6

Mechanical Installation: Rooftop

Goal Install the rooftop array, including racking, modules, microinverters or DC optimizers (as applicable), and grounding wires

Rally the crew and cinch up those harnesses — it’s solar time! This chapter walks you through the installation of a standard flush-mount PV array on a sloped roof with asphalt (composition) shingles. It applies to both grid-tied and off-grid systems. (In addition, there’s an overview of flat-roof installation here. Ground-mount installation is covered in chapter 7, but you’ll come back here for the module details.) Of course, as you surely learned during the system design, PV hardware isn’t exactly standard. While PV modules are relatively interchangeable, racking systems are highly product-specific. Every manufacturer has its own special doohickies and techniques for mounting and leveling the racking, securing the modules, grounding, and performing other essential functions. That’s why your true guide for the installation will be the manufacturers’ instructions and the local building code. If you have questions about your hardware or how to use it with your roofing material, contact the manufacturer or a solar installation professional.

Laying Out the Racking

This phase of the array installation consists of three stages:

1. 1. Completing the basic layout of footers and rails: mapping the location of the racking and modules on paper
2. 2. Locating rafters (or trusses) for footers: measuring for the precise locations of the roof framing members that you will anchor into
3. 3. Snapping chalk lines and marking pilot holes: creating reference lines on the rooftop and marking pilot holes for installing the footers

The footers of your racking system are anchored into the roof rafters or trusses with lag screws. The size, length, and number of lag screws, as well as the number of footers, may need to be specified by an engineer, to satisfy the building department. These specs are based on the size and spacing of the rafters/trusses as well as the local building code requirements for wind and snow loads.

To ensure a strong connection, the lags must go into the meat of the framing, at the centers of the rafters. For beginners, the most accurate and foolproof way to locate rafters is to measure the rafter spacing from inside the attic (or from inside the house, if you have vaulted ceilings). Pros usually locate rafters from above the roof, using the old carpenter’s trick of hammering and listening for the right sound. If you know how to do this effectively, you can use this technique instead. Just be careful to hammer only in areas that will be covered with flashing, as the hammer blows are hard on the shingles and you’ll want the extra protection of the flashing to prevent further damage over time.

Before you climb onto the roof, make sure your ladder is tied off and everyone working on the roof is outfitted with fall-arresting equipment (see Rooftop Safety). Never work on a roof that is slippery due to rain, snow, ice, or debris. Make sure your roof is dry and clean (no tree branches or leaves) before starting the installation.

Tools

• Tape measure
• Drill
• ¹⁄₄-inch drill bit, 12 inches long

Materials

• ¹⁄₄- or ¹⁄₂-inch graph paper
• Pencil
• Ruler
• Straight wire or coat hanger
• Tape
• Marking crayon
• Stud finder (for finished/vaulted ceilings)
• Chalk line

Laying Out the Footers and Rails

1. 1. Create a rooftop map. Draw a new map on ¹⁄₄- or ¹⁄₂-inch graph paper, or use a copy of the array layout from your system design. Include the roof dimensions from ridge to eave and from side to side (typically west to east). Also mark all of the rafters; their spacing doesn’t have to be precise, but it must show the correct number of rafters in the installation area. Use one of the basic measuring techniques shown in Locating the Rafters to find the rafter spacing (see Framing Dimensions, opposite); then use this spacing to lay out the rafters on your map. You will measure for the exact locations of the rafters in the next stage of the layout.

Rooftop Map

Tip

Framing Dimensions

Spacing of rafters, trusses, and other framing members typically follows on-center layout (abbreviated “o.c.”), meaning they are spaced according to the distance between the center of one rafter and the center of the next (not the open space between the rafters). Most rafters are spaced 16 or 24 inches on center, while trusses are more commonly spaced 24 inches on center. Both rafters and trusses are typically made with “2-by” lumber (2×4, 2×6, etc.), which is about 1¹⁄₂ inches wide. When you measure or mark a layout from the outside faces of the rafters, you locate the center of each rafter by measuring over ³⁄₄ inch from each layout measurement or mark.

