrounded electrical system, voltage-to-ground is taken to be equal to the maximum voltage that exists between any two conductors of the system. This is based on the reality that an accidental ground fault on one of the ungrounded conductors of the system places the other system conductors at a voltage aboveground that is equal to the value of the voltage between conductors. Under such a ground-fault condition, the voltage to ground is the phase-to-phase voltage between the accidentally grounded conductor and any other phase leg of the system. On, say, a 480-V, 3-phase, 3-wire ungrounded delta system, voltage to ground is, therefore, 480 V, as shown in Fig. 100-34.
Fig. 100-34.
In many Code rules, it is critically necessary to distinguish between references to “voltage” and to “voltage to ground.” The Code also refers to “voltage between conductors,” as in 210.6(A) through (D), to make very clear how rules must be observed.
110.1. Scope. This article provides a variety of general regulations that govern the installation of equipment and conductors. Part I applies to installations rated 600 V or less and those rated over 600 V, unless specifically modified by another rule in part III or IV. Part II applies only to systems rated 600 V or less, while part III provides general rules for systems operating at over 600 V, and part IV covers electrical systems rated over 600 V used in “tunnel installations.” Part V covers the requirements for manholes.
110.2. Approval. As indicated by this section and the companion definition given in Art. 100, all equipment used must be “acceptable to the authority having jurisdiction” (AHJ). That generally means that the local inspector is the final judge of what equipment and conductors may be used in any given application.
Review the discussion in Art. 100 on the three standards for product acceptance (approved, identified, and listed) for more information on this point.
The intent of the NEC is to place strong insistence on third-party certification of the essential safety of the equipment and component products used to assemble an electrical installation. And, many Code sections specifically require the equipment to be “listed.” But, where any piece of equipment is not required to be listed, then the local inspector is the one who determines if such equipment can be used. It should be noted that many inspectors require equipment to be “listed” if there exists a “listed” version of the type of equipment you’re using. Such action helps ensure that the equipment used is inherently safe.
The NEC is not the only controlling standard covering electrical installations. The Occupational Safety and Health Administration (OSHA) has a standard, the Electrical Design Safety Standard, that must also be satisfied. Because the NEC is the basis for the OSHA standard, and the NEC is more dynamic in terms of change, in the vast majority of cases, NEC requirements are more stringent than those of the OSHA design standard. And satisfying the NEC will ensure compliance with the OSHA regulations. But, while the NEC doesn’t always mandate the use of listed equipment, the OSHA standard requires that listed equipment be used to the maximum extent possible. That is, as far as OSHA is concerned, if there exists a “listed” piece of equipment of the type you are installing, then you must use the “listed” equipment instead of a nonlisted counterpart. Failure to do so is a direct violation of OSHA’s Electrical Design Safety Standard.
In addition, OSHA addresses those instances where there exists no “listed” version of the type of equipment you need. In such cases, the local inspector, plant safety personnel, the manufacturer, or other authority must perform a safety inspection. Although the OSHA standard does not provide any guidance with respect to “what” the safety inspection must entail, it seems reasonable to assume that consideration of the points delineated in 110.3(A)(1) through (8) should serve to satisfy the intent of the OSHA requirement for a safety evaluation.
The use of custom-made equipment is also covered in OSHA rules. Every piece of custom equipment must be evaluated as essentially safe by the local inspector, plant safety personnel, the manufacturer, or other authority and documentary safety-test data of the safety evaluation should be provided to the owner on whose premises the custom equipment is installed. And it seems to be a reasonable conclusion from the whole rule itself that custom-equipment assemblies must make maximum use of “listed,” “labeled,” or “certified” components, which will serve to mitigate the enormous task of conducting the safety evaluation.
The bottom line is that if an OSHA review is a serious concern, look for listed equipment. However, this usual, overly simplistic, one-size-fits-all approach of a government bureaucracy undermines the integrity of the NEC process. In the 2005 NEC cycle, NEMA made a serious, official proposal to require all pull boxes to be listed. The panel chair put the question to CMP 9, and out of courtesy, invited the NEMA representative to make the motion, which was to accept the proposal. It was greeted with an extended dead silence, followed by an announcement by the chair that the motion failed because it did not so much as receive a second. This current author then moved to reject the proposal, on the grounds that it was excessive to require a listing, especially on pull boxes that may be made in local sheet metal shops to meet specific dimensional requirements. CMP 9 overwhelmingly followed suit, the second time during this author’s tenure on that panel that it has refused to require listings on this equipment.
The NEC process is a transparent, open process, fully subject to opportunities for public participation and comment. It is a consensus process requiring a two-thirds vote of a panel for which no interest can have more than a third of the membership, and for which the actual proportion is far lower than that. Every time some bureaucracy tries to require universal listings, it is tantamount to an attempt to make thousands of amendments throughout the NEC, in this case removing “approved” and “identified” and substituting “listed,” all without going through the consensus process. The OSHA rules are what they are. This author believes such agencies would be better off staying out of the way of consensus standards development efforts by agencies that work as well as NFPA does. If OSHA has specific information that a listing is needed where it is not now specified, they should submit a proposal like anyone else. To their credit, the U.S. Consumer Product Safety Commission has been participating in this way for many years.
110.3. Examination, Identification, Installation, and Use of Equipment. This section presents general rules for establishing what equipment and conductors may be used. Part (A) lists eight factors that must be evaluated in determining acceptability of equipment for Code-recognized use. It’s worth noting that these criteria may be used as a basis for the evaluation that is required, but not defined, by OSHA rules for “unlisted” equipment. Remember, as far as OSHA is concerned, use of “unlisted” equipment is only permitted in those cases where no commercially available product of the type to be used is “listed.” Where no “listed” version is available, then OSHA would permit the use of the “unlisted” piece of equipment, BUT OSHA requires a safety evaluation be performed.
Part (A)(1) states that the “suitability” of the equipment in question must be evaluated with respect to the intended use and installation location. The FPN to 110.3(A)(1) notes that, in addition to “listing” or “labeling” of a product by UL or another test lab to certify the conditions of its use, acceptability may be “identified by a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application.” This is a follow-through on the definition of the word “identified,” as given in Art. 100. The requirement for identification of a product as specifically suited to a given use is repeated at many points throughout the Code.
With the exception of items (3) and (8) in 110.3(A), listing standards will generally cover the concerns listed in items (2) and (4) through (7). Where “unlisted” equipment is used, these factors must be considered and adequately addressed. Item (3), an important consideration for electrical inspectors to include in their examination to determine suitability of equipment for safe and effective use, is “wire-bending and connection space.” See Fig. 110-1. This factor is a function of field-installation and addresses the concern for adequate gutter space to train conductors for connection at conductor terminal locations in enclosures for switches, CBs, and other control and protection equipment. This general mention of the need for sufficient conductor bending space is aimed at avoiding poor terminations and conductor damage that can result from excessively sharp conductor bends required by tight gutter spaces at terminals. Specific rules that cover this consideration are given in 312.6 on “Deflection of Conductors” at terminals or where entering or leaving cabinets or cutout boxes—covering gutter widths and wire-bending spaces. Item (8) is essentially a “catchall” requirement that depends on the designers and installers to use common sense and their knowledge of safe application to identify and correct any condition that may exist or develop relative to the installation they are performing.
Fig. 110-1. Equipment must be evaluated for adequate gutter space to ensure safe and effective bending of conductors at terminals. [See. 110.3(A)(3).]
Part (B) of this section is a critically important Code rule because it incorporates, as part of the NE Code itself, all the application regulations and limitations published by product-testing organizations, such as UL, Factory Mutual, ETL, etc. That rule clearly and certainly says, for instance, that any and every product listed in the UL Electrical Construction Materials Directory (Green Book) must be used exactly as described in the application data given with the listing in the book. Because the Electrical Construction Materials Directory and the other UL books of product listings, such as the Hazardous Location Equipment Directory (Red Book) and the Electrical Appliance and Utilization Equipment Directory (Orange Book), contain massive amounts of installation and application instruction, all those specific bits of application data become mandatory NE Code regulations as a result of the rule in 110.3(B). The data given in the UL listing books supplement and expand upon rules given in the NE Code. In fact, effective compliance with NE Code regulations can be assured only by careful study and observance of the limitations and conditions spelled out in the application instructions given in the UL listings books or similar instructions provided by other national testing labs.
UL now publishes, in a separate directory called the “White Book,” the guide card information from the Green, Red, and Orange Books. This publication also includes all of the materials in the UL Marking Guides, and a special index of product categories arranged by NE Code section references. For example, if you are handed a “no-niche” underwater luminaire covered in NEC 680.23(D), this index allows you to go directly from the 680.23(D) entry in this index to the relevant topic and its page number in the White Book, namely, “Luminaires and Forming Shells”, Guide Card entry “WBDT” under the “Swimming Pool and Spa Equipment” general category. This publication is also available on CD, and specific, current information is always available online.
In the Electrical Equipment for Use in Ordinary Locations (AALZ) guide card information, UL points out certain basic conditions that apply to many listed products, some of which are excerpted here to provide a sense of the information contained in this directory.
1. In general, individual appliances and equipment have been investigated only for use indoors, in dry locations. An exception is where outdoor use is specifically permitted by the Article of the NEC concerned with the product installation. See also the general Guide Information for the product category or included in the individual listing. In some cases, the title (e.g., Snow Movers, Swimming Pool Fixtures) indicates the conditions for which the product has been investigated.
Cord- and plug-connected appliances, obviously intended for outdoor use such as gardening appliances, are not intended for use in the rain, and should be stored indoors when not in use.
2. Marked ratings of utilization equipment include ampere, wattage, or volt-ampere ratings. Motor-operated utilization equipment may also be marked with a horsepower rating. The actual marked ratings (other than the horsepower rating) and other markings or instructions, if any, are to be used to select branch circuit conductors, branch circuit overcurrent protection, control devices, and disconnecting means.
The ampere or wattage marking on power-consuming equipment is valid only when the equipment is supplied at its marked rated voltage. In general, the current input to heating appliances or resistance heating equipment will increase in direct proportion to an increase in the supply voltage, while the current input to an induction motor supplying a constant load will increase approximately in direct proportion to a decrease in the supply voltage. These increases in current can cause overcurrent protection devices to open even when these devices are properly selected on the basis of nameplate ratings.
