Chapter 10

Staying Secure: Wiring and Safety Components

IN THIS CHAPTER

Figuring out circuits

Examining conductors and conduit

Taking disconnects and overcurrent protection into account

Protecting your systems from ground faults

Labeling for the benefit of safety personnel

When you install a PV system, you must make safety your top priority. That means giving the wiring and safety components of your PV designs special consideration, particularly during installation. In this chapter, I describe some common wiring and safety components used in PV systems, specifically conductors, conduit, disconnects, overcurrent protection devices, ground fault protection, and labels. (See Chapter 13 for how to size these items.)

Every utility and jurisdiction, the government entity (typically a city or county office) that dictates the requirements for building and electrical work, you deal with will likely have slightly different wiring and safety component requirements that you must follow. I make it a point to show you the requirements as laid out by the National Electrical Code^® (NEC^®), but when it comes down to it, a local jurisdiction can make its own rules or have its own interpretations of the NEC^®. If you ever have doubts as to what the requirements are in a particular area, talk with the local inspectors to find out what they want to see. (Of course, you should also purchase of copy of the NEC^® and research it so you can become as familiar with it as possible. See Chapter 1 for how to obtain a copy of the NEC^® Handbook.)

Defining the Circuits in a PV System

A circuit is the path needed to complete the circle the electrons must run in to do useful work. (For instance, in a grid-direct PV system, the DC circuit is the path the electrons take from the PV modules, through the inverter, and back to the modules.) Different conductors, or wires, throughout a PV system connect circuits, and these conductors are commonly placed inside conduit, or pipes, that protect them as they run from one point to the next.

The size of the conductors needed and the specific safety devices used are determined by the circuit type and location. Figure 10-1 shows a basic schematic of a PV system with the following circuits:

• PV source circuit: The circuit that comes from the individual PV strings is the PV source circuit. These circuits are often run along the back of the strings during installations; they lead to a junction box (a box where you can transition from outdoor wiring to indoor wiring) or a combiner box (a box where you can transition your wiring and also place strings in parallel; typically these boxes also have fuses inside).
• PV output circuit: The PV output circuit consists of the wires between the junction box or combiner box and the DC disconnect(s), which are often referred to as the DC disconnecting means, indicating that this is the method to disconnect the DC (PV) conductors from the rest of the system.
• Inverter input circuit: The conductors from the DC disconnect(s) to the inverter make up the inverter input circuit.
• Inverter output circuit: The conductors on the AC side of the system on the output side of the inverter comprise the inverter output circuit.

FIGURE 10-1: Different circuits in a PV system.

Note: The NEC^® defines conductors based on their role in circuits. For example, the NEC^® makes references to grounded current-carrying conductors (for both AC and DC circuits) and grounded conductors (for both types of circuits). I suggest you become familiar with the Code language and terms for your PV installations by digging into the NEC^® and picking up a book on understanding the NEC^®.

Checking Out Types of Conductors

For the components of a PV system (meaning the array, batteries, charge controller, and inverter) to work together and produce energy, something needs to join them together — enter wiring. The wires used to connect all the individual pieces of equipment in a PV system are known as conductors. The PV side of the system has positive and negative conductors, and each one is connected to its respective part of the PV modules.

The most common material for the conductors used in PV systems is copper because it has high conductivity (the ability to pass current) and is compatible with the conductor terminations (the place where the conductors are attached to the disconnects, circuit breakers, inverters, and charge controllers).

Less expensive aluminum conductors can be an option, but aluminum is often viewed as an inferior wiring method. If you choose to use aluminum, the conductors generally need to be larger because aluminum doesn’t possess the same conductivity as copper, and all the conductor terminations in a PV system would require explicit aluminum ratings. (Flip to Chapter 13 for full details on sizing conductors and safety devices.) The terminations portion of the requirement can be the most difficult part to meet. If aluminum conductors (or copper conductors, for that matter) are connected to terminals that aren’t rated for that material, the connections can eventually fail, increasing the risk of fire. My verdict: In comparison to the overall system costs, the savings associated with aluminum conductors in residential and commercial PV installations just isn’t worth it.

Selecting the proper conductor isn’t difficult, so there’s no excuse for ever installing a conductor just because it was in the truck. Numerous conductor types are available, and each one has its proper uses and limitations, as you find out in the following sections.