1. 2. Add the modules. Sketch the module layout for the entire array onto your map. Maintain the required spacing at the ridge, eave, and side edges of the roof. Include all of the modules, but don’t worry about making them perfect; you’ll likely make some adjustments before finalizing the map. Refer to the racking specs for the spacing between adjacent modules in each row (determined by the module mid clamps) and the spacing between modules in different rows (typically ¹⁄₈ inch or more).
2. 3. Add the footers and rails. Mark the footer locations on the map, centering each over a rafter. It’s okay to vary the spacing to accommodate the actual framing layout, provided you don’t exceed the maximum span between footers specified by the racking manufacturer, the building department, or engineer. Space the rows of footers evenly, according to the manufacturer’s specs. Typically, the rails (which sit atop the footers) must be within the top/bottom 30% of the modules’ length. There’s also a maximum length that the rails and modules can extend beyond the outermost footers; this is typically 12 to 18 inches but varies by manufacturer. Draw the rails spanning across the footers. Depending on the type of module end clamps you use, rails may be flush with the outside edges of the outer modules, or they may need to extend 1¹⁄₂ inches or more beyond the modules to accommodate the end clamps.

Locating the Rafters

Use the following steps to locate the rafters on your roof. If you have a vaulted, finished ceiling (so that the rafters are not visible from below), read For Finished Ceilings (opposite) along with the following steps.

1. 1. Drill a locator hole. Your rafter measurements will start from a locator hole drilled through the roof at the top, outside corner of the racking layout. From inside the attic, identify the outer rafter that will receive footers. Measure down from the ridge board or peak of the roof frame and mark the approximate location of the first footer onto this first rafter. Drill a hole up through the roof sheathing and roofing, using a long ¹⁄₄-inch (or smaller) drill bit, keeping the bit aligned with the outside face of the first rafter. Stick a straight piece of wire up through the hole and tape the wire to the side of the rafter. This will help you find the hole from the top side of the roof. Alternatively, you can have a helper mark the hole with a crayon.
   
2. 2. Measure the rafters in the first (uppermost) row. Hook your tape measure on the outside face of the first rafter and measure straight across the rafters perpendicularly. Note the precise location of the outside face of each rafter you will anchor footers into, recording the dimensions on your rooftop map.
   
3. 3. Mark the remaining rows. Back at the first rafter where you drilled the starter hole, measure down (toward the eave) along the length of the rafter and mark the locations of each of the remaining rows of footers, according to your plan. At the bottom row, drill another hole through the roofing and mark its location with another piece of wire (or have your helper mark it on the rooftop). When you’re up on the roof, you’ll snap chalk lines between the holes to create a vertical reference line. If the rafter bows (curves sideways) quite a bit, you may want to drill an additional hole for some of the intermediate rows. This first rafter is the reference point for all of your rafter measurements, and a bow of more than ¹⁄₂ inch or so can result in missing some rafters with the footers’ lag screws.
   
4. 4. Locate the remaining rafters. Measure across the rafters for each remaining row, noting the locations of the anchoring rafters, as before. It’s necessary to measure each row because the rafters may not be perfectly parallel to one another and/or each rafter may not be straight.

Tip

For Finished Ceilings

To locate rafters on vaulted ceilings, use a stud finder to locate each rafter and mark its side edges. Before drilling your starter hole, make sure there are no electrical wires or plumbing pipes in the area. Measure over from the starter hole to each anchor rafter; then confirm the rafter location with the stud finder. For more accuracy, drive a small finish nail through the drywall at ¹⁄₄-inch intervals to pinpoint the rafter edges; you’ll know you’re on the rafter when you hit wood.

No Rafter Where You Need One?

Sometimes the roof framing doesn’t agree with your racking layout, due to inconsistent spacing, roof penetrations, or modifications to the framing. The standard solution is to add wood blocking between two rafters where you need an anchor point. Use framing lumber that is the same size as the rafters (2×6, 2×8, etc.), and cut the blocking to fit snugly between the rafter pairs. Tap the blocking into place and fasten it by driving two 3¹⁄₂-inch screws or two 16d nails through the rafters and into each end of the blocking. Alternatively, you can use 4×4 lumber for blocking, which gives you a little more wiggle room for anchor placement.

Tip: If you discover you need blocking while installing the module racking, drill a small pilot hole down through the top of the roof where you need the anchor point. Inside the attic, position the blocking so that it is centered under the hole.

Marking the Roof

1. 1. Snap a vertical line. On the rooftop, measure over ³⁄₄ inch from each starter hole of the array layout and make a mark, indicating the center of the rafter. With a helper, snap a chalk line through the marks to create a vertical (ridge to eave) reference line representing the center of the first anchor rafter. If you made additional holes to follow a bowed rafter, snap a chalk line between each adjacent pair of holes, like connect-the-dots.
   