3. Except as noted in the general Guide Information for some product categories, most terminals, unless marked otherwise, are for use only with copper wire. If aluminum or copper-clad aluminum wire can be used, marking to indicate this fact is provided. Such marking is required to be independent of any marking on terminal connectors, such as on a wiring diagram or other visible location. The marking may be in an abbreviated form, such as “AL-CU.”
4. The ampere or wattage marking on power-consuming equipment is valid only when the equipment is supplied at its marked rated voltage. In general, the current input to heating appliances or resistance heating equipment will increase in direct proportion to an increase in the supply voltage, while the current input to an induction motor supplying a constant load will increase approximately in direct proportion to a decrease in the supply voltage. These increases in current can cause overcurrent protection devices to open even when these devices are properly selected on the basis of nameplate ratings.
A very important qualification is indicated for the temperature ratings of terminations. Although application data on maximum temperature ratings of conductors connected to equipment terminals are given in 110.14(C), this matter is more clearly and comprehensively covered in the UL General Information Directory.
The foregoing data are just a tiny fraction of the many and varied requirements that are delineated in the UL listing instructions. Always take the time to review the general listing instructions given in the appropriate UL Directory (White, Green, Orange, or Red Book) and the manufacturer’s installation instructions to ensure that all prohibitions and limitations placed upon the equipment or conductors in question are observed. Failure to do so is a clear and direct violation of the requirement given in 110.3(B).
110.4. Voltages. In all electrical systems there is a normal, predictable spread of voltage values over the impedances of the system equipment. It has been a common practice to assign these basic levels to each nominal system voltage. The highest value of voltage is that at the service entrance or transformer secondary, such as 480Y/277 V. Then considering voltage drop due to impedance in the circuit conductors and equipment, at midsystem the actual voltage would be 460Y/265, and finally a “utilization” voltage would be 440Y/254. Variations in “nominal” voltages have come about because of (1) differences in utility-supply voltages throughout the country, (2) varying transformer secondary voltages produced by different and often uncontrolled voltage drops in primary feeders, and (3) preferences of different engineers and other design authorities.
It’s worth noting that for the purpose of calculating the Code-required minimum load to be served, the Code permits the use of “nominal” system voltages to establish the minimum load, as stated in Sec. 220.5(A), (e.g., 120/240, 208/120, 480/277). Use of “utilization” voltages (e.g., 110/220, 440/254) will satisfy the Code inasmuch as the lower “utilization” voltage values will result in a higher value of load current than if the “nominal” voltages are used. Because the Code is aimed at establishing a “minimum” load that must be served, the higher load that results from use of the lower “utilization” voltages will satisfy the Code and provide an additional measure of safety.
110.6. Conductor Sizes. In this country, the American Wire Gage (AWG) is the standard for copper wire and for aluminum wire used for electrical conductors. The American Wire Gage is the same as the Brown & Sharpe (B & S) gage. The largest gage size is 4/0 AWG; above this size the sizes of wires and cables are stated in thousands of circular mils (kcmil). The circular mil is a unit used for measuring the cross-sectional area of the conductor, or the area of the end of a wire which has been cut square across. One circular mil (commonly abbreviated cmil) is the area of a circle 1/1000 in. in diameter. The area of a circle 1 in. in diameter is 1,000,000 cmil; also, the area of a circle of this size is 0.7854 sq in.
To convert square inches to circular mils, multiply the square inches by 1,273,200.
To convert circular mils to square inches, divide the circular mils by 1,273,200 or multiply the circular mils by 0.7854 and divide by 1,000,000.
In interior wiring the gage sizes 14, 12, and 10 are frequently solid wire; 8 AWG and larger conductors in raceways are required to be stranded if pulled into a raceway, although in practice this is the usual configuration anyway, even in cable assemblies. (See 310.3.)
A cable (if not larger than 1,000,000 cmil) will have one of the following numbers of strands: 7, 19, 37, or 61. In order to make a cable of any standard size, in nearly every case the individual strands cannot be any regular gage number but must be some special odd size. For example, a Class B stranded 2/0 AWG cable must have a total cross-sectional area of 133,100 cmil and is made up of 19 strands. No. 12 AWG has an area of 6530 cmil and 11 AWG, an area of 8234 cmil; therefore each strand must be a special size between Nos. 12 and 11.
110.7. Insulation Integrity. This general rule requires that the integrity of the conductor insulation must be maintained. This can be accomplished by observing conduit fill limitations as well as proper pulling techniques. However, basic knowledge of insulation-resistance testing is important.
Measurements of insulation resistance can best be made with a megohmmeter insulation tester. As measured with such an instrument, insulation resistance is the resistance to the flow of direct current (usually at 500 or 1000 V for systems of 600 V or less) through or over the surface of the insulation in electrical equipment. The results are in ohms or megohms, but where the insulation has not been damaged, insulation-resistance readings should be in the megohm range.
110.8. Wiring Methods. All Code-recognized wiring methods are covered in Chap. 3 of the NE Code.
110.9. Interrupting Rating. Interrupting rating of electrical equipment is divided into two categories: current at fault levels and current at operating levels.
Equipment intended to clear fault currents must have interrupting rating equal to the maximum fault current that the circuit is capable of delivering at the line (not the load) terminals of the equipment. See Fig. 110-2. The internal impedance of the equipment itself may not be factored in to use the equipment at a point where the available fault current on its line side is greater than the rated, marked interrupting capacity of the equipment.
Fig. 110-2. (Sec. 110.9.)
If overcurrent devices with a specific AIR (ampere interrupting rating) are inserted at a point on a wiring system where the available short-circuit current exceeds the AIR of the device, a resultant downstream solid short circuit between conductors or between one ungrounded conductor and ground (in grounded systems) could cause serious damage to life and property. Since each electrical installation is different, the selection of overcurrent devices with a proper AIR is not always a simple task. To begin with, the amount of available short-circuit current at the service equipment must be known. Such short-circuit current depends upon the capacity rating of the utility primary supply to the building, transformer impedances, and service conductor impedances. Most utilities will provide this information. But, be aware that 110.9 essentially implies such calculations be performed for all electrical systems, and 110.10 mandates consideration of the available fault current at every point in the system where an overcurrent protective device is applied.
Downstream from the service equipment, AIRs of overcurrent devices generally will be reduced to lower values than those at the service, depending on lengths and sizes of feeders, line impedances, and other factors. However, large motors and capacitors, while in operation, will feed additional current into a fault, and this must be considered when calculating short-circuit currents.
Manufacturers of overcurrent devices have excellent literature on figuring short-circuit currents, including graphs, charts, and one-line-diagram layout sheets to simplify the selection of proper overcurrent devices.
In the last paragraph, the Code recognizes that equipment intended only for control of load or operating currents, such as contactors and unfused switches, must be rated for the current to be interrupted, but does not have to be rated to interrupt available fault current, as shown in Fig. 110-3.
Fig. 110-3. (Sec. 110.9.)
110.10. Circuit Impedance and Other Characteristics. This section requires that all equipment be rated to withstand the level of fault current that is let through by the circuit protective device in the time it takes to operate—without “extensive damage” to any of the electrical components of the circuit as illustrated in Fig. 110-4.
Fig. 110-4. (Sec. 110.10.)
The phrase “the component short-circuit current ratings” was added to this rule a few editions back. The intent of this addition is to require all circuit components that are subjected to ground faults or short-circuit faults to be capable of withstanding the thermal and magnetic stresses produced within them from the time a fault occurs until the circuit protective device (fuse or CB) opens to clear the fault, without extensive damage to the components.
The Code-making panel (CMP) responsible for Art. 110 has indicated that this section is not intended to establish a quantifiable amount of damage that is permissible under conditions of short circuit. The general requirement presented here is just that, a general rule. Specifics, regarding what damage is or is not acceptable under fault conditions, are established by the product test standard. For example, as stipulated in UL 508, Industrial Control Equipment, which covers combination motor-starters, the permissible damage for a Type E (the so-called self-protected) motor starter is different from the requirements for other types of motor starters. The Type E unit must satisfy a more rigorous performance criterion than the others. Therefore, it is the UL Standard, not 110.10, that requires a more rigorous performance criterion for the Type E starter than is required for the other types of listed motor-starters. The last sentence helps clarify that the prevention of “extensive damage” can be achieved by applying listed devices within their listed ratings. That is to say, the NEC does not intend to regulate product safety. Such regulation is the function of the NEMA/UL Product Standards. Any product that satisfies the controlling standard, and is applied in accordance with its ratings, is acceptable to the NEC and will satisfy the intent of this general rule.
110.11. Deteriorating Agents. Equipment must be “identified” for use in the presence of specific deteriorating agents, as shown on the typical nameplate in Fig. 110-5. In addition, equipment not normally suitable for use in wet locations must be protected from permanent damage while exposed to outdoor conditions during construction. The NEC has long stated that a dry location may be temporarily wet during building construction; this provision does not contradict that principle, but requires appropriate care during the construction process.
Fig. 110-5.
110.12. Mechanical Execution of Work. This statement has been the source of many conflicts because opinions differ as to what is a “neat and workmanlike manner.”
The Code places the responsibility for determining what is acceptable and how it is applied in the particular jurisdiction on the authority having jurisdiction. This basis in most areas is the result of:
1. Competent knowledge and experience of installation methods.
2. What has been the established practice by the qualified journeyman in the particular area.
3. What has been taught in the trade schools having certified electrical training courses for apprentices and journeymen.
Examples which generally would not be considered as “neat and workman-like” include nonmetallic cables installed with kinks or twists; unsightly exposed runs; wiring improperly trained in enclosures; slack in cables between supports; flattened conduit bends; or improvised fittings, straps, or supports. See Fig. 110-6.
Fig. 110-6. Irregular stapling of BX to bottoms of joists and ragged drilling of joists add up to an unsightly installation that does not appear “workmanlike.” (Sec. 110.12.)
It has long been required in specific Code rules that unused openings in boxes and cabinets be closed by a plug or cap and such rules were presented in what was then Art. 370, now Art. 314, and Art. 373, now Art. 312. The requirement, now given in part (A) for such plugging of open holes is also a general rule to provide fire-resistive integrity of all equipment—boxes, raceways, auxiliary gutters, cabinets, equipment cases, or housings (Fig. 110-7). This rule does not extend to mounting holes in the back of boxes, etc. Not specifically mentioned, but presumably still permitted, would be weep holes drilled in outdoor enclosures.
