Lesson 7
PV System Installation

In the initial site visit, the approximate locations for the inverter, disconnect switches, the combiner box(es), and the junction box(es) are identified. The distance between each device and from the inverter to the building’s electric panel is noted. Using the PV system’s peak DC voltage and current output, the inverter’s AC electrical characteristics, and the distances between the equipment the appropriate size of wire for each run can be determined (see the voltage drop table in Lesson 6).

Given that the building has passed the site survey and is a good candidate for installing the solar electric system, it is time to start the paper work. Keep in mind that various municipalities across Pennsylvania enforce different codes and may have restrictive covenants that need to be checked and cleared with the local officials. The building and electrical permits need to be obtained before proceeding with the PV system installation.


example of pv array on a house
Safety is of the utmost importance. Even before unloading the solar equipment and installation hardware and tools, the crew should have a safety meeting. It is recommended that everyone on the crew be trained in CPR and basic first aid. This meeting should include safety issues presented in the solar water-heating section, such as preventing falls from cluttered work areas, setting up and using ladders correctly, wearing gloves and safety glasses, and being careful to not drop tools or equipment. The meeting also should include a lengthy discussion of electric shock and its potential when working around PV systems should be presented. The following information is excerpted from Sandia National Laboratory’s Photovoltaic Systems Research & Development website www.sandia.gov/pv/syso/ESafety1.html.

Common electrical accidents result in shocks and/or burns, muscle contractions, and traumatic injuries associated with falls after the shock. These injuries can occur any time electric current flows through the human body. The amount of current that will flow is determined by the difference in potential (voltage) and the resistance in the current path. At low frequencies (60 Hz or less) the human body acts like a resistor but the value of resistance varies with conditions. It is difficult to estimate when current will flow or the severity of the injury that might occur because the resistivity of human skin varies from just under a thousand ohms to several hundred thousand ohms depending primarily on skin moisture.

If a current greater than 0.02 amperes (only 20 milliamperes) flows through your body, you are in serious jeopardy because you may not be able to let go of the current-carrying wire. This small amount of current can be forced through sweaty hands with a voltage as low as 20 volts, and the higher the voltage the higher the probability that current will flow. High voltage shock (>400 volts) may burn away the protective layer of outer skin at the entry and exit points. When this occurs, the body’s resistance is lowered and lethal currents may cause instant death. The data in the following table shows the reaction of the human body to various levels of current flow.

Electric Shock Hazard - Current Level
Reaction AC Current (ma) DC Current (ma)
Perception - Tingle, Warmth 1.0 6.0
Shock - Retain muscle control; reflex may cause injury 2.0 9.0
Severe Shock - Lose muscle control; cannot let-go; burns; asphyxia 20 90
Ventricular Fibrillation 100 500
Heart Frozen - Body temperature rises; death occurs in minutes 1000 1000

Electrical shock is painful and a potentially minor injury is often aggravated by the reflex reaction of jumping back away from the source of the shock. Anytime a PV array contains more than two PV modules, a shock hazard should be presumed to exist.

To avoid shock, always measure the voltage from any wire to any other wires, and to ground. Use a clamp-on ammeter to measure the current flowing in the wires. Never disconnect a wire before you have checked the voltage and current. Do not presume everything is in perfect order. Do not trust switches to operate perfectly and do not trust that schematics will always tell everything you need to know. Use a voltmeter often—it could save your life.

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Roof: Select Installation Site

On the roof, the physical site for the PV array needs to be chosen and within this area, the rafters must be located. The composition of the roof will dictate how the mounting rails or rack—which hold the modules in the array—should be installed. The mounting procedure is basically the same as for the solar water-heating installation. You have the choice of using lag screws into the center of the rafters; using the J-bolt next to the rafter; or using a spanner board, all thread, and compression block to mount the clips for holding the rack system which holds the modules securely in place.

Locating the rafters and their centers from up on the roof could involve using a stud finder, tapping with a hammer, using the fascia board/rafter nail connection, or a combination of these methods.

If you cannot locate the rafters from the roof, you will have to get into the attic space. Once you get oriented in the attic space, use a long 1/16th inch drill bit to drill a hole up through the roof right next to one of the rafters that you will be using to attach the mounting clips. If you do not have someone on the roof to note where the drill bit comes up through the roof, push a piece of wire up through the hole to allow you to easily see the hole when you go back up on the roof. Drill two holes, one at the upper level of the mounting rack’s location and one at the lower level of the rack. Be sure to measure the distance between the rafters at each level of the drilled hole—this will allow you to locate the center of rafters, including those that were not installed parallel to each other.
For a roof covered with masonry or ceramic tiles, the installation process needs some adjustments. The roof structure is commonly designed near the limit of the dead load of the tiles. In this case, the rafters must be enhanced to support the additional dead load of the PV system and the live load associated with the installation. An alternative is to remove the tiles where the PV system is to be installed and transition to a roof of asphalt or composition shingles. Doing this should allow the installation of the PV system without enhancing the roof structure.