2. 2. Snap the top horizontal line. The horizontal lines mark the anchor locations for the footers. Measure down from the roof ridge at both ends of the layout and mark the height of the top row of footers. Snap a chalk line through the marks to create a horizontal reference line, perpendicular to the vertical chalk line.

   Note: Footers must be centered over rafters for proper anchoring, but often the rail locations can be moved up or down the rafters a bit so that the footers sit properly on the shingles (provided the modules don’t overhang the rails by more than 30% of their length, or as specified). If necessary, adjust your horizontal line as needed so that the footer flashing will sit flat atop the shingles (see here).

3. 3. Mark the remaining rows. Measure down from the top horizontal line to mark the locations of the remaining rows of footers, measuring at both ends to make sure all lines are parallel. Adjust the row locations as needed to accommodate the shingles. Snap chalk lines through each set of marks.
4. 4. Mark the anchor points. Using the dimensions from your rafter layout, measure over from the vertical chalk line and mark the location of each anchor point on the horizontal lines. Because you measured over ³⁄₄ inch from the starter holes to center the vertical line over the first rafter, you don’t need to do that with the rest of them; just use the rafter measurements on your map, and all your marks will be centered over a rafter.

   Note: Doing this portion of the installation (measuring and marking) accurately is essential for the final PV array to end up looking straight and true, with all the rails and modules in perfect alignment.

   

Tip

Draw the Array Corners

Use a marking crayon to clearly mark the four corners of the array (that is, the outside corners of the outer modules) onto the roof. This will help you visualize the array’s position until the modules are up. It can be an invaluable reminder, as you’ll see the first time it prevents you or one of your crew from drilling in the wrong place. You can also mark the locations of the rails, but use a different color of chalk to distinguish the rail markings from the array markings.

Installing Footers and Rails

Installing the racking hardware itself often is the most product-specific aspect of the entire PV installation. Always follow the manufacturer’s instructions for your hardware and roofing type. The steps shown here may or may not apply to your project, but the procedure demonstrates some of the essential basics — namely, preparing shingles for flashing, sealing around the penetrations, and torquing the bolts on the racking system.

A couple of specialty tools are required: a flat pry bar or a shingle ripper for removing nails from underneath shingles, and a torque wrench for making sure the fasteners are tight enough without overtightening. Also be sure to use the highest-quality roofing sealant you can find. Do not use caulk (silicone or other) or any other type of sealant not specifically designed for roofing. You can buy all of these supplies at local hardware stores and home centers.

Tools

• Drill
• Drill bit (sized for lag screws)
• Socket driver bit and socket (for lag screws)
• Caulking gun
• Flat pry bar or shingle ripper
• Ratchet wrench
• Torque wrench
• String line
• Wood blocks (2, must be the same thickness)

Materials

• Racking system components
• Lag screws (size and material specified by racking manufacturer/building department/engineer)
• Roofing sealant

Removing Nails from Shingles

To remove a nail, slide a flat pry bar or shingle ripper (a $20 tool designed for this purpose) under the butt edge of the first shingle above the anchor point, breaking the adhesive seal where the shingles overlap. Slide the bar up under the nail (there will be a shingle between the bar and the nail head) and twist and pry the bar slightly to raise the nail head about ¹⁄₈ inch. Pull out the bar, slide it under the second shingle above the base, and pry up the nail directly. Remove the nail and cover the nail hole with a small amount of roofing sealant.

Installing Footers and Flashing

1. 1. Drill pilot holes. Drill a pilot hole through the roofing and into the center of the rafter at each anchor point, using a bit that’s sized for the specified lag screw. A common size for the lag is ⁵⁄₁₆ inch, which typically calls for a ³⁄₁₆-inch pilot hole.
   
2. 2. Break the shingle seal. Asphalt shingles have an adhesive strip underneath their butt edges that sticks to the shingles below. When the roof is hot, the tarlike adhesive is really soft and you can usually just slide the flashing through it. Otherwise, carefully break the seal with a flat pry bar. The flashing must slide up under two courses of shingles above the anchor point. Remove any nails in the way of the flashing, as needed (see Removing Nails from Shingles, opposite). Repeat this step as you install each piece of flashing (step 3).
   