Part (B) presents a requirement for current-carrying parts—buswork, terminals, etc.—that is similar to the rule of 250.12. Both rules effectively prohibit conductive surfaces from being rendered nonconductive due to the introduction of paint, lacquer, or other substances. It should be noted that this rule is not intended to prohibit the use of “cleaners.” Use of cleaning agents is recognized, but only those agents that do not contaminate conductive surfaces or deteriorate nonmetallic structures within the enclosure, as some spray lubricants are capable of doing. Be certain that any type of cleaners used for maintenance purposes is suitable for the specific application.
This section also indicates that defective equipment may not be used. Although wording prohibiting the use of damaged or otherwise defective equipment may seem superfluous, apparently many installers were using or reusing damaged equipment. At complete odds with common sense, such practice puts those who use and maintain the system at risk and is expressly forbidden. And although not specifically mentioned, any equipment that is damaged during the construction phase should be considered as covered by the rule of 110.12(B) and should be replaced.
Fig. 110-7. Unused openings in any electrical enclosure must be plugged or capped. Any punched knockout that will not be used must be closed, as at arrow.
110.14. Electrical Connections. Proper electrical connections at terminals and splices are absolutely essential to ensure a safe installation. Improper connections are the cause of most failures of wiring devices, equipment burndowns, and electrically oriented fires. Remember, field installation of electrical equipment and conductors boils down to the interconnection of manufactured components. The circuit breakers, conductors, cables, raceway systems, switchboards, MCCs, panelboards, locknuts, bushings—everything—only need be connected together. With that in mind it can be easily understood why the most critical concern for any designer and installer should be the actual interconnection of the various system components—especially terminations of electrical conductors and bus. Although failure to properly terminate conductors is presently the primary cause of system failure throughout the country, it can easily be overcome by attention to, and compliance with, the rules of this section as well as applicable listing and installation instructions.
Terminals and splicing connectors must be “identified” for the material of the conductor or conductors used with them. Where in previous NEC editions this rule called for conductor terminal and splicing devices to be “suitable” for the material of the conductor (i.e., for aluminum or copper), the wording now requires that terminal and splicing devices must be “identified” for use with the material of the conductor. And devices that combine copper and aluminum conductors in direct contact with each other must also be “identified for the purpose and conditions of use.”
The NEC definition of identified does not specifically require that products be marked to designate specific application suitability, as noted in the discussion in Art. 100 on the topic of “Identified.” However, the general information from the UL directory quoted in the discussion of 110.3 does say that terminations are generally suitable for copper wire only, and where aluminum is suitable there will be a marking. In addition, the installation instructions furnished with the equipment will clearly indicate whether aluminum terminations are permitted, and how they are to be made and torqued.
In general, pressure-type wire splicing lugs or connectors bear no marking if suitable for only copper wire. If suitable for copper, copper-clad aluminum, and/or aluminum, they are marked “AL-CU”; and if suitable for aluminum only, they are marked “AL.” Devices listed by Underwriters Laboratories Inc. indicate the range or combination of wire sizes for which such devices have been listed. Terminals of 15- and 20-A receptacles not marked “CO/ALR” are for use with copper and copper-clad aluminum conductors only. Terminals marked “CO/ALR” are for use with aluminum, copper, and copper-clad aluminum conductors.
The vast majority of distribution equipment has always come from the manufacturer with mechanical set-screw-type lugs for connecting circuit conductors to the equipment terminals. Lugs on such equipment are commonly marked “AL-CU” or “CU-AL,” indicating that the set-screw terminal is suitable for use with either copper or aluminum conductors. But, such marking on the lug itself is not sufficient evidence of suitability for use with aluminum conductors. UL requires that equipment with terminals that are found to be suitable for use with either copper or aluminum conductors must be marked to indicate such use on the label or wiring diagram of the equipment—completely independent of a marking like “AL-CU” on the lugs themselves. A typical safety switch, for instance, would have lugs marked “AL-CU,” but also must have a notation on the label or nameplate of the switch that reads like this: “Lugs suitable for copper or aluminum conductors.”
UL-listed equipment must be used in the condition as supplied by the manufacturer—in accordance with NE Code rules and any instructions covered in the UL listing in the Guide Card information for the product category—as required by the NE Code’s 110.3(B). Unauthorized alteration or modification of equipment in the field is not covered by the UL listing and can lead to very dangerous conditions. For this reason, any arbitrary or unspecified changing of terminal lugs on equipment is not acceptable unless such field modification is recognized by UL and spelled out very carefully in the manufacturers’ literature and on the label of the equipment itself.
For instance, UL-listed authorization for field changing of terminals on a safety switch might be described in manufacturers’ catalog data and on the switch label itself. It is obvious that field replacement of set-screw lugs with compression-type lugs can be a risky matter if great care is not taken to assure that the size, mounting holes, bolts, and other characteristics of the compression lug line up with and are fully compatible for replacement of the lug that is removed. Careless or makeshift changing of lugs in the field has produced overheating, burning, and failures. To prevent junk-box assembly of replacement lugs, UL requires that any authorized field replacement data must indicate the specific lug to be used and also must indicate the tool to be used in making the crimps. Any crimp connection of a lug should always be done with the tool specified by the lug manufacturer. Otherwise, there is no assurance that the type of crimp produces a sound connection of the lug to the conductor.
The last sentence of 110.14(A) also prohibits use of more than one conductor in a terminal (see Fig. 110-8) unless the terminal is identified for the purpose (meaning generally recognizable as suitable for the purpose by appropriate markings or instructions).
Fig. 110-8. [Sec. 110.14(A).]
Use of the word “identified” in the last sentence of 110.14(A) could be interpreted to require that terminals suited to use with two or more conductors must somehow be marked. This is a frequent example of where installers confuse “identified” with “marked” as covered in the discussion of the definition of “identified.” For a long time terminals suited to and acceptable for use with aluminum conductors have been marked “AL-CU” or “CU-AL” right on the terminal. Twist-on or crimp-type splicing devices are “identified” both for use with aluminum wires and for the number and sizes of wires permitted in a single terminal—with the identification marked on the box in which the devices are packaged or marked on an enclosed sheet.
For set-screw and compression-type lugs used on equipment or for splicing or tapping-off, suitability for use with two or more conductors in a single barrel of a lug could be marked on the lug in the same way that such lugs are marked with the range of sizes of a single conductor that may be used (e.g., “No. 2 to No. 2/0.”). But the intent of the Code rule is that any single-barrel lug used with two or more conductors must be tested for such use (such as in accordance with UL 486B standard), and some indication must be made by the manufacturer that the lug is properly suited and rated for the number and sizes of conductors to be inserted into a single barrel. Again, the best and most effective way to identify a lug for such use is with marking right on the lug, as is done for “AL-CU.” But the second sentence of 110.3(A)(1) also allows such identification to be “provided with” a product, such as on the box or on an instruction sheet. See Fig. 110-9.
A fine-print note after the first paragraph of 110.14 calls attention to the fact that manufacturers are marking equipment, terminations, packing cartons, and/or catalog sheets with specific values of required tightening torques (pound-inches or pound-feet). Although that puts the installer to the task of finding out appropriate torque values, virtually all manufacturers are presently publishing “recommended” values in their catalogs and spec sheets. In the case of connector and lug manufacturers, such values are even printed on the boxes in which the devices are sold. In 110.3(B), the NEC requires that all listed equipment be used as indicated by the listing instructions that are issued with the product. In virtually all cases, where a mechanical type terminating device is used, the manufacturer will indicate a prescribed torque value. That is the value that was used during product testing. In order for one to be certain that the installed equipment will operate as it did during product certification testing, the equipment must be used in the same manner it was during testing. And that includes torquing the terminating devices to the values prescribed in the manufacturer’s installation instructions. Failure to torque every terminal to the manufacturer-prescribed value is a clear and direct violation of this Code section.
Fig. 110-9. A terminal with more than one conductor terminated in a single barrel (hole) of the lug (at arrows) must be “identified” (marked, listed, or otherwise tested and certified as suitable for such use).
Torque is the amount of tightness of the screw or bolt in its threaded hole; that is, torque is the measure of the twisting movement that produces rotations around an axis. Such turning tightness is measured in terms of the force applied to the handle of the device that is rotating the screw or bolt and the distance from the axis of rotation to the point where the force is applied to the handle of the wrench or screwdriver:
Torque (lb ft) = force (lb) × distance (ft)
Torque (lb in.) = force (lb) × distance (in.)
Because there are 12 in. in a foot, a torque of “1 lb-ft” is equal to “12 lb-in.” Any value of “pound-feet” is converted to “pound-inches” by multiplying the value of “pound-feet” by 12. To convert from “pound-inches” to “pound-feet,” the value of “pound-inches” is divided by 12.
Note: The expressions “pound-feet” and “pound-inches” are preferred to “foot-pounds” or “inch-pounds,” although the expressions are used interchangeably. When the unit leads with the distance (such as foot-pounds), it is supposed to be referring to a unit of energy in classical physics, where energy is defined as the product of applied force and the distance over which that force is acting.
Torque wrenches and torque screwdrivers are designed, calibrated, and marked to show the torque (or turning force) being exerted at any position of the turning screw or bolt. Figure 110-10 shows typical torque tools and their application.
Section 110.14(B) covers splice connectors and similar devices used to connect fixture wires to branch-circuit conductors and to splice circuit wires in junction boxes and other enclosures. Much valuable application information on such devices is given in the UL Electrical Construction Materials Directory, under the heading of “Wire Connectors and Soldering Lugs.” The new last sentence of 110.14(B) states that connectors or splices used with directly buried conductors must be listed for the application.
This wording makes the use of listed connectors and splice kits mandatory where used directly buried. As indicated by the submitter of this proposal for a change, such equipment is listed, is commercially available, and should be used.
Part (C) of 110.14 reiterates the UL rules regarding temperature limitations of terminations, which are made mandatory by 110.3(B). It is worth noting that the information given by UL in the guide card information for “Electrical Equipment for Use in Ordinary Locations” is more detailed. The FPN following this section is intended to indicate that if information in a general or specific UL rule permits or requires different ratings and/or sizes, the UL rule must be followed.
The last sentence indicates acceptability of 90°C-rated wire where applied in accordance with the temperature limitations of the termination.