A few things to note regarding conductor classification:

• Conductors are always designated by an acronym that describes the conductor’s properties. For example, USE is an underground service entrance cable.
• Conductors typically have multiple acronyms listed on them. As long as one of the properties noted by the acronyms meets the requirements for your installation, you can use that conductor.
• Some conductors have the designation -2 at the end. This designation indicates that the conductors are rated for 90 degrees Celsius (194 degrees Fahrenheit) in wet or dry conditions. (I explain the significance of this temperature rating in Chapter 13.)

USE-2

Standard PV modules come with two copper conductors preinstalled with quick-connect plugs for connection to adjacent modules and to help make the installation process easier. These factory-installed conductors, which are often underground service entrance (USE-2) cables, connect from module to module (or directly to a microinverter, which I describe in Chapter 9) and are secured to the backside of the PV array using specially designed clips and wire ties, as I describe in Chapter 17.

Because USE-2 is often installed along the backside of a PV array as part of the PV source circuit wiring, it gets exposed to some extreme conditions, including full sunlight and temperatures exceeding 60 degrees Celsius (140 degrees Fahrenheit). USE-2 conductors work in these extremes because they’re inherently sunlight-resistant as well as heat- and moisture-resistant.

USE-2 conductors are readily available in a variety of sizes and can even be ordered in various colors (which can help when color-coding the wiring). For the portion of the PV source circuit that runs from the modules to the combiner box, you can buy rolls of USE-2 cable and install the quick-connect plugs yourself, or you can purchase USE-2 conductors that have the plugs preinstalled on the ends.

Don’t run USE-2 inside conduit and then inside buildings. It doesn’t have the proper fire retardants in the insulation and isn’t rated for these locations. For wires that go through buildings, you need to transition to building wiring (which I describe later in this chapter).

PV wire

A double-insulated conductor known as PV wire uses cross-linked polyethylene for the insulation. This durable conductor was born out of the need for a conductor that could be used with a transformer-less inverter (see Chapter 9) because the NEC^® requires additional protection in the form of conduit or multiple layers of insulation for the conductors on the PV side of these inverters. The two layers of insulation in PV wire help protect the copper conductors more than the single-insulated USE-2 conductor.

PV wire is usually used inside PV source circuit wirings, but you can also use it as PV output circuit wiring. However, because it’s more expensive than building wiring, most PV pros don’t opt for this application.

PV wire isn’t used in PV systems as often as the USE-2 conductor, but it has begun to gain popularity, and I recommend using it rather than USE-2 when you can. You can use it regardless of the inverter technology associated with the PV array, thereby reducing the different types of conductor you have to buy and manage on-site. Although PV wire is more expensive than USE-2, it’s also more durable. Just like USE-2, for the conductors running from the modules to the combiner box or junction box, you can purchase PV wire in plug-free rolls or with preinstalled plugs.

Building wiring

Conductors that leave the junction box or combiner box on their way to the inverter or charge controller are usually transitioned into a standard type of building wiring, such as heat-resistant thermoplastic (THHN) or moisture and heat-resistant thermoplastic (THWN-2). They can be aluminum or copper.

Building wiring is usually used for PV output circuits, inverter input circuits, and inverter output circuits. You can use it on both the DC and AC sides of a PV system, but you need to make sure you protect the conductors from damage by running them inside conduit (see the later related section). Note that building wiring can’t be run in exposed locations because it doesn’t have the sunlight-resistant characteristics of USE-2 and PV wire.

I recommend always using the THWN-2 conductor because it has additional temperature ratings. The -2 at the end indicates that the conductor is rated for 90 degrees Celsius (194 degrees Fahrenheit) in wet and dry conditions. Because you may have these conductors running in conduit along a roof, inside an attic, or down the side of a building, they may be exposed to some of the most extreme temperatures imaginable. Sticking with THWN-2 lets you use the same conductors in a variety of locations without too much concern.

You can buy rolls of wire that are rated as both THHN and THWN-2, but this isn’t considered standard. So if you go to your electric supply house and order THHN without specifying the THWN-2 requirement, you probably won’t get what you want.

Battery wiring

The most widely available conductors for use from the battery bank to the inverter in battery-based systems are moisture-resistant thermoset (RHW), moisture- and heat-resistant thermoplastic (THW), and underground service entrance (USE). All of these conductors are commonly made with copper.

Another battery wiring alternative is a highly flexible cable, which can have thousands of small, finely stranded copper wires wrapped together to make a large cable. People like to use these cables because tight spaces require a minimal bending radius, and the flexible wires are much easier to work with than the relatively stiff RHW, THW, and USE types.