3. 3. Install the flashing. Fill the pilot hole (for the footer lag bolt) with roofing sealant. Install the footer flashing as directed by the manufacturer. You may need to apply roofing sealant to one or both sides of the flashing. Typically, the flashing slides up under the two courses of shingles above the footer base, and its bottom edge is near or slightly above the butt edge of the shingle it rests on. Tip: Use a screw as a guide to align the hole in the flashing with the pilot hole.
   
4. 4. Mount the footers. Position each footer and footer base (as applicable) over a flashing piece and secure them with a lag screw through the hole in the flashing and into the pilot hole in the roof. If the footer can be adjusted on its base, leave it a little loose so you can move it as needed when positioning the rails (you will tighten the lag screws later).

   Note: Lag screws must be long enough to penetrate the rafters at least the minimum distance required by the racking manufacturer and local building department. Often the structural engineer (if you use one) will specify the size of lag screw required.

   

Assembling and Adjusting the Rails

1. 1. Attach the rails. Attach the first section of rail onto the footers as directed by the manufacturer. Leave the rail connections loose for now so you can adjust the rails later. If the row is longer than a single rail, attach the second rail to the footers, then splice the rails together using the provided splicing plate or clamp. Be sure to leave the specified gap between rail sections to allow for thermal expansion (¹⁄₄ inch is typical). Tighten the lag screws on all of the footers in the row. Repeat the same process to assemble all the rails in every row.
   

Tip

Bottom Up

Most PV installation pros install the rails and modules from the bottom up, starting near the eave and working toward the ridge. The rails make for handy supports for your feet, your tools, or even a module or two. Just don’t stack a bunch of modules on the roof, creating the potential for one really expensive avalanche!

1. 2. Set the rail height at the ends. Starting at the outside footer at one end of the bottom rail, position the rail so its mounting bolt is roughly centered top-to-bottom in the elongated hole in the footer (the elongated hole allows for adjustment up or down). Tighten the bolt to the specified torque setting. Repeat with the last footer at the other end of the rail.

   Note: The rails — and ultimately the modules — are installed parallel to the roof and in a flat plane. They do not have to be level (perfectly horizontal), like a countertop or a pool table. Roofs are not always level from side to side. If the roof is out of level and the modules follow it, they will look fine. If the roof isn’t level but the modules are, you might see the difference from the ground.

   
2. 3. Set up a string line. Tie a string line onto the lag bolt of one of the outside footers. Pull the line around the end of the rail and over the top. Run the line to the other end of the rail and tie it off in the same way, making sure it is taut. Slip a block of wood under the line at each end (blocks must be the same thickness; as shown, the blocks are ³⁄₄ inch thick) so the string is held above the rails.
   
3. 4. Straighten the rail. Working from one end to the other, measure between the string line and the top of the rail. Adjust the rail height at each footer so the rail is ³⁄₄ inch (or whatever your block thickness is) from the line, then tighten the rail mounting bolt to secure it to the footer, torquing the bolt as directed. Repeat at each footer until you reach the other end of the rail.
   
4. 5. Straighten the remaining rails. If there is only one row in the array (two rails total), move the string line to the upper rail and repeat the same process to straighten and secure the upper rail. If there are more rows in the array, straighten and secure the top-most rail as you did the bottom rail, and then tie the string line to the top and bottom rails so it runs perpendicularly over all of the rails. Use the same measuring technique to position the intermediate rails (those between the top and bottom rails), so they are the same distance from the string line as the top and bottom rail. This ensures that all of the rails are in the same plane.
   
5. 6. Check all fasteners. Double-check all of the racking connections (footers and rails) before moving on to the module installation. Start at one end of the top or bottom rail, and move down each rail or row, retorquing all the nuts/bolts with the torque wrench.

Installing Modules

Once the footers and rails are in place, aligned, and secured, installing modules is a straightforward process of assembling prefabricated parts. The steps and techniques vary by the type of system and the specific products used. The general procedure involves four tasks:

• Mounting microinverters or DC optimizers (as applicable) to the racking rails or to the module frames (see here). With some microinverter systems, this step may include securing AC trunk cables along the rails (the microinverters then connect to the trunk cables). With string-inverter systems, it may be easiest to install the long home-run cables along the racking before installing the modules.
• Grounding (or bonding) modules, rails, and microinverters/DC optimizers to a system ground. While module clamps and WEEBs typically bond modules to rails, bonding the rails requires a separate ground wire. Microinverters may or may not need to be attached to this ground wire, depending on the model and the system wiring.
• Installing a junction/combiner box (which may be either DC or AC).
• Installing modules, wiring them together as you go and securing them to the rails with mid and end clamps.