Install the mounting clips according to the manufacturer’s instructions. Use a sealant that is UV-resistant to prevent leaks when screwing down the mounting clips. If you had to drill up through the roof to locate the rafters, be sure to force sealant into those holes also. For an installation with the array installed parallel to the roof slope, the mounting clips should be such that the PV modules are situated 3 to 4 inches above the roof. This allows cooling air to circulate below the modules, allows rain to run off under the array, and helps prevent ice dams from forming during freezing conditions. Depending on the mounting equipment, rails are commonly attached to the mounting clips and serve as a secure structure for attachment of the modules and sub-arrays. Connect all dissimilar metals (such as steel and aluminum) using non-conductive washers to prevent galvanic corrosion. Make sure that all connections of the mounting clips and mounting rails are tight—once the modules or sub-arrays are attached, it might not be possible to get to the nuts and bolts.

Visually check the modules for any cracked glazing, and check that the frame, wiring box, and the back’s potting material are intact. Check the open circuit voltage and current of each module before hauling them up onto the roof. Depending on the mounting technique and the inverter’s input voltage, you may be able to assemble a group of modules into a sub-array on the ground. The modules can be connected with the proper size, color, and type of wiring to form the sub-array, and then the sub-array’s open circuit voltage and current can be checked before being moved to the roof as a unit. Label the each wire pair for connection in the junction—this is used to document and identify the string and circuit going to the inverter and can be used for troubleshooting purposes at some point in the future. To prevent shocks, be sure to use wire nuts and/or electrician’s tape to cover the ends of wires coming off the modules or sub-array. When the modules are exposed to sunlight, they are electrically hot and are capable of providing the closed-circuit (normal) operating voltage and current levels to any material that can become a circuit. Before closing the electrical connector box on each module, check that the wiring connections are tight.

Getting the modules or sub-array to the roof can be as simple as one man on a step-ladder lifting a module up to a second person on the roof of a single story building. In the case of a sub-array, two people could be on the roof using ropes or straps to pull the sub-array up a ladder or two with another person below stabilizing and pushing the sub-array upward. In the picture below, an articulated manlift is used to move sub-arrays to the roof of a three-story building.
Note two points of interest:
1) there are two people on the left-facing roof—they are in harness and tied off and will move to the south roof to attach the sub-array when it ispositioned near the mounting rails; and
2) the table to the right of the white truck is used to assemble the modules into sub-arrays.

NCAT headquarters array

Before the module or sub-array is attached to the mounting rack on the roof, the frame ground wire needs to be attached. Modules have a designated spot to attach the equipment ground wire. You’ll need to procure the correct type of stainless steel fastener and connector (sometimes supplied with the module). The wire for the equipment ground shall be bare copper. The ground needs to be continued to the common DC equipment ground bus. This can be accomplished by transitioning to a green THHN ground wire in the roof junction box which is carried thru the steel conduit to the PV DC disconnect or inverter. The bare copper wire can be run outside the conduit to the junction box, then the combiner box and on to the inverter or other disconnect means where it is connected to a equipment-grounding screw. The size of the equipment grounding wire is dictated by the rating of the overcurrent device (breaker/fuse) protecting the circuit. If for example the overcurrent breaker is rated at 30 amps, the grounding conductor should be AWG #10 copper. Check the tightness of the equipment ground connection and tighten as necessary – you probably will not be able to reach all of them later when the modules or sub-arrays are mounted securely on the rails.

example of a combiner box
The locations for the junction box(es) and combiner box(es) were determined during the initial site survey. Re-evaluate the sites, install the hardware, and connect the pieces of equipment with conduit. The combiner box is basically a device that connects the input PV strings in parallel to produce one circuit. Remember that paralleling electric circuits increases the current flow in the downstream circuit. The combiner box output wire must be sized larger for the higher current and to minimize voltage drop and line losses of the PV generated DC power going to the inverter. The conduit between the junction box(es) and the combiner box(es) must be sized to safely hold the number of module or sub-array wire pairs passing through.
Note: steel conduit is recommended for PV source circuits and required per National Electric Code section 690.xx.

In the fused combiner box(es), the fuses should be removed and the appropriate number of properly sized wire pairs should be fished from the combiner box(es) to the junction box(es). Each wire pair should be labeled at each end for connection in the combiner and junction boxes. Connect the wire pairs in the combiner box(es) first and then connect them to the wires from modules or sub-arrays in the junction box(es).

CAUTION: When making PV source circuit electrical connections in junction and combiner boxes, make absolutely sure that each source circuit is broken by keeping a series connection disconnected for each source circuit (typically made with MC style plugs).

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The very last connection should be the plugging together these series connections. In this way, you will never be exposed to the lethal voltages involved. The junction box(es) should be sized larger than required for the number of wire pairs to be connected in them. Over sizing allows safe connection of the wire pairs outside the box and then the long leads are coiled neatly inside the box. The combiner box above has 10 input strings (this box is installed in an attic space - note the green equipment ground wires). Before you close up the junction box(es) make sure that the wire connections between the PV strings and the extension wires to the combiner box(es) are tight.

