Array Wiring for Rooftop Systems

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Wiring Your Solar Array

The electrical wiring for your solar PV installation is not complex. New Hampshire regulations allow a homeowner to do electrical work on their own property without being a licensed electrician. But please consider this carefully. 

You must have a thorough understanding of wiring and how electrical systems work. Bad wiring can kill, or could burn your house down. If you have any doubts about your ability to do the work, hire an electrician. If you don’t know how to safely add a circuit to your home’s distribution panel, hire an electrician. 

Consider the following carefully:

• • Hiring an electrician does not always guarantee a good result. There are great professionals out there, but sadly that is not always true. 

  • Don’t accept a wiring job that meets the minimum specifications. Affordable improvements are the best value if future expansions are added to the system. 

  • A system that is inspected and approved may not be safe. Building inspectors are often overloaded and cannot take the time to trace every circuit and inspect every detail. 

  • Every system is unique. The design will be determined by your existing house wiring and the requirements of your solar installation. 

  • If you are doing it yourself, be sure you understand and follow National Electric Code (NEC) requirements. This information is readily found online. You should know which version of codes is adopted by your local town building department. 

  • Proper safety labels are required on many parts of the system. Inexpensive adhesive labels for solar components can be purchased at pvlabels.com. 

The information on this page is not a complete guide. It describes a general design for the most common PV installations: a net metered system tied to the public utility grid using microinverters attached to each PV panel. For other designs (off grid, single inverter systems, and systems with a backup generator) you will need to find more information. This guide can help you discuss your needs with your electrician, who may be new to solar installations. For experienced DIY types, it may get you started with your design.

We have generic wiring diagrams for the electrical design of a grid tied small microinverter system, large microinverter system as well as a string inverter system. These Microsoft PowerPoint files may be downloaded and easily modified for your specific design. 

We’ll discuss particular concerns starting with the PV panels and ending with the utility interconnection. 

Overall system considerations

The size of your system (number of panels, voltage and current output) will affect many of the subcomponents described below. The size will dictate the gauge of the conductors and rated capacities of components that you choose. In addition to current carrying capacity, you also need to consider the total resistance of all wires and components from the microinverters to your main electrical panel, and calculate the voltage drop of the complete electrical path. If the voltage drop is too high, your system may not operate efficiently. You can adjust your design by using larger conductors (lower AWG gauge wire) to reduce the resistance and voltage drop. Compensating for voltage drop is a greater issue for ground or pole mount arrays at some distance from the house. The total length of wires from your PV panels location to your home’s main electrical panel will have a big impact on the design and conductor requirements.

Voltage drop is easily calculated from Ohm’s Law: I = E/R, where I = current in Amperes, E is voltage and R is resistance measured in ohms. So the voltage drop will be E = R(I). To calculate the voltage drop in a wiring segment, multiply the segment’s current by the total resistance, and the units of measurement will cancel out to give you the volts: 

          Voltage drop = (current in volts per ohm)(ohms/foot)(2-way length of wiring in feet)

The total current is the combined maximum output of all the microinverters. The resistance values (ohms/foot) are specific to the wire being used based on the gauge, the conductor metal (copper or aluminum), and how many strands are used to make the wire. These resistance values are available on line. You need to double the length of a conductor run to account for both the hot lead and the return path.

After you find the voltage drop for a segment of your system, determine what the percentage voltage drop is by dividing that voltage by 240v. Do this for every segment of the system (inverters to junction box, junction box to collector, collector to main), since different segments may use different types and sizes of conductors. A circuit line before the collector box will have a lower current – only the inverters on that circuit are counted. But after the collector box the current will be the sum from all inverters in the system. The sum of all the percentage voltage drops should not exceed 2% for best system performance. To learn more, see this Enphase technical brief.

Panel and Microinverter wiring.

Our example assumes you are using Enphase Microinverters, the most popular grid tied inverter and one of the products HAREI members can purchase at a discount. Solar panels have standardized electrical connectors that simply plug in to the microinverter. There are 2 cables for the positive and negative DC voltage output from the panel, and the design of the cables makes it impossible to connect them incorrectly to the microinverter input. (Note there are 3 different cable interconnect designs for different panel manufacturers. We will make sure our members order compatible models.) The 240v AC output from the microinverters plugs into a proprietary cable from the same manufacturer.

The panel/inverters are connected serially along the Enphase cable, up to a maximum of 16 inverters. A string like this would constitute 1 circuit that will lead to your circuit collector box. The Enphase cable will have a custom cable terminator at the distal end (supplied by Enphase), and will lead into a junction box at the near end where it connects to cable you supply for the run to the collector box.

Although you can have up to 16 inverters on 1 segment of Enphase cable depending on the inverter output power, for systems at a great distance from the main service panel you need to reduce the voltage drop as much as possible. The voltage drop through the Enphase cable with its integrated connectors is relatively high, and increases non-linearly with the number of connectors. A circuit of 16 panels can be broken up into parallel sub circuits that join together in the junction box before entering the collector box. Reducing the number of connectors in half actually reduces the voltage drop more than 70%. For detailed information, see this Enphase technical brief.

Junction box wiring

Each Enphase cable leads to a junction box where the four individual wires are spliced to wire that runs to the collector box. The wires are spliced using standard wire nuts – be sure to use the correct size nut for the size and quantity of leads in the splice. The box should be watertight and have watertight fittings for the cable entrances. PVC boxes with gaskets and covers are readily available at hardware stores. These boxes will not have punch out openings like metal boxes do, but clean openings are easily drilled with a Forstner drill bit. This is a big advantage since you can position the openings in the best locations. 