90°C-insulated conductors may be used in virtually any application that 60°Cor 75°C-rated conductors may be used and in some that the lower-rated conductors cannot. But the ampacity of the 90°C-rated conductor must never be taken to be more than that permitted in the column that corresponds with the temperature rating of the terminations to which the conductor will be connected. And that applies to both ends of the conductor. For example, consider a 6 AWG THHN copper conductor, which has a Table 310.16 ampacity of 55 A in the 60°C column. But, if the 6 AWG is supplied from a CB with, say, a 60/75°C-rating, it may be considered to be a 65-A wire, provided the equipment end is also rated 60/75°C or 75°C. But, if the equipment at either end of the wire is rated at 60°C,
Fig. 110-10. Readily available torque tools are (at top, L–R): torque screwdriver, beam-type torque wrench, and ratchet-type torque wrench. These tools afford ready compliance with the implied requirement of the fine-print note in the Code rule. [Sec. 110.14.]
or unmarked and therefore rated that way by default, the 6 AWG THHN copper conductor may carry no more than the 60°C-ampacity (55 A) shown in Table 310.16 for a 6-AWG copper wire. The wording in parts (C)(1)(a)(3) and (C)(1)(b)(2) is intended to indicate as much (Fig. 110-11). Note also that most motors go directly to the 75°C ratings regardless of wire size, per (C)(1)(a)(4).
Fig. 110-11. 90°C-insulated conductors may be used even where derating is not required, provided they are taken as having an ampacity not greater than the ampacity shown in the column from Table 310.16 that corresponds to the temperature rating of the terminations—at both ends—to which the conductors will be connected.
110.15. High-Leg Marking. Here the Code mandates a specific “color-coding” for the “high leg” in a 4-wire delta-system. These systems, used in certain areas, create two 120-V “legs” by center-tapping one of the secondary windings of a 240-V delta-wound transformer. That is, by grounding the center point of one winding, the phase-to-phase voltage remains 240 V, 3-phase, and the voltage between phase A and the grounded conductor, and between phase C and the grounded conductor will be 120 V, single phase. BUT the difference of potential between phase B and the grounded conductor will be 208 V, single-phase. The fact that phase B is at 208 V, with respect to the grounded center-tapped conductor, while phases A and C are at 120 V, is the reason that this type of distribution system is know as a “high-leg” delta system.
As indicated, this rule requires color-coding of the “high leg” (i.e., phase B—the one that’s at 208 V to the grounded conductor) or other “effective means” to identify the “high leg.” This “identification” must be provided at any point in the system where “a connection is made” and the high leg is run with the other circuit conductors. That would include enclosures where the high leg is itself not connected but merely “present” within the enclosure, and would exclude enclosures where the grounded conductor is not present, such as where the three phase-legs supply a motor load. Although the wording used in this section would permit identifying the high leg by tagging with numbers or letters, or other “effective means,” where color-coding is used, the high leg must be colored orange. Use of other colors to identify the high leg would seem to be a violation of the wording used here.
110.16. Flash Protection. This section calls for a field marking of electrical equipment such as switchboards, panelboards, motor control centers, meter socket enclosures, and industrial control panels—provided by the installer at the time of installation—that indicates “flash protection” is required when maintaining such equipment. The marking is intended to alert maintenance personnel of the need for protective gear when working on the equipment while it is energized. Such marking must be on the exterior of covers and doors that provide access to energized live parts to satisfy the requirement for the warning to be “clearly visible” “before examination, etc.” Refer to NFPA 70E® for detailed information on selecting personal protective equipment (PPE) that is appropriate for the degree of arc flash exposure involved.
110.18. Arcing Parts. Complete enclosures are always preferable, but where this is not practicable, all combustible material must be kept well away from the equipment.
110.20. Enclosure Types. This section and table cover selection criteria on types of enclosures. The second paragraph of this section makes the selection criteria set forth in the table mandatory. The material has been relocated from 430.91 because it does not just cover motor control centers but has general applicability for all installations. This section gives selection data, with characteristics tabulated, for application of the various NEMA types of motor controller enclosures for use in specific nonhazardous locations operating at 600 V and below. Note that this table is incorrectly located because it does not apply to medium-voltage equipment, but the rule occurs in Part I (General) and not Part II (600 V, Nominal, or Less) of the Article. This will need attention in the next code cycle.
110.21. Marking. The marking required in 110.21 should be done in a manner that will allow inspectors to examine such marking without removing the equipment from a permanently installed position. It should be noted that the last sentence in 110.21 requires electrical equipment to have a marking durable enough to withstand the environment involved (such as equipment designed for wet or corrosive locations).
110.22. Identification of Disconnecting Means. As shown in Fig. 110-12, it is a mandatory Code rule that all disconnect devices (switches or CBs) for load devices and for circuits be clearly and permanently marked to show the purposes of the disconnects. This is a “must” and, under OSHA, it applies to all existing electrical systems, no matter how old, and also to all new, modernized, expanded, or altered electrical systems. This requirement for marking has been widely neglected in electrical systems in the past. Panelboard circuit directories must be fully and clearly filled out. And all such marking on equipment must be in painted lettering or other substantial identification.
This rule now appears as 110.22(A) because the section has been divided into three lettered paragraphs. Effective identification of all disconnect devices is a critically important safety matter. When a switch or CB has to be opened to de-energize a circuit quickly—as when a threat of injury to personnel dictates—it is absolutely necessary to identify quickly and positively the disconnect for the circuit or equipment that constitutes the hazard to a person or property. Painted labeling or embossed identification plates affixed to enclosures would comply with the requirement that disconnects be “legibly marked” and that the “marking shall be of sufficient durability.” Paste-on paper labels or marking with crayon or chalk could be rejected as not complying with the intent of this rule. Ideally, marking should tell exactly what piece of equipment is controlled by a disconnect (switch or CB) and should tell where the controlled equipment is located and how it may be identified. Figure 110-13 shows a case of this kind of identification as used in an industrial facility where all equipment is marked in two languages because personnel speak different languages. And that is an old installation, attesting to the long-standing recognition of this safety feature.
Fig. 110-12. All circuits and disconnects must be identified. OSHA regulations make NE Code Sec. 110.22 mandatory and retroactive for existing installations and for all new, expanded, or modernized systems—applying to switches as well as circuit breakers. (Sec. 110.22.)
Fig. 110-13. Identification of disconnect switch and pushbutton stations is “legibly marked” in both English and French—and is of “sufficient durability to withstand the environment”—as required by the Code rule. (Sec. 110.22.)
The rule of 110.22 has long required that every disconnect be marked to indicate exactly what it controls. And that marking must be legible and sufficiently durable to withstand the environment to which it will be exposed. And, the rule of 408.4 requires that any modifications also be reflected in the circuit directory of panelboards. While most are aware of the requirement in 110.22, it seems as if very few pay any attention to this part of the rule of 408.4.
It should be noted that the marking of disconnects is one of the few requirements that is made retroactive by OSHA. That is, regardless of when the disconnect was installed, or when a modification was performed, the purpose of every disconnect must be marked at the disconnect. If your facility or your customer’s facility does not have such markings for each and every disconnect, every effort should be made to ensure that a program to provide such markings is initiated and completed. Failure to do so could result in heavy fines should you be subject to an inspection by OSHA (Fig. 110-13).
The second paragraph, 110.22(B), covers the field marking requirements for series combinations of circuit breakers or fused switches used in an “engineered series combination” with downstream devices that do not have an interrupting rating equal to the available short-circuit current but are dependent for safe operation on upstream protection that is rated for the short-circuit current; enclosure(s) for such “series rated” protective devices must be marked in the field “Caution—Engineered Series Combination System Rated _____ Amperes. Identified Replacement Components Required.”
This provision correlates with the new procedure in 240.86(A) that allows, under strict engineering supervision and only in existing installations, the use of upstream overcurrent protective devices with a let-through current under fault conditions that does not exceed the interrupting rating of a downstream overcurrent device. This allows for adapting existing installations to increases in available fault current resulting from changes in the infrastructure of the serving utility or other factors. The engineer must be able to certify that the lower-rated downstream device will not begin to open or melt during the operating period of the upstream device, or the downstream device may be subjected to the full available fault in excess of its rating, instead of merely the let-through current. This requires a very sophisticated evaluation, and will be completely defeated if the wrong replacement component is selected.
The third paragraph, 110.22(C), says that where circuit breakers or fused switches are used in a “tested series combination” with downstream devices that do not have an interrupting rating equal to the available short-circuit current but are dependent for safe operation on upstream protection that is rated for the short-circuit current, enclosure(s) for such “series rated” protective devices must be marked in the field “Caution—Series Combination System Rated _____ Amperes. Identified Replacement Components Required.”
Such equipment is typically employed in multimeter distribution equipment for multiple-occupancy buildings—especially residential installations—with equipment containing a main service protective device that has a short-circuit interrupting rating of some value (e.g., 65,000 A) that is connected in series with feeder and branch-circuit protective devices of considerably lower short-circuit interrupting ratings (say 22,000 or 10,000 A). Because all of the protective devices are physically very close together, the feeder and branch-circuit devices do not have to have a rated interrupting capability equal to the available short-circuit current at their points of installation. Although such application is a literal violation of NEC 110.9, which calls for all protective devices to be rated for the short-circuit current available at their supply terminals, “series rated” equipment takes advantage of the ability of the protective devices to operate in series (or in cascade as it is sometimes called) with a fault current interruption on, say, a branch circuit being shared by the three series protective devices—the main, feeder, and branch circuit. Such operation can enable a properly rated main protective device to protect downstream protective devices that are not rated for the available fault. When manufacturers combine such series protective devices in available distribution equipment, they do so on the basis of careful testing to assure that all of the protective devices can operate without damage to themselves. Then UL tests such equipment to verify its safe and effective operation and will list such equipment as a “Series Rated System.”
Because UL listing is based on use of specific models of protective devices to assure safe application, it is critically important that all maintenance on such equipment be based on the specific equipment. For that reason, this Code rule demands that the enclosure(s) for all such equipment be provided with “readily visible” markings to alert all personnel to the critical condition that must always be maintained to ensure safety. Thus, all series-rated equipment enclosure(s) must be marked.
Note that the enhanced requirements for series-rated systems only apply where the series rating is required to satisfy 110.9. For example, on a 22 kA available fault current system at the service disconnect, if it can be shown that there is sufficient static impedance in the length of feeder between the service and a downstream panel so that the available fault current at that panel is only 10 kA, then no series connection listing is involved and this marking requirement does not apply. This is true even though the downstream panel is still in series with the service protective device. Series-connected listings are an economical way to avoid a fully rated system, but they are not required when the conditions in the field are such that a downstream overcurrent device could not be subjected to a fault current in excess of its rating, and therefore comply with 110.9 without any upstream assistance. Since a very little impedance goes a long way in reducing an otherwise very high available fault current, this is a very frequent practical result. Many engineers take this into account when they position downstream equipment.