Tools

• Ratchet wrench and sockets
• Torque wrench
• Wire cutters
• 4- or 6-foot level (or straightedge)
• Jigsaw
• Needle-nose pliers

Materials

• Grounding lugs
• 6 AWG solid-copper ground wire (or as specified by local code)
• Black UV-resistant wire ties or outdoor cable clips
• PV modules
• Module end and mid clamps
• WEEBs (as needed)
• Rail caps (as applicable)
• Array screening system (optional)

Additional Materials for Microinverter Systems

• NEMA 3R electrical AC junction box
• AC trunk cable (one per branch circuit, as applicable)
• Microinverters

Additional Materials for String-Inverter Systems

• Disconnecting DC combiner box
• 10 AWG PV cables (one two-ended cable per series-string, for home runs)
• DC optimizers (as applicable)
• Electrical tape
• Marker

The system shown in the following steps includes microinverters that serve two modules (rather than one) and connect together with their own wire leads; they do not connect to a separate AC trunk cable. The microinverters and rails are grounded/bonded with a single ground wire. Wiring details for string inverters are discussed in Home Runs for String-Inverter Systems.

Mounting Microinverters (or DC Optimizers) to Modules

Installing microinverters or DC optimizers to the backsides of modules is called frame mounting and is an alternative to rail mounting. Frame mounting, available with most models, is primarily used with railless racking systems and ballasted (flat-roof) systems, which don’t have rails to mount to, but it can also be used with conventional rail systems. It typically requires a separate mounting bracket supplied by the inverter or optimizer manufacturer.

To frame-mount a microinverter or DC optimizer, choose a location where the unit will not interfere with the rail when the module is installed. Secure the unit to the module frame and torque the fasteners as specified. Connect the wire leads from the module to the microinverter or DC optimizer as directed. When you install the modules on the rooftop, you will connect the outgoing leads from the microinverters to one another or to the AC trunk cable; optimizers also will connect to one another.

Installing the Microinverters and Ground Wire

1. 1. Mount the microinverters to the rails. Mark where the centers of the modules will fall on each of the rails that will receive microinverters (every other rail, since the units mount to only one rail for each row of modules). Install a microinverter at each mark, following the manufacturer’s directions. Usually this involves one or two brackets with holes and bolts that fasten the microinverter bracket to the rail.
   
2. 2. Connect the microinverters together. Join each microinverter to the next one in the row using the connectors on their wire leads. Alternatively, if the system includes an AC trunk cable, extend each trunk cable along the rails with the microinverters. Align the cable connectors on the trunk cables with the microinverters so that the inverter leads can reach the connectors. Plug in each inverter lead to a connector, as directed by the manufacturer.
   
3. 3. Cap off the last microinverter lead. The last microinverter in each row or branch circuit will have an unused lead, since there’s no other inverter down the line to connect to. Cover this lead with a termination cap supplied by the manufacturer. Most simply fit over the lead connector and twist to lock. The first module in the row or branch circuit will connect to the junction box with a special cable that has a module connector at one end and loose wires at the other end. AC trunk cables have similar termination caps; install these as directed by the manufacturer.
   
4. 4. Mount the grounding lugs. Install a grounding lug on each rail, and torque as directed by the manufacturer. Lugs are typically arranged in a straight line across and perpendicular to the rails — to save on wiring — but can be installed anywhere on each rail, as desired.
   
5. 5. Install the ground wire. Secure the 6 AWG solid-copper ground wire to the first lug (typically on the rail farthest from where the array’s junction/combiner box will be). Tighten the lug over the wire to the specified torque setting. Extend the wire in a straight line to the next connection point. The ground wire must connect to all of the rails. Tip: Make 90-degree turns, as needed, for a neat installation. In the lower left photo, a little pigtail spiral (made with pliers after the wire is secured) is one installer’s finishing touch at the start of the wire run.
   
   
6. 6. Ground the microinverters (as applicable). Connect the ground wire to each microinverter case or mounting bracket, as directed. Run the wire to the junction/combiner box location, leaving enough slack for wiring into the box later.

   Note: As mentioned here, some microinverters are designed with internal grounding and do not need to use the external ground wire. However, the system must include a ground wire in all of the cabling between the microinverters and into the junction box. If any of this cabling lacks a ground, you’ll need to connect the microinverters to the external ground wire.

   
7. 7. Tie up the wiring. Secure the cables to the rails with cable clips or wire ties (see Wire Management).
   
8. 8. Install the junction box. Mount a NEMA 3R (outdoor-rated) electrical box of a suitable size to the racking rail or to a conduit located near the rail (seeRooftop Conduit Runs). Note: Use only the predrilled holes and/or mounting tabs for fastening the box to the rail or conduit; do not drill new holes or otherwise alter the box. If you have a string inverter system, you will install a disconnecting combiner box instead of a junction box. This must be within 10 feet of the array to satisfy the NEC’s rapid shutdown requirement.

   With microinverters, you will be bringing AC into the junction box. With string inverters, you will be bringing DC into the disconnecting combiner box.

   
9. 9. Map the installation. Create a simple map or chart of the entire array, noting the location and serial number of each microinverter and module. Most microinverters come with stickers that you can peel off and add to your map. This record will prove invaluable if an inverter has a problem and you need to know where it is located in the array. Also, your inspector and/or the utility may request to see the map.
   

Home Runs for String-Inverter Systems

Each series-string in the array gets two home run cables: one runs from the last module in the string and along a rail back to the combiner box; the other (typically shorter) runs from the first module in the string to the combiner box.

Note: If you’re adding DC optimizers to your system, you will install the optimizer units to the rails (similar to microinverter installation) and connect the units to one another. The modules connect to the optimizers. The home runs for the strings connect to the first and last optimizers in the strings.

An easy way to make home runs is to use long PV cables with factory-installed quick- connect fittings on both ends. (The fittings are typically MC4 connectors but must be compatible with your modules.) Extend each cable from the far end of each string back to the combiner box. Cut off the end at the combiner box and use the leftover cable, with its connector, to make the shorter run from the near end of the string to the combiner box. Plan carefully so that you have the proper fittings at the ends of the strings: if the last module will have a female lead for connecting to the home run, start that home run with a male fitting, and vice versa.

Label the ends of the home runs at the combiner box, using light-colored electrical tape and a marker. Assign a number to each series-string, and note the string number and whether the cable is connected to a positive (+) or negative (–) module lead. Use a tape color that is not red or green. Red usually means positive (+), and green usually indicates ground, in standard electrical installations.

WARNING: Leave the home run cables disconnected until the end of the PV system installation. Do not connect the home run cables to the modules when the modules are installed. Modules create electricity under any daylight conditions. Leaving these cables disconnected is the best way to ensure safety until the system installation is complete.

Note: With microinverter systems, it’s okay that the modules and microinverters are connected to the AC trunk cable, because the inverters don’t output electricity unless the PV system is connected to the utility grid.

Installing the Modules

1. 1. Position the first module. Set the first module onto the rails at one end of the row (if the array has multiple rows, start at the bottom row). Fit two end and mid clamps onto the rails, using WEEB washers, if necessary (see Grounding Your Modules and Racking). Measure to confirm that the module is centered top-to-bottom and is square to the rails. Connect the module leads to the rail-mounted microinverter or DC optimizer, if applicable. If you have frame-mounted microinverters, connect the microinverter to the AC trunk cable, as applicable.
   
2. 2. Position the second module. Set the second module into place and snug it up against the first module with the mid clamps (with WEEBs, as applicable) in between. Align the bottom edges of the first two modules, using a 4- or 6-foot level (simply as a straightedge, not as a level). Secure both modules with the mid clamps, and torque the clamp bolts as specified by the manufacturer. Connect the wiring as applicable.

   Note: Since WEEBs are installed two for every two modules, you will place the WEEBs under every other pair of mid clamps, and under the end clamps on rows with an odd number of modules.

   • Microinverter (rail-mounted): Connect second module leads to microinverter (microinverters already connected together or to trunk cable).
   • Microinverter (frame-mounted): Connect microinverters together or to AC trunk cable (modules already connected to microinverters).
   • String inverter without DC optimizers: Connect first module + lead to second module – lead, or vice versa (remember you are wiring them in series).
   • String inverter with rail-mounted DC optimizers: Connect each + and – module lead to its own optimizer unit (optimizers are already connected together).
   • String inverter with frame-mounted DC optimizers: Connect first module optimizer to second module optimizer (modules are already connected to optimizers).
   