Sizing the junction box is important. The wire splices should not be crammed in too tight, and the wire leads inside the box should have enough length to easily move them around or allow repair splices in the future. Boxes are sized according to the number of wire splices and the wire gauge. You can calculate the minimum volume for each splice (or look up the value in a table) and then sum all the values. Select a box that meets or exceeds the volume requirement. Size requirements can be found online.

Circuit collector box wiring

The circuit collector box may be a model specifically designed for solar installations or it may be an exterior grade load panel. All the solar circuits will feed into the collector box, and each circuit will be protected with a circuit breaker. A main breaker in the box is not usually required – your design may differ. 

The collector is a subpanel to your home’s main distribution panel, so it should be wired accordingly: the common bar should not be bonded to the ground, and both common and ground should have their own path back to the main panel. For an installation not on the house, the box should also be directly grounded to a buried ground rod. Adding a lightening arrestor is recommended.

The solar circuit wires are connected to the breakers and busses the same way a circuit is wired into a home distribution panel. If you don’t know what that means or how to do it, hire an electrician.

The collector box is combining all the circuits of your solar array. Compared to how distribution panels are usually wired, it is back feeding current into your home’s electrical distribution, and you should only use circuit breakers unaffected by the direction of current flow. The output of the collector box is 4 conductors off the panel bus bars heading towards the house: 2 “hot” leads, the neutral common and the ground

REC meter wiring

Several inverter systems integrate a revenue grade REC meter.   If your inverter does not, a separate REC meter can be located anywhere between the collector box and the integration into your home’s electrical system. Being located adjacent to the collector box is often a convenient position, but each layout is unique. Used REC meters from utilities are readily available for as little as $20 - $25. They should be from a reliable source that has tested and certified their performance before reselling. The 4 prong meters commonly used bridge the 2 hot conductors, and have input and output lugs for those 2 conductors. The ground conductor should be bonded to the meter box as it passes through. The common conductor should pass through the box fully insulated – do not bond the common to the ground anywhere other than your home’s main panel.

Service conduit

The service conduit is the longest run of wire or cables between the solar panel array and the home’s main service panel. This path and location will be unique for every installation, and may even be a different section of the wiring plan in different designs. For example, a ground mount system located at some distance from the house will likely have large conductors in a buried PVC conduit between the collector box and system cut-off switch. But on a roof-mounted array the collector box could be co-located with the cutoff switch, with 1 or more conduits bringing the solar circuit wires down to the collector.

There are many types of electrical wires and cables. It’s very important to select a type suitable for the voltage and current loads, ambient temperatures, exposure to water and exposure to uv radiation from sunlight.

• • Wiring rated to be installed in wet areas is always important, even if your wire runs through conduit. Buried conduit may fill with water, and there is always condensation inside the conduit. 

  • Wire and cable is rated up to a maximum temperature. If the ambient temperature is too high, the current carrying capacity of the cable may be de-rated to a lower value. This is unlikely to be a problem for buried cables in the cool temperatures of the ground, but may be a concern for some roof top installations heated by the summer sun. 

  • Conduits should not be overfilled with wires. Electrical codes determine what maximum percentage of fill is allowed, based on the cross sectional area of the conduit. You must add up the cross sectional areas of each conductor to assure the maximum fill is not exceeded. The data for these calculations, and the code requirements, are easily found on line. 

Whatever the design, the conduit should be large enough for the wires it contains for proper heat dissipation and for pulling wires through the conduit. Long service paths will have the greatest impact on voltage drop for the system. Refer to the voltage drop discussion above.

System cutoff switch

The cutoff switch should provide easy access and instant shutdown of the photovoltaic system for emergency or maintenance situations. Locating it close to the electric utility service meter makes it easier for emergency responders (fireman, for example) to safely disconnect all electrical inputs to the house. Most grid tied PV systems are single phase, 240V AC installations, and the switch should meet that requirement. (Note that 3 phase 208V PV systems are also possible, and would require a matching switch. However, these systems are more likely to be found in commercial properties rather than a residential home.) A single phase switch’s wiring is similar to the REC meter box. There are input and output lugs for the 2 hot conductors. The ground conductor should be bonded to the switch box. The common conductor should pass through the box fully insulated.

The interconnection with the home’s electrical system

The interconnection will occur in the main distribution panel of the home’s electrical system, or in a subpanel connected to the main panel. The interconnection is wired in the panel just like any other circuit using a circuit breaker for the 2 hot leads, and the common and ground wires attached to their respective bus bars. Since the system is back feeding 240V AC, the circuit breaker will be a double pole breaker that connects with both hot bus bars.

If the interconnect is made in a subpanel, don’t forget to include the wiring from the subpanel to the main panel in your total system voltage drop calculations.

Note that service panels have a maximum rating for how much current their bus bars can support. The maximum current your system could provide should not exceed 20% of the service panel rating. For example, a 200A panel can support an additional 40A from the solar array. If your solar system will exceed that value, then you must compensate in some manner to keep the total current from getting too high. For example, you could replace the main panel breaker with a lower rated breaker so that utility and solar combined inputs are not too high.

Try to locate the solar system breaker at the opposite end of the panel bus bars. Under maximum loads the household circuits can draw current from both ends of the bars and that will also prevent overloading of current passing through the metal bars.