110.26. Spaces About Electric Equipment (600 V, Nominal, or Less). The basic rule of 110.26 calls for “sufficient access and working space” to be provided in all cases to permit ready and safe operation and maintenance of electrical equipment. This rule applies to receptacles and all electrical equipment. However, the specific work space dimensions and other rules that are in the lettered paragraphs following only apply under the conditions set forth in those paragraphs. For example, a hydromassage bathtub motor and receptacle that has no access door provided is in violation of this section (as well as some prescriptive criteria in Art. 680). However, the full panoply of required work space widths and depths do not apply because such equipment does not need to be worked hot.
The wording of 110.26(A) calls for compliance with parts (A)(1), (2), and (3). Those three subparts define the work space zone needed at electrical equipment. These rules are slightly modified and expanded upon by parts (B), (C), (D), and (E). Part (F) has nothing whatever to do with work space and covers the dedicated wiring space above (and below) certain pieces of equipment.
As indicated by 110.26(A)(1), the dimensions [shown in Table 110.26(A)] of working space in the direction of access to live electrical parts for equipment operating at 600 V or less—where live parts are exposed—or to the equipment enclosure—in the usual case where the live parts are enclosed—must be carefully observed. The minimum clearance is 3 ft. The minimum of 3 ft was adopted for Code Table 110.26(A) to make all electrical equipment—panel-boards, switches, breakers, starters, etc.—subject to the same 3-ft (914-mm) minimum to increase the level of safety and assure consistent, uniform spacing where anyone might be exposed to the hazard of working on any kind of live equipment. Application of Code Table 110.26(A)(1) to the three “conditions” described in Table 110.26(A) is shown in the sketches making up this hand-book’s Table 110-1. Figure 110-14 shows a typical example of Condition 3.
Table 110-1. Clearance Needed in Direction of Access to Live Parts in Enclosures for Switchboards, Panelboards, Switches, CBs, or Other Electrical Equipment—Plan Views [110.26(A)]
Fig. 110-14. Condition 3 in Code Table 110.26(A) for the rule covered by Sec. 110.26(A) applies to the case of face-to-face enclosures, as shown here where two switchboards face each other. The distance indicated must be at least 3 or 4 ft depending on the voltage of enclosed parts. [Sec. 110.26(A).]
According to 110.26(A)(1)(a), a “minimum” depth of work space behind equipment rated 600 V, and less, must be provided where access is needed when deenergized. The past few editions of the NEC have required a minimum depth of work space behind equipment rated over 600 V where access was required only when the equipment was deenergized. For equipment rated over 600 V requiring rear access only when deenergized, 110.34(A)(1) mandates that the depth of work space must not be less than 762 mm (30 in.). The same rule applies to equipment rated 600 V or less (Fig. 110-15).
Fig. 110-15. Where access is needed, but only when the equipment is deenergized, the work space need only be 30 in. deep. This addition to part (a) in Sec. 110.26(A) applies only to those cases where access is needed only when the equipment is deenergized. As always, if access is needed at the rear for “examination, adjustment, servicing, or maintenance” when the equipment is energized, the depth, as well as the other aspects of work space must satisfy the basic rule. And, if there is never a need to gain access to the rear of the equipment, there is no minimum depth required by the Code, but careful attention should be paid to any clearances required by the equipment manufacturer.
110.26(A)(1)(b) allows working clearance of less than the distances given in Table 110.26(A) for live parts that are operating at not over 30 V RMS, 42 V peak, or 60 V DC. The last phrase recognizes the inherent safety of low-voltage circuits like the Class 2 and Class 3 control and power-limited circuits covered by Art. 725, as well as certain other low-voltage systems recognized in Chaps. 7 and 8. This exception allows less than the 3-ft minimum spacing of Table 110.26(A) for live parts of low-voltage communication, control, or power-limited circuits. BUT, only where “special permission” is granted. Remember, any such “special permission” must be in writing.
Part (1)(c) to 110.26(A) permits smaller work space when replacing equipment at existing facilities, provided procedures are established to ensure safety. This allowance responds to widespread misinterpretation of these rules at the time 480- and 600-V motor control centers were being installed in industrial occupancies with Condition 2 dimensioned aisles. Many engineers considered this to be a 3½-ft clearance because when staff would be working one side hot the other side would be closed, and therefore present a grounded and not energized surface. This was never the intent of the rule, because often both sides are worked hot at the same time. The NEC has been reworded to preclude this misinterpretation from continuing.
However, when it came time to upgrade such existing nonconforming installations, facilities were in a quandary because now the room and the conduits were in place. The solution was to provide limited relief for existing applications that allows Condition 2, but only if qualified personnel are involved and there are written procedures in place that preclude working both sides of the aisle hot.
In part (A)(2) of 110.26, the Code mandates a minimum width for the required clear work space. For all equipment, the work space must be 762 mm (30 in.) or the width of the equipment, whichever is greater. Note that there is no requirement to center the equipment in the clear space, only the requirement to provide the space, which may even begin right at one edge of the equipment and then extend beyond the equipment on the other side.
And, as required by the second sentence of Sec. 110.26(A)(2), clear work space in front of any enclosure for electrical equipment must be deep enough to allow doors, hinged panels, or covers on the enclosure to be opened to an angle of at least 90°. Any door or cover on a panelboard or cabinet that is obstructed from opening to at least a 90° position makes it difficult for any personnel to install, maintain, or inspect the equipment in the enclosure safely. Full opening provides safer access to the enclosure and minimizes potential hazards (Fig. 110-14). Although this rule seems to be related to “depth” of work space, it is covered in 110.26(A)(2), “Width of Working Space.” (See Fig. 110-16.)
110.26(A)(3), “Height of Working Space,” relates to part (E), “Headroom.” Here the Code defines the height of the required working space “depth” and “width” described in parts (A) and (B) of 110.26. The depth and width is supplemented by a third dimension that constitutes the zone at electrical equipment that must be provided and kept clear.
The wording here does permit a limited intrusion into the required work space. Equipment associated with the electrical installation, such as wireways, pull boxes, etc., may protrude into the work space, but not more than 150 mm (6 in.) beyond the front of the electrical equipment that requires the dedicated space. This intrusion is permitted either below or above the equipment in question. It is also permitted for even the items specified in 110.26(E) (service equipment, panelboards, distribution boards, and motor control centers) because all 110.26(E) does is to establish the extent of the vertical dimension. The rule in this location, and no other, determines the extent to which the required work space may be intruded upon. Note that this allowance does not permit large transformers or other equipment that extend further into the work space.
Fig. 110-16. Working space required in front of electrical equipment for side-to-side clearance and door opening. [Sec. 110.26(A).]
Part (B) in 110.26 presents a very important requirement regarding the use of work space. The three-dimensional area identified by parts (A)(1), (A)(2), and (A)(3) of 110.26 must not only be provided but must be maintained! That is, such space must be viewed and treated as an “exclusion zone.” There may be no other things in the work space—not even on a “temporary” basis.
The second sentence of 110.26(B) addresses maintenance situations on equipment located in “passageways or general open space.” The concern here is for unqualified personnel, coming into the proximity of and potentially in contact with live electrical parts. Where equipment located in areas accessible to the general population of a building or facility is opened to perform maintenance or repair, the work space area must be cordoned off to keep unqualified persons from approaching the live parts. Failure to do so clearly constitutes a violation of the NEC.
In 110.26(C), “Entrance to Working Space,” the Code regulates the necessary entrance/exit to the work space. In part (A), a general statement calls for at least one entrance, of sufficient size, be provided to allow the work space to be entered/exited. Although the wording used here—“of sufficient area”—does not clearly define the dimensions needed to ensure compliance, it seems safe to assume that compliance with the dimensions spelled out in 110.26(C)(2) (i.e., 610 mm (24 in.) wide × 2.0 m (6½ ft) high) would be acceptable to the vast majority of electrical inspectors, if not all electrical inspectors.
As an added safety measure, to prevent personnel from being trapped in the working space around burning or arcing electrical equipment, the rule of 110.26(C)(2) requires two “entrances” or directions of access to the working space around any equipment enclosure that contains “overcurrent devices, switching devices, or control devices,” where such equipment is rated 1200 A or more, and which is also larger than 1.8 m (6 ft) wide; both conditions must be met before the enhanced access rules in this provision apply. This rule covers all types of equipment. That is, 110.26(C)(2) requires two “entrances” or directions of access to the working space around switchboards, motor-control centers, distribution centers, panelboard lineups, UPS cubicles, rectifier modules, substations, power conditioners, and any other equipment that is rated 1200 A or more.
At each end of the working space at such equipment, an entranceway or access route at least 610 mm (24 in.) wide and at least 2.0 m (6½ ft) high must be provided. Because personnel have been trapped in work spaces by fire between them and the only route of exit from the space, rigid enforcement of this rule is likely. Certainly, design engineers should make two paths of exit a standard requirement in their drawings and specs. Although the rule does not require two doors into an electrical equipment room, it may be necessary to use two doors in order to obtain the required two entrances to the required work space—especially where the switchboard or control panel is in tight quarters and does not afford a 24-in.-wide path of exit at each end of the work space.
In Fig. 110-17, sketch “A” shows compliance with the Code rule—providing two areas for entering or leaving the defined dimensions of the work space. In that sketch, placing the switchboard with its front to the larger area of the room and/or other layouts would also satisfy the intent of the rule. It is only necessary to have an assured means of exit from the defined work space. If the space in front of the equipment is deeper than the required depth of work space, then a person could simply move back out of the work space at any point along the length of the equipment. That is confirmed by the wording in 110.26(C)(2)(a). A similar idea is behind the objective of part (b) to 110.26(C)(2), which recognizes that where the space in front of equipment is twice the minimum depth of working space required by Table 110.26(A) for the voltage of the equipment and the conditions described, it is not necessary to have an entrance at each end of the space (Fig. 110-18). In such cases, a worker can move directly back out of the working space to avoid fire. For any case where the depth of space is not twice the depth value given in Table 110.26(A) for working space, an entranceway or access route at least 24 in. wide must be provided at each end of the working space in front of the equipment.
