Photovoltaic Tutorial: Balance of System (BOS)

BOS Items -- continued -- return to page 1

Electrical Devices, Wire & Conduit

Here's at look at the equipment you'll need to connect your photovoltaic array and inverter to the main service panel in your home:

Junction Box or Combiner

The National Electric Code (NEC) stipulates that any splice or other connection between separate sets of wires must must be contained within a junction box or other electrical enclosure. This insures protection from moisture, wind, sunlight, friction and critters. For PV systems, a junction box or combiner is typically placed at or near the array site.

Solar Living, Inc.
While the junction box or combiner for the array source circuit is not shown in this graphic, other required devices are featured in their order of appearance along a PV electrical circuit. A net meter is also required by many utility companies, enabling an accurate tally of the solar power output. In some home systems, the DC disconnect is attached to the central inverter and does not have to be purchased separately. Be sure to check with your local building inspector and utility for their specific requirements.

Junction boxes and combiners, like all enclsoures, come equipped with easy to drill holes called knockouts. Conduit is attached to one of these openings to carry wire down from the roof. Heavy-duty PV or USE-2 wire typically