The conduits between the combiner box(es), DC disconnect switch, and the inverter must be sized to accommodate the size and number of wires passing through them. With the fuses out of the combiner box(es), the inverter and the DC disconnect switch in the off position, and the appropriate wires can be pulled through the conduit and connected to the combiner box(es), disconnect switch, and the appropriate inverter inputs. Note that only the positive wire is switched by the disconnect. The equipment ground wire can also be connected at the combiner box(es), the DC disconnect switch, and the inverter.

an inverter
A DC grounding electrode should be installed that connects with the equipment ground at the inverter. This ground should in turn be connected to the existing AC grounding system in the building’s electric panel. The size of this equipment grounding should be #8 Copper minimum. Conductor size for the DC output is determined by the amp rating of the circuit protection from the combiner box(es) and the DC disconnect switch. The grounding wire size for the AC side of the inverter to the building’s electric panel is determined by the amp rating of the circuit breaker and must be #8 copper at a minimum.

If the installation crew is large enough, the inverter and disconnect switches can be installed in the building while the modules are being installed on the roof. The meter that looks like a utility meter is the building owner’s way of keeping track of the number of kWhs produced by the PV system installed on the roof. This inverter does not have the capability to display the number of kWhs produced by the PV system.

an inverter
This picture shows the inverter mounted on a board. The AC disconnect switch is mounted to the left of the inverter and the DC disconnect switches are mounted to the inverter’s right. This is a clean arrangement and having critical switches near each other to be easily shutdown in an emergency or whenever work has to be done on the solar electric system is ideal. Assembling the board with the inverter and disconnect switches already mounted saves install time in the building. Mounting the inverter board close to the combiner box(es) reduces the amount of large gauge wire needed between the combiner box(es) and the inverter. Thus the longer run is smaller gauge wire for the AC circuit from the inverter to the building’s electric panel. A note here, if the inverter has multi-speed cooling fans, the noise when the fans operate on high is often too loud to hold a meeting in the same room. It is best to install the inverter board on a wall in a room that is not normally occupied.

The AC output from the inverter must be connected to an overcurrent device that is rated at 1.25 times the maximum continuous output current of the inverter. Run conduit from the inverter to the AC disconnect switch and to the building’s electric panel. The utility may require a separate, lockable utility disconnect switch be installed near the utility’s meter. This disconnect switch is for utility personnel use to take the PV system off line when utility work is done in the area. The wire size for these runs is determined by the inverter’s AC current output and the distance to the breaker being back-fed in the electric panel.

Turn the electric panel breaker(s) to the off position, check the disconnect switches to be sure they are in the off position, then fish the appropriate wires to each device back to the inverter. Connect the wires securely to the panel breakers, switches, and the inverter. The installation is ready for a final check before the system is turned on.

Although it is probably not possible to check the rooftop connections, check the tightness of all electrical connections and tighten as necessary. In the process of checking the electrical connections, check that the conduit runs are supported according to code, and check the tightness of all mounting screws and bolts used to mount the inverter, disconnect switches, conduit, and the equipment grounding connections.

Use the voltmeter to check the polarity and the open circuit voltage and current of each string coming into the combiner box(es). Write down the open circuit volt and current values for each string. These will be used as reference numbers to check the performance of each string and help in the troubleshooting process. Check the open circuit DC voltage at the inverter when the DC disconnect switch is closed. The open circuit current in this circuit is probably more than can be checked with a digital voltmeter (they usually max out at 10 amps), so use a DC ammeter to determine the current level. Record this value for future reference. Check the utilities line-to-line-and line to-ground voltage at the breakers in the electric panel and label the front of the electric panel and the breakers to identify the solar circuits.

Once the final physical and electrical inspections are complete, follow the inverter manufacturer’s instructions to get the inverter turned on. If all is installed correctly, the PV system should start to produce power. Turn off the inverter, the panel breakers, and all the disconnect switches and contact the inspector to schedule the final system inspection.

Finally, you will need to prepare a general electrical schematic to give to the building owner (and the electrical inspector), along with copies of the equipment descriptions, operating and troubleshooting instructions, and warranties. On the electrical schematic, include a drawing of the PV array layout with the specific circuits going to the combiner box(es) labeled.


  1. Why is the equipment ground necessary between the modules and the inverter? Can the same reason be used for installing the equipment ground between the inverter and the electric panel?

  2. Why is low-voltage power dangerous?

  3. Does a PV module with an open circuit voltage and amperage of 27 and 3.5 respectively under full sun conditions, present a shock hazard for someone who comes in contact with the wires? Explain your answer.

  4. What is the function of the combiner box?

  5. Why is there disconnect switch between the inverter and the PV array?

  6. Why is there a disconnect switch between the inverter and the building’s electric panel?

  7. What are the two factors used to determine the size of wire to install in the PV system?

  8. Why is it important to evaluate the voltage drop in DC circuits?

  9. What three methods are commonly used to locate the rafter centers when on the roof?

  10. Why mount the PV array at a level 3 to 4 inches above the roof?