Sketch “B” in Fig. 110-17 shows the layout that must be avoided. With sufficient space available in the room, layout of any equipment rated 1200 A or more and over 1.8 m (6 ft) wide with only one exit route from the required work space would be a clear violation of the rule. As shown in sketch “B,” a door at the right end of the working space would eliminate the violation. But, if the depth D in sketch “B” is equal to or greater than twice the minimum required depth of working space from Table 110.26(A) for the voltage and “conditions” of installation, then a door at the right is not needed and the layout would not be a violation.
Fig. 110-17. There must be two paths out of the work space required in front of any equipment containing fuses, circuit breakers, motor starters, switches, and/or any other control or protective devices, where the equipment is rated 1200 A or more and is more than 1.8 m (6 ft) wide. [Sec. 110.26(C).]
Fig. 110-18. This satisfies Exception No. 2 to Sec. 110.26(C).
The last sentence in 110-12(C)(2)(b) states that when the defined work space in front of an electrical switchboard or other equipment has an entranceway at only one end of the space, the edge of the entrance nearest the equipment must be at least 3, 3½, or 4 ft (900 mm, 1.0 m, or 1.2 m) away from the equipment—as designated in Table 110.26(A) for the voltage and conditions of installation of the particular equipment. This Code requirement requires careful determination in satisfying the precise wording of the rule. Figure 110-19 shows a few of the many possible applications that would be subject to the rule.
The third numbered paragraph in 110.26(C)(2) puts forth an additional requirement for installations where doors are used as a means of access. Here the NEC mandates that any such door be provided with special hardware to facilitate exit where a maintenance person has lost the use of their hands, as could be the case in a fire. The Code calls for the “egress” doors to open in the direction of egress, which of course is “out” of the spaces. Additionally, such doors must be fitted with “panic bar” or “pressure plate” type opening hardware. The open-out and the panic-hardware rules now apply to all egress doors within 25 ft of the work space area. The issue is making sure that an injured electrician who has been burned and cannot use his hands to turn a knob can get far enough away from the burndown to seek help, without unduly burdening the rest of the facility with special hardware requirements. The NEC is unclear as to how the 25 ft is to be measured, whether in a straight line on a plan view or by proceeding through a reasonably assumed route of travel, but the route of travel would seem to be more closely related to the motivation for the requirement.
Fig. 110-19. Arcing burndown must not block route of exit. [Sec. 110.26(C).]
This requirement only applies to large equipment, where it is assumed that the risks are greater just as it is assumed that enhanced egress rules are appropriate. Although not required for other than “Large Equipment” work space access, recommending and providing such a means of egress is not a Code violation and could be viewed as an added safety feature.
110.26(D) requires lighting of work space at “service equipment, switchboards, panelboards, or motor control centers installed indoors.” The basic rule is shown in Fig. 110-20.
Fig. 110-20. Electrical equipment requires lighting and 6½-ft headroom at all work spaces around certain equipment. [Secs. 110.26(D) and (E).]
The second to last sentence in 110.26(D) points out the Code-making panel’s intent. That is, if an adjacent fixture provides adequate illumination, another fixture is not required. In dwelling units where the identified equipment is located in a habitable room, a switched receptacle outlet, as permitted by 210.70(A)(1) Exception No. 1, would also satisfy the requirement for illumination given here. And, lastly, control of the required lighting outlet at electrical equipment by automatic means, only, is prohibited. But, the use of automatic control along with a manual override would meet the spirit and the letter of this rule.
It should be noted that although lighting is required for safety of personnel in work spaces, nothing specific is said about the kind of lighting (incandescent, fluorescent, mercury-vapor), no minimum footcandle level is set, and such details as the position and mounting of lighting equipment are omitted. All that is left to the designer and/or installer, with the inspector the final judge of acceptability.
In 110.26(E), minimum headroom, which must extend from the floor or work platform to 6½ ft (2 m) or the height of the equipment, in the working spaces is required around electrical equipment. The rule applies to “service equipment, switchboards, panelboards, or motor control centers.” The Exception permits “service equipment or panelboards, in existing dwelling units, that do not exceed 200 amperes” to be installed with less than 6½ ft of headroom—such as in crawl spaces under single-family houses. But the permission for reduced headroom of the equipment described in the Exception applies only in existing “dwelling units” that meet the definition of that phrase. In any space other than an existing dwelling unit, all indoor service equipment, switchboards, panel-boards, or control centers must have headroom at the equipment that is at least 6½ ft high, but never less than the height of the equipment.
Details on lighting and headroom are shown in Fig. 110-20. But, in that sketch, it should be noted that the 2.0 m (6½ ft) headroom must be available for the entire length of the work space. There must be a 2.0 m (6½ ft) clearance from the floor up to the bottom of the light fixture or to any other overhead obstruction—and not simply to the ceiling or bottom of the joists.
110.26(F). Dedicated Equipment Space. Pipes, ducts, etc., must be kept out of the way of circuits from panelboards and switchboards. This rule is aimed at ensuring clean, unobstructed space for proper installation of switchboards and panel-boards, along with the connecting wiring methods used with such equipment.
The wording of this rule has created much confusion among electrical people as to its intent and correct application in everyday electrical work. And, it seems that one can develop a complete understanding of this rule only by repeated readings. On first reading, there are certain observations about the rule that can be made clearly and without question:
1. Although the rule is aimed at eliminating the undesirable effects of water or other liquids running down onto electrical equipment and entering and contacting live parts—which should always be avoided both indoors and outdoors—the wording of the first sentence limits the requirement to switchboards, panelboards, “distribution boards”—whatever they are—and to motor control centers. Individual switches and CBs and all other equipment are not subject to the rule—although the same concern for protection against liquid penetrations ought to be applied to all such other equipment. The reason for this is rooted in the history of the requirement and not technical merit. The rule originated in what is now Art. 408 under the control of a different code-making panel, and in that location the other types of equipment were not within the scope of that article. The material was relocated by order of the Correlating Committee, but the wording here still is focused on the old applications.
2. The designated electrical equipment covered by the rule (switchboards, panelboards, etc.) does not have to be installed in rooms dedicated exclusively to such equipment, although it may be. This rule applies only to the area above the equipment, for the width and depth of the equipment.
Part (F)(1)(a) (for indoor installations) of this rule very clearly defines the “zones” for electrical equipment to include any open space above the equipment up to 1.8 m (6 ft) above the top of the gear, or the structural ceiling, whichever is lower. In any case, the dedicated clear space above switchboards, panelboards, distribution panels, and motor control centers extends to the structural ceiling if it is less than 1.8 m (6 ft) above the equipment. And, where the structural ceiling is higher than 1.8 m (6 ft) above the equipment, this rule permits water piping, sanitary drain lines, and similar piping for liquids to be located above switchboards, etc., if such piping is at least 1.8 m (6 ft) above the equipment. The permission for switchboards and panelboards to be installed below liquid piping that is located more than 6 ft above the equipment must be carefully considered, even though containment systems, etc., are required to prevent damage from dripping or leaking liquids. The object is to keep foreign piping (chilled-water pipes, steam pipes, cold-water pipes, and other piping) from passing directly over electrical equipment and thereby eliminate even the possibility of water leaking from the piping and overflowing the drain onto the equipment (Fig. 110-21). This rule completely prohibits any intrusion on the dedicated area, up to 6 ft above the equipment. And, where piping, etc., is to be run over the 6-ft minimum dedicated area, it must be provided with some means to prevent any discharge or condensation from coming into contact with the equipment below.
The exception recognizes that suspended ceiling systems with removable tiles may occupy the dedicated space above switchboards, etc.
110.26(F)(1)(b) identifies the area where “foreign systems” are permitted to be installed. As one would imagine, this zone begins at a distance of 6 ft above the top of the electrical equipment and extends to the structural ceiling. As indicated, protection against damage due to leaking of the foreign piping systems must be provided.
Fig. 110-21. Water pipes and other “foreign” piping must not be located less than 6 ft above switchboard. [Sec. 110.26(F).]
Note carefully: It is not a requirement of this rule that “foreign” piping, ducts, etc., must always be excluded from the entire area above electrical equipment. Although the rules require that the “foreign” piping, ducts, etc., must be kept out of the “space” dedicated to the electrical equipment, the rule, literally, permits such “foreign” piping, ducts, etc., to be installed above the dedicated space, above the equipment. BUT, protection must be provided in the form of drain gutters or containment systems of some sort to prevent damage to the electrical equipment, below. However, it is much wiser to eliminate any foreign piping—even sprinkler piping used for fire suppression—from the area above electrical equipment. Where such installation is not possible, then take great care to ensure an adequate system of protection for the electrical equipment. (See Fig. 110-21.)
As covered in part (F)(1)(c), sprinkler piping, which is intended to provide fire suppression in the event of electrical ignition or arcing fault, would not be foreign to the electrical equipment and would not be objectionable to the Code rule. Another confirmation of Code acceptance of sprinkler protection for electrical equipment (which means sprinkler piping within electrical equipment and even directly over electrical equipment) is very specifically verified by 450.47, which states, “Any pipe or duct system foreign to the electrical installation shall not enter or pass through a transformer vault. Piping or other facilities provided for vault fire protection or for transformer cooling shall not be considered foreign to the electrical installation.” BUT, the wording here only permits installation of sprinkler piping above the dedicated space “where the piping complies with this section.” That means the sprinkler piping would have to be at least 6 ft above the equipment to comply with 110.26(F)(1)(a) and be provided with the “protection”—a gutter or containment system—required by 110.26(F)(1)(b). As long as the containment system only falls below the sprinkler pipe and not underneath the sprinkler head, the sprinkler will still be able to perform its function. In other words, route the sprinkler piping in such a manner that it is offset and not directly over the equipment, or at the least, make sure that the suppression system is arranged so the sprinkler head is not directly over the electrical equipment, allowing it to discharge on a fire in the gear from either the front or sides or both. Layouts of piping can be made to assure effective fire suppression by water from the sprinkler heads when needed, without exposing equipment to shorts and ground faults that can be caused by accidental water leaks from the piping. That will prevent any conflict with the rule given here and provide for the desired fire suppression.
The final lettered paragraph, 110.26(E), used to be the last sentence of 110.26. It makes clear that the use of a locked door or enclosure is acceptable, where the key or combination is available to “qualified personnel” (e.g., the house electrician or serving contractor’s journeyman). Under such conditions, a lock does not inhibit the ready accessibility contemplated in the definition of that term.