If you opt for flexible cables, make sure the connectors you use with them are properly listed for use with flexible stranded cables. In the past, flexible cables were used with terminals and crimp lugs that weren’t properly rated. The result? The conductors failed and put the systems at risk of starting fires.

Any conductors used for battery wiring are in corrosive environments, so you need to verify with the manufacturer and the NEC^® that the conductor type you want to use is appropriate for the location. Chapter 3 in the NEC^® covers wiring methods and materials, so refer to the Code for more info.

Ground wiring

All the exposed metal parts of PV systems need a connection to ground, which means you need to connect the PV module frames, racking, and metal boxes to a conductor that’s connected to a large conductor that’s in contact with the earth. (I get into grounding in Chapter 17.) The conductors used to make the ground connections between PV modules are almost always bare copper. After the PV system’s conductors are transitioned into conduit, either bare copper or insulated ground conductors can be used.

Ground wiring can be run with all the circuits mentioned earlier in this chapter: PV source, PV output, inverter input, and inverter output. Using bare ground wire to ground the PV modules is preferable because it’s properly rated for that environment and readily accessible. Using building wiring, like THWN-2, for ground wire in conduit is great; it’s easier to pull and has the appropriate color-coding on the insulation. (I explain the importance of color-coding in Chapter 17.)

Considering Kinds of Conduit

With the exception of the PV source circuit wiring, the conductors used in PV systems need proper protection from potential damage; this protection is referred to as conduit. (PV source circuit wiring doesn’t require conduit because NEC^® allows you to use USE-2 or PV wire in these locations.) You can see some examples of conduit found in PV systems in Figure 10-2. Conduit can come in a wide range of sizes, as you discover in Chapter 13, and a variety of materials. Selecting the right conduit for the system you’re designing depends on the location where the conduit will be used and any additional NEC^® requirements. (Chapter 17 outlines some of the specific requirements surrounding conduit installation in PV systems.)

FIGURE 10-2: Conduit used in PV systems.

You have two main options for conduit: metallic and nonmetallic. Both provide protection for the conductors they house, but they have different installation and usage requirements.

Metallic conduit

According to the NEC^®, you can install PV circuits (I explain what these are in the earlier “Defining the Circuits in a PV System” section) for the PV output circuits inside a building prior to a disconnecting means as long as they remain in metallic conduit. A variety of metallic conduit options are available for use in PV installations, and each one offers excellent physical protection. Two of the more popular metallic conduit types are as follows:

• Electrical metallic tubing (EMT): A thin-walled, stiff pipe, EMT is used for the majority of the PV output circuit. It’s an inexpensive choice that you can use both indoors and outside. You can bend it (with the help of a tool) when you need to change the conduit’s direction, and you can change the conduit run using a wide variety of fittings. EMT requires more labor to install than flexible metallic tubing, but when you get the hang of it, working with EMT is easy.
• Flexible metallic tubing (FMT): As the name indicates, this is a flexible metallic conduit, which makes it great when you’re working in a confined space. It’s more expensive than EMT, but sometimes the time savings in using FMT versus EMT more than makes up for the initial cost. FMT is typically used for short runs, and it can be used in conjunction with EMT to make your job easier. FMT isn’t allowed in wet locations, though, so you can only use it inside buildings.

One wiring method for PV circuits run inside a building that isn’t explicitly allowed by the 2008 NEC^® is the use of metal-clad cables. Metal-clad cables are a cable assembly (conductors bundled together and wrapped with an outer protective covering); consequently, they don’t comply with the wording of the NEC^® in this situation. (Note that this guideline may change in the 2011 version of the Code.)

Nonmetallic conduit

Nonmetalllic conduit is acceptable in PV systems as long as the conduit has the proper ratings (such as UV protection) and is listed for installation in a particular location (such as exposed wet locations). You can use nonmetallic conduit for any of the circuits in PV systems. Common types of nonmetallic conduit include the following:

• Rigid polyvinyl chloride (PVC): An inexpensive and easy conduit to work with, PVC is similar to EMT (see the preceding section) in the sense that the material is stiff and can be bent or attached to specific fittings. One feature to be aware of is that PVC expands and contracts a great deal with changes in temperature, which requires you to install special expansion fittings on long PVC runs. (The NEC^® has specific requirements for this expansion and contraction.) PVC is used for PV output circuits that run underground or on the outside of a building.
• Liquid-tight flexible nonmetallic (LFNC): LFNC is appropriate when you need to use a flexible conduit in an outdoor location. It’s easy to work with and used to connect PVC conduit to a disconnect or inverter. It’s more expensive than PVC conduit, and the fittings used aren’t cheap either, which means you’ll generally run PVC as close as possible to the desired endpoint and use LFNC to make the final connection.