3. 3. Continue down the row. Install more modules in the row, using the same techniques and adding WEEBs as needed. Check the alignment of each module with the level. When the row is complete, it’s a good idea to view the entire row from various angles and from the ground to make sure the modules are visually aligned with the roof. Visual alignment, as seen from the ground, is more important than precise alignment and squareness to the rails.
   
4. 4. Complete the row. Secure the outside module at each end of the row with a pair of end clamps, using WEEBs, as applicable. Retorque all of the clamps in the row. Note: Remember to call for your rough inspection at the specified time, based on what the inspector wants to see of the modules and racking.
   

   WARNING: Do not connect the home-run cables to the modules (string-inverter systems). Leave them disconnected until the end of the PV system installation, when they can be connected by, or under the guidance of, your electrician. Modules create electricity under any daylight conditions (a series-string of connected modules produces a dangerously high voltage). Leaving these cables disconnected is the best way to ensure safety until the system installation is complete.

5. 5. Install the remaining modules (as applicable). If there are multiple rows in the array, install the modules in the next row up from the bottom, using the same techniques as before. Maintain the manufacturer’s minimum specified gap between the rows (typically ¹⁄₈ inch or more). Sight down the row as you go to keep the modules aligned. Complete the remaining rows, then trim the rails to length, as specified, using a jigsaw or reciprocating saw. Cover the ends of the rails with caps from the manufacturer, if applicable. Add screening to enclose the space below the modules to keep out animals, if desired (see Critter-Proof Your Array). Sometimes the ends of the rails are spray-painted to match the color of the module frames.
   

Wire Management

Neatness is one of the hallmarks of quality on any electrical job, but it’s particularly critical for PV systems. Dangling wires look bad — even if you can’t see them from the ground, you’ll know they’re there, and your inspector might frown on them. They’re also prone to collecting dirt and debris and are vulnerable to damage from sliding snow and ice. Make sure that wires do not dangle down from the modules or rails and do not touch the roof. As a good rule of thumb, you should be able to look under your array and see no wiring below the bottom edges of the module rails.

To help tame and protect wires, some racking systems have rails with channels for laying in the wires. Rail manufacturers also offer cap pieces for enclosing the channels if there’s a large gap between modules. If your rails don’t have channels for wire management, secure the wiring to the rails with outdoor-rated wire clips or black, UV-resistant plastic wire ties (zip ties).

Rails with channels offer the cleanest installation. The channels are open at the top but are almost entirely covered by the modules.

Plastic wire ties can wrap all the way around rails and are handy for tying up coils of wire.

Rail clips (typically supplied by racking or microinverter manufacturer) fasten directly to rails.

Wire clips for module frames help tame long module leads. These are usually metal or plastic, ¹⁄₄ inch in size.

Installing Flat-Roof Systems

Ballasted flat-roof systems offer a nice advantage over sloped-roof systems: you don’t have to locate and anchor into rafters. Most installations follow an assemble-as-you-go process. Here’s an overview of the basic steps (installation details can vary widely from system to system):

1. 1. Snap a chalk line onto the roof to represent the front edge of the first row. Position two front module brackets on the line. Position two brackets for the rear of the module, spacing them according to the module size. If there will be multiple rows in the array, use connector-type rear brackets. If there will be one row only, use standard rear brackets. Weight the brackets with one ballast block (concrete block) each, as applicable. Note: Some systems include metal trays for ballast blocks; others may require protective material between the blocks and the roof.
2. 2. Set the first module onto the brackets, and adjust the brackets as needed. Add module end clamps and mid clamps to the brackets. Note: Since there are no rails, microinverters and DC optimizers are frame-mounted to the modules prior to installing the modules (see here).
3. 3. Position and weight the next two brackets; then add the second module. Secure both modules with the module clamps. Connect the modules/microinverters/DC optimizers together electrically as required for the system type. Continue the same process to complete the first row.

WARNING: Do not connect the home run cables to the modules for string-inverter systems (see WARNING here).

1. 4. Begin the next row behind the first. The spacing between rows is automatically set by the connector brackets. Complete the remaining rows in the array. For the last row, use standard rear brackets (not connector brackets) to support the rear edges of the modules.
2. 5. Complete the ground wire and AC/DC prewiring runs per the system design.
3. 6. Install wind deflector panels to the rear and connector brackets, as applicable. Add ballast blocks as needed to meet the design specifications. Specs should include the quantity, weight, and location of ballast blocks for the given tilt and wind and snow loads.