110.27. Guarding of Live Parts. Part (A) of this rule generally requires that “live parts” (i.e., energized parts of equipment) be “guarded” to prevent accidental contact. It applies to all systems operating at 50 V or more. This is typically accomplished through the use manufacturer-provided enclosures. However, where live parts are not enclosed with a suitable enclosure, the alternate methods described in parts (A)(1) through (A)(4) can be employed to satisfy this requirement.
Part (B) of 110.27 addresses an additional concern for protection of electrical equipment. After the 1968 NEC, old Sec. 110.17(a)(3), accepting guardrails as suitable for guarding live parts, was deleted. It was felt that a guardrail is not proper or adequate protection in areas accessible to other than qualified persons. However, where electrical equipment is exposed to physical damage—such as where installed alongside a driveway, or a loading dock, or other locations subjected to vehicular traffic—the use of guardrails is clearly acceptable and required by this rule. Failure to protect equipment against contact by vehicles is a violation of this section.
Live parts of equipment should in general be protected from accidental contact by complete enclosure (i.e., the equipment should be “dead-front”). Such construction is not practicable in some large control panels, and in such cases the apparatus should be isolated or guarded as required by these rules.
110.30. General. Figure 110-22 notes that high-voltage switches and circuit breakers must be marked to indicate the circuit or equipment controlled. This requirement arises because 110.30 says that high-voltage equipment must comply with preceding sections in part I of Art. 110. Therefore, the rule of 110.22 calling for marking of all disconnecting means must be observed for high-voltage equipment as well as for equipment rated up to 600 V.
The second sentence states clearly, and emphatically, that the rules given in part II of Art. 110 apply only to equipment on the load side of the service. That is, only high-voltage equipment installed on the load side of the “service point” is covered. In no case shall these rules be applied to high-voltage equipment that is owned and operated by the utility.
Fig. 110-22. High-voltage switches and breakers must be properly marked to indicate their function. (Sec. 110.30.)
110.31. Enclosure for Electrical Installations. The last sentence of the first paragraph indicates that there may be instances where additional precautions or special design would be necessary, due to the specifics related to the application. Always check the manufacturer’s installation instructions and appropriate UL data to ensure that the enclosure meets the specific hazard encountered.
Table 110.31 provides minimum clearance requirements between the required fencing to live parts.
Section 110.31(A) repeats key construction requirements from the basic rules in Part III of Art. 450 for transformer vaults and makes them applicable to comparable rooms without transformers, but with other medium-voltage equipment. It is presently unclear, however, when such a vault requirement would be triggered by a medium-voltage installation. Note also that there is no comparable allowance for the reduction of the required fire rating if an automatic fire suppression system is installed.
Section 110.31(B) covers enclosed areas or rooms in interior locations. Figure 110-23 illustrates the rules which cover installation of high-voltage equipment indoors in places accessible to unqualified persons in part (B)(1). Installation must be in a locked vault or locked area, or equipment must be metal-enclosed and locked. The Code is quite clear that a lock and key is the only acceptable means to provide positive control [110.34(C)]. The basic concern is related to unqualified persons coming into proximity or contact with high voltage. This rule states what is considered as adequate to provide the desired exclusion of other than “qualified personnel.”
For equipment that is not enclosed, as described in 110.31(C), another enclosure—or, more accurately, a barrier—must be constructed around the entire area where unenclosed high-voltage equipment is “accessible” to other than qualified persons. Such fencing must be no lower than 7 ft in total height. This may be 7 ft of fencing, or a 6-ft fence supplemented by at least three strands of barbed wire, or the “equivalent.”
Fig. 110-23. NE Code rules on high-voltage equipment installations in buildings accessible to electrically unqualified persons. [Sec. 110.31(A).]
Part (B)(1) also calls for “appropriate caution signs” for all enclosures, boxes, or “similar associated equipment.” This is a field marking that must be provided by the installer. For any equipment, rooms, or enclosures where the voltage exceeds 600 V, permanent and conspicuous warning signs reading DANGER—HIGH VOLTAGE, KEEP OUT must always be provided. It is a safety measure that alerts unfamiliar or unqualified persons who may, for some reason, attempt to gain access to a locked, high-voltage area. Note that Sec. 110.31(B)(1) does not require locking indoor metal-enclosed equipment that is accessible to unqualified persons, but such equipment is required to be marked with “WARNING” signs [see 110.34(C)].
And, the last sentence of part (A)(1) requires that manufacturers design their equipment to ensure that unqualified persons can’t come into contact with live parts of the high-voltage equipment.
Part (B)(2) applies to areas accessible only to qualified persons. In such areas, no guarding or enclosing of the live high-voltage parts is called for. The rule simply requires compliance with the rules given in 110.34, 110.36, and 490.24. In 110.34, the Code covers clear “working space” and the methods of “guarding” for systems rated over 600 V. Section 110.36 describes the acceptable wiring methods and 490.24 covers internal spacings in medium-voltage equipment that are field wired, and fabricated. Note that the spacings in Table 490.24 do not apply to internal spacings on equipment “designed, manufactured, and tested in accordance with accepted national standards.”
In 110.31(C)(1), the Code requires compliance with the rules for equipment rated over 600 V, given in all parts of Art. 225. And, (C)(2) requires compliance with 110.34, 110.36, and 490.24 where outdoor high-voltage equipment is accessible only to qualified persons.
110.31(D) essentially specifies that outdoor installations with exposed live parts must not provide access to unqualified persons.
For equipment rated over 600 V, nominal, 110.31 requires that access be limited to unqualified persons only, by installing such equipment within a “vault, room, closest, or in an area that is surrounded” by a fence, etc., with locks on the doors. In part (D) of Sec. 110.31, the Code identifies additional methods for preventing unauthorized access to metal-enclosed equipment where it is not installed in a locked room or in a locked, fenced-in area.
110.31(D) provides a variety of precautions that are needed where high-voltage equipment, installed outdoors, is accessible to unqualified persons. They include: design of openings in the equipment enclosure, such as for ventilation, to be such that they prevent “foreign objects” from being inserted; and “guards” must be provided where the equipment is subject to damage by cars, trucks, and so forth. Enclosed equipment must be equipped with nuts and bolts that are not “readily removed.” In addition, elevation may be used to prevent access [110.34(E)], or equipment may be enclosed by a wall, screen, or fence under lock and key, as shown in Fig. 110-24. Where the bottom of high-voltage equipment is not mounted at least 8 ft above the floor or grade, the equipment enclosures must be kept locked. And covers on junction boxes, pull boxes, and so forth, must be secured using a lock, bolt, or nut.
That sentence in 110.31(D) recognizes a difference in safety concern between high-voltage equipment accessible to “unqualified persons”—who may not be qualified as electrical personnel but are adults who have the ability to recognize warning signs and the good sense to stay out of electrical enclosures—and “the general public,” which includes children who cannot read and/or are not wary enough to stay out of unlocked enclosures (Fig. 110-25).
Fig. 110-24. High-voltage equipment enclosed by a wall, screen, or fence at least 7 ft (2.13 m) high with a lockable door or gate is considered as accessible only to qualified persons. [Sec. 110.31(B).]
Fig. 110-25.Metal-enclosed high-voltage equipment accessible to the general public—such as pad-mount transformers or switchgear units installed outdoors or in indoor areas where the general public is not excluded—must have doors or hinged covers locked (arrow) if the bottom of the enclosure is less than 8 ft (2.5 m) above the ground or above the floor.
The rationale submitted with the proposal that led to this change in the Code rule noted:
Where metal-enclosed equipment rated above 600 V is accessible to the general public and located at an elevation less than 8 ft, the doors should be kept locked to prevent children and others who may be unaware of the contents of such enclosures from opening the doors.
However, in a controlled environment where the equipment is marked with appropriate caution signs as required elsewhere in the NE Code, and only knowledgeable people have access to the equipment, the requirement to lock the doors on all metal-enclosed equipment rated above 600 V and located less than 8 ft (2.5 m) above the floor does not contribute to safety and may place a burden on the safe operation of systems by delaying access to the equipment.
For equipment rated over 600 V, 110.31(D) has required that the equipment cover or door be locked unless the enclosure is mounted with its bottom at least 8 ft off the ground. In that way, access to the general public is restricted and controlled. In addition, the bolts or screws used to secure a cover or door may serve to satisfy the rule of Sec. 110.31(D), provided the enclosure is used only as a pull, splice, or junction box. Where accessible to the general public, an enclosure used for any other purpose must have its cover locked unless it is mounted with its bottom at least 8 ft (2.5 m) above the floor (Fig. 110-26).
In the last two sentences of 110.31(D), “bolted or screwed-on” covers, as well as in-ground box covers over 100 lb (45.4 kg) are recognized as preventing access to the general public. The last sentence recognizes that there is no need to secure the cover on an in-ground box that weighs at least 100 lb. This correlates with the rule of 314.72(E), which states that covers weighing 100 lb (45.4 kg) satisfy the basic requirement for securing covers given in this section.
Fig. 110-26. Access by the general public to any metal enclosure containing circuits or equipment rated over 600 V, nominal, must be prevented. The wording recognizes the bolts or screws on the covers of boxes used as pull, splice, and junction boxes as satisfying the requirement for preventing access. And, additional wording in this rule recognizes that covers weighing over 100 lb are inherently secured and do not require bolts or screws for the cover or door. Remember, the permission given in the basic rule is for pull, splice, and junction boxes, only.
110.32. Work Space about Equipment. Figures 110-27 and 110-28 point out the basic Code rule of 110.32 relating to working space around electrical equipment.
Figure 110-29 shows required side-to-side working space for adequate elbow room in front of high-voltage equipment.
Fig. 110-27. This is the general rule for work space around any high-voltage equipment. (Sec. 110.32.)
Fig. 110-28. Sufficient headroom and adequate lighting are essential to safe operation, maintenance, and repair of high-voltage equipment. (Sec. 110.32.)
Fig. 110-29. Working space in front of equipment must be at least 3 ft (900 mm) wide, measured parallel to front surface of the enclosure. (Sec. 110.32.)