Delving into Disconnects

The conductors used in PV systems carry the current from the array down to the inverter and then to the loads. For safety and maintenance reasons, you need to install a way for those conductors to disconnect themselves from all sources of power. The exact location and specifications for disconnects depend on the specifics of the system installed, but all PV systems, regardless of their size, must possess the ability to disconnect the conductors.

Figure 10-3 shows two types of disconnects used in PV systems:

• Figure 10-3a is a disconnect integrated into an inverter. These can often disconnect both the PV output circuit (DC) and the inverter output circuit (AC) at the same time. They’re an excellent way to meet the NEC^® disconnect requirements for inverters, but they don’t meet the requirements of utilities that insist upon visible, lockable disconnects.
• Figure 10-3b shows two disconnects positioned outside of an inverter. Each disconnect is dedicated to a single circuit (PV output and inverter output, respectively). Installing two disconnects requires more space around the inverter and increases your installation time, but if the utility calls for a visible, lockable disconnect on the inverter output circuit, these disconnects satisfy both the utility and the NEC^® requirements.

FIGURE 10-3: A disconnect integrated into an inverter (a) and separate disconnects (b).

According to the NEC^®, DC and AC disconnects must be provided to stop power flowing in the conductors and allow for safe access to all components of the PV system. Generally, this guideline means supplying disconnects at the inverter location(s) as well as at points of interconnection with the utility (this last part applies only to utility-interactive systems).

One common (and unpopular) required disconnect is a visible, lockable disconnect at the utility meter location. The utility typically requests this disconnect for its personnel. It must be physically placed next to the meter (so the utility can access it 24 hours a day) and electrically placed between the inverter and the utility point of interconnection. The idea is that if a utility power outage occurs and a utility worker wants to, she can disconnect the PV array from the grid and lock the disconnect for safety. When you ask the utility to send a net-metering agreement, look through the packet you receive for disconnect requirements. If you have any doubt or want to install the disconnect(s) slightly differently than specified by the utility, be sure to contact the utility for clarification.

The NEC^® doesn’t require any disconnects at the PV array, but some local jurisdictions do. Always check with the electrical inspector to see whether your client’s jurisdiction has any additional requirements for disconnects.

Note: Disconnects can also serve as the form of overcurrent protection, like a circuit breaker that makes the utility interconnection for a utility-interactive inverter. See the next section for the full scoop on overcurrent protection.

Perusing Overcurrent Protection Devices

When conductors are installed in PV systems, they need protection from the possibility of too much current passing through them. Enter overcurrent protection devices (OCPDs). When too much current begins to pass through a conductor, an OCPD opens the circuit, preventing additional current from passing through until someone manually resets or replaces the device.

The following sections cover the two types of OCPDs used in PV systems — circuit breakers and fuses — so you can get a feeling for exactly how they protect conductors and where you should install them. (I show you how to size these devices in Chapter 13.)

Circuit breakers

Circuit breakers are the devices you most likely have installed in the main distribution panel at your home. The circuit breakers used in PV systems are very similar. They may have a different look based on the manufacturer, but for the most part they operate in the same way: When too much current passes through a circuit breaker, the breaker trips, opening the circuit and stopping the current from flowing. Most breakers are thermally activated, so they trip when they reach a certain temperature. The nice feature about circuit breakers is that when they trip, you can simply reset them instead of replacing them.

Circuit breakers are manufactured for specific enclosures. Consequently, you can’t install just any circuit breaker inside a load center; instead, you have to buy the breaker that was manufactured for the exact panel you’re using. Circuit breakers are also manufactured for specific current and voltage levels as well as the current type (AC or DC). Always make sure the circuit breakers are listed for the circuit you want to install them in.

Circuit breakers are typically used as the OCPD on the AC side of PV systems (in other words, the inverter output circuit) as well as on the DC side of low-voltage, battery-based arrays (meaning the PV source circuit, inverter input circuit, and inverter output circuit).

Don’t use circuit breakers on the DC side of high-voltage PV installations because they don’t carry the proper ratings for high-voltage applications (which is anything greater than 150 VDC).

Fuses

When you’re dealing with a high-voltage DC circuit or when the OCPD needs to be placed outside of a load center, you need to install a fuse. Fuses come in all shapes and sizes, and they can fit any PV system requirement.

Fuses work pretty similarly to circuit breakers, which makes them a good substitute when you can’t use circuit breakers. Inside every fuse is a filament that remains intact as long as it doesn’t get too hot. If too much current passes through the fuse, the fuse overheats and pops, and the current is interrupted. Note: A popped fuse must be replaced; it can’t be reset like a circuit breaker.

Like circuit breakers, fuses have specific listings associated with them. The main one you need to be aware of is the DC rating. Many fuses carry both AC and DC current ratings but at very different voltage levels. You need to make sure you order the fuses with the proper DC voltage and current ratings when you use them on the DC side of a system (typically the PV source circuit).

The most common locations for fuses are in a combiner box for the PV source circuit and inside an inverter for the connection between the PV output conductors and the inverter. Fuses are also good for when an inverter’s output circuit can’t be connected to the utility through a circuit breaker. In this last example, a fused disconnect provides both overcurrent protection and a method for disconnecting the inverter in one device (I cover disconnects earlier in this chapter).

Focusing on Ground Fault Protection

To help reduce the risk of fires when a PV array has experienced a fault, ground fault protection (GFP) devices have become a requirement for all but a few PV installations. When you deal with grid-direct systems, you need only consider the proper installation techniques because GFP comes standard in grid-direct inverters (see Chapter 9). When you’re working with battery-based systems, however, expect to have an additional component to install. (Fortunately you only have one option in terms of current ratings. Check the NEC^® for its specific requirements regarding interrupting faults.)

GFP is added to PV systems at the inverter inside a DC wiring box to reduce the risk of fire if an array’s conductors become compromised. If either the positive or the negative conductor is damaged, that conductor can come into contact with a metal component in the PV system such as a module, the racking, or the inverter. This contact between the conductor (which contains the current from the PV array) and metal pieces allows current to flow and presents a fire (and shock) hazard.

The GFP merely reduces the fire risk and alerts the system owner of the problem; it does nothing to reduce the risk of shock. In fact, after the GFP has been activated, the shock hazard is even higher because there’s now a voltage potential between anything metal and ground. So if you were to grab the portion of the array that was faulted, the current would flow through your body, causing a shock throughout your body at best and death at worst.

A GFP works by purposely connecting one of the current-carrying conductors across the GFP circuit. This circuit is kept closed through a small-amperage fuse (generally just 1 A). The circuit is looking for any current that’s flowing from one of the current-carrying conductors to ground. Under normal circumstances, there’s no current to ground, but as soon as a wire comes into contact with a grounded portion of the array (anything metallic), current then flows to ground. This improper current flow causes the fuse inside the GFP to pop and a visible sign to appear, like a warning light or a circuit breaker in the off position, indicating the presence of the ground fault. So when the GFP is open, a reference to ground is no longer present for the conductors. This situation is known as floating, and it’s a dangerous one for whoever’s troubleshooting the array.

If the GFP has been activated, you must track down and fix the ground fault before the array can continue operation. This process can take some time and skill because tracking down a ground fault can be difficult and complicated. (One resource for you to research the methodology and safety precautions is SolarPro, solarprofessional.com, a magazine that has covered this topic in great detail.) After the ground fault has been fixed, you can replace the fuse with a properly rated replacement and restart the system.

Looking at the Basics of Labels

Proper labeling is growing increasingly important as PV systems become more popular. Inspectors are now requiring PV system installers to do a better job of meeting Article 690 of the NEC^®, which lists a number of labeling requirements for PV systems. I outline the exact language needed on system-specific labels in Chapter 18, but in this section, I outline the general requirements and the generic labels needed for every job.

PV system labels give safety personnel, such as firefighters, quick information they can use in an emergency. Consequently, you should always design your system labels so they present the necessary information simply and accurately.

Here are some of the major labeling requirements you must meet in any PV system you design (you can purchase premade versions of the generic labels, but you’ll need to have the system-specific ones made at a local sign shop):

• The DC disconnect needs labeling with specific information about the different voltage and current levels (see Figure 10-4a).
• The point of interconnection with the utility needs labeling indicating voltage and current levels (see Figure 10-4b).
• The building exterior needs a plaque or label identifying the secondary power source (the PV array) and all disconnects for the system (see Figure 10-4c).

FIGURE 10-4: Sample labels for PV systems.