110.33. Entrance and Access to Work Space. Entrances and access to working space around high-voltage equipment must comply with the rules shown in Fig. 110-30. Section 110.33(A)(1) says that if the depth of space in front of a switchboard is at least twice the minimum required depth of working space from Table 110.34(A), any person in the working space would be capable of moving back out of the working space to escape any fire, arcing, or other hazardous condition. In such cases there is no need for a path of exit at either end or at both ends of the working space. But where the depth of space is not equal to twice the minimum required depth of working space, there must be an exit path at each end of the working space in front of switchgear or control equipment enclosures that are wider than 6 ft (1.8 m). And what applies to the front of a switchboard also applies to working space at the rear of the board if rear access is required to work on energized parts.
Fig. 110-30. Access to required work space around high-voltage equipment must be ensured. (Sec. 110.33.)
The wording of 110.33(A)(1)(b) specifies minimum clearance distance between high-voltage equipment and edge of entranceway to the defined work space in front of the equipment, where only one access route is provided. Based on Table 110.34(A)—which gives minimum depths of clear working space in front of equipment operating at over 600 V—the rule in this section presents the same type of requirement described by 110.26(C)(2). Based on the particular voltage and the conditions of installation of the high-voltage switchgear, control panel, or other equipment enclosure, the nearest edge of an entranceway must be a prescribed distance from the equipment enclosure. Refer to the sketches given for 110.26(C).
Section 110.33(A) concludes with a new numbered paragraph (3) on personnel doors, how they must open and what hardware is required for them. This rule is identical to the comparable rule for large equipment in 110.26(C)(3), and raises the same issues. Refer to that discussion for more information. Section 110.33(B) requires permanent provisions for access in the form of ladders or stairways to the required work space about medium-voltage equipment on balconies, rooftops, attics, platforms, etc.
110.34. Work Space and Guarding. Application of Code Table 110.34(A) to working space around high-voltage equipment is made in the same way, as shown for Code Table 110.26(A)—except that the depths are greater to provide more room because of the higher voltages.
As shown in Fig. 110-31, a 30-in. (762-mm)-deep work space is required behind enclosed high-voltage equipment that requires rear access to “deenergized” parts. Section 110.34(A)(1) notes that working space is not required behind dead-front equipment when there are no fuses, switches, other parts, or connections requiring rear access. But the rule adds that if rear access is necessary to permit work on “deenergized” parts of the enclosed assembly, the work space must be at least 30 in. (762 mm) deep. This is intended to prohibit cases where switchgear requiring rear access is installed too close to a wall behind it, and personnel have to work in cramped quarters to reach taps, splices, and terminations. However, it must be noted that this applies only where “deenergized” parts are accessible from the back of the equipment. If energized parts are accessible, then Condition 2 of 110.34(A) would exist, and the depth of working space would have to be anywhere from 4 to 10 ft (1.2 to 3.0 m) depending upon the voltage [see Table 110.34(A)].
Fig. 110-31. Space for safe work on deenergized parts. [Sec. 110.34(A).]
Section 110.34(B) covers the common occurrence of medium-voltage equipment or transformer rooms with exposed live parts, and what that means for 600 V and lower equipment that may be in the same room. In such cases the medium-voltage equipment must be separated by a screen, fence, or other partition. However, this separation rule does not apply to lower voltage equipment that is only serving the room where the medium-voltage equipment is located. For example, a snap switch and a luminaire in the room would not provoke the separation rule.
Section 110.34(C) requires that the entrances to all buildings, rooms, or enclosures containing live parts or exposed conductors operating in excess of 600 V be kept locked, except where such entrances are under the observation of a qualified attendant at all times. The last paragraph in this section requires use of warning signs to deter unauthorized personnel. The rule of 110.34(D) on lighting of high-voltage work space is shown in Fig. 110-28. Note that the rule calls for “adequate illumination,” but does not specify a footcandle level or any other characteristics.
Figure 110-32 shows how “elevation” may be used to protect high-voltage live parts from unauthorized persons.
110.34(F). Protection of Service Equipment, Metal-Enclosed Power Switchgear, and Industrial Control Assemblies. The basic rule of the first sentence in this section excludes “pipes or ducts foreign to the electrical installation” from the “vicinity of the service equipment, metal-enclosed power switchgear, or industrial control assemblies.” Then, addressing the case where foreign piping is unavoidably close to the designated electrical equipment, the next sentence calls for “protection” (such as a hood or shield above such equipment) to prevent damage to the equipment by “leaks or breaks in such foreign systems.”
Piping for supplying a fire protection medium for the electrical equipment is not considered to be “foreign” and may be installed at the high-voltage gear. The reason given for that sentence was to prevent the first sentence from being “interpreted to mean that no sprinklers should be installed.” Fire suppression at such locations may use water sprinklers or protection systems of dry chemicals and/or gases specifically designed to extinguish fires in the equipment without jeopardizing the equipment. Water is sometimes found to be objectionable; leaks in piping or malfunction of a sprinkler head could reduce the switchgear integrity by exposing it to a flashover and thereby initiate a fire.
Fig. 110-32. Elevation may be used to isolate unguarded live parts from unqualified persons. [Sec. 110.34(E).]
110.40. Temperature Limitations at Terminations. Terminations for equipment supplied by conductors rated over 2000 V must carry not more than the 90°C ampacity values given in Tables 310.67 through 310.86, unless the conductors and equipment to which the conductors are connected are “identified” for the 105°C ampacity.
The proposal to include this section pointed out that the ampacity values given in tables for conductors rated above 2000 V were all 90°C-rated values. And, with the increased attention that has been focused on the coordination between conductor ampacity and temperature limitations of the equipment, some question had been raised regarding the use of the 90°C ampacity values in Tables 310.67 through 310.86 with equipment intended to be supplied by conductors rated over 2000 V. The rule of 110.40 allows the conductors covered in Tables 310.67 through 310.86 to carry the full 90°C ampacity and be connected virtually without concern for the equipment terminations, unless otherwise marked to indicate that such application is not permitted.
This rule was accepted based, in part, on information provided in the proposal regarding American National Standards Institute (ANSI) acceptance of the use of such conductors at their full 90°C ampacity, where tested for such operation. The Code-making panel (CMP) added the qualifying statement “unless otherwise identified” to indicate that such application is permitted where equipment has been so tested. In fact, the wording accepted actually assumes that equipment intended to be supplied by conductors rated over 2000 V—i.e., the conductors covered by Tables 310.67 through 310.86—is tested at the full 90°C ampacity. But, if the equipment is otherwise “identified,” it must be used as indicated by the manufacturer. It should be noted that although the rule is contained in part III of Art. 110, which covers equipment rated over 600 V, because the tables mentioned cover conductors rated over 2000 V, it only applies to the terminations on equipment intended to be supplied by conductors rated over 2000 V. The terminations on all other equipment supplied by conductors rated from 601 to 2000 V must be coordinated with the ampacity value corresponding to the temperature rating of the terminations (Fig. 110-33).
Fig. 110-33. When derating with conductors rated over 2000 V, the temperature rating of the terminations may be assumed to be 90°C unless otherwise marked.
Part IV of Art. 110 covers high-voltage—600 V or more—installations in tunnels. Given that mines and surface mining equipment are not regulated by the NEC, the types of tunnels covered here are those used for trains, cars, irrigation, or whatever—but NOT for mines. Installation of all high-voltage power and distribution equipment, as well as the tunneling equipment identified here, must be protected and installed in accordance with 110.51 through 110.59.
Part V. Manholes and Other Electric Enclosures Intended for Personnel Entry, All Voltages.
110.70. General. The rules in this part apply unless an industrial occupancy demonstrates appropriate engineering supervision that supports design differences; those differences being subject to documentation and review by the inspector. The NEC requires that manholes must be designed under engineering supervision and that they must withstand the loading likely to be imposed.
110.72. Cabling Work Space. A clear work space must be provided not less than 3 ft (900 mm) wide where cables run on both sides, and 2½ ft (750 mm) wide if the cables are on only one side, with vertical headroom not less than 6 ft (1.8 m) unless the opening is within 1 ft (300 mm) of the adjacent interior side wall of the manhole. If the only wiring in the manhole is power-limited fire alarm or signaling circuits, or optical-fiber cabling, then one of the work space dimensions can drop to 2 ft (600 mm) if the other horizontal clear work space is increased so the sum of both dimensions is not less than 6 ft (1.8 m).
110.73. Equipment Work Space. If the manhole includes equipment with live parts that will require work while energized, then the normal NEC work space rules apply. The headroom rises to 6½ ft (2.0 m), and there must be clear work space at least 3 ft deep and 30 in. (762 mm) wide (wider if the equipment is wider). The depth rises to 3½ ft or 1.1 m over 150 V to ground, and then goes up to 4 ft (1.2 m) for up to 2.5 kV; 5 ft (1.5 m) for up to 9 kV, 6 ft for up to 25 kV, and deeper for higher voltages, as covered in Table 110.34(A).
110.74. Bending Space for Conductors. Essentially, manholes are pull boxes that are large enough for personnel to enter, and therefore they need to be sized to accommodate the installed conductors without violating the rules that normally apply to sizing pull boxes, as covered for comparable applications in Art. 314. For medium-voltage applications, where the same conductor passes straight through the manhole (e.g., entering the south wall and leaving the north wall) the minimum distance is 48 times the largest shielded cable diameter (32 times the largest unshielded cable diameter). If a conductor enters one side of a manhole and then makes a right-angle turn, the dimension drops to 36 times (24 times for unshielded conductors), but it is measured in both directions, and it is increased by the sum of the outside diameters of all other cable entries on the same wall. If there are multiple rows or columns of duct openings in any direction, use the row or column that gives you the largest sizing calculation and ignore all other cable entries.
110.75. Access to Manholes. The NEC requires access to be at least 26 × 22 in., (650 × 550 mm) if rectangular, otherwise at least 26 in. (650 mm) in diameter. This allows for a ladder to rest against the edge of the opening; if the manhole has a fixed ladder permanently mounted, then the diameter can be reduced to 24 in. (600 mm). A similar reduction is permitted if the only wiring in the manhole is power-limited fire alarm or signaling circuits, or optical-fiber cabling. The access opening must not be directly over electrical equipment or conductors, but if this is not practicable, the manhole must be fitted with a protective barrier or a fixed ladder. The cover, if rectangular, must be restrained so it cannot fall into the manhole. Covers must “prominently” identify the manhole’s function by wording (e.g., “ELECTRIC”) or a logo, and they must weigh at least 100 lb (45 kg); if not, then they must be secure so a tool will be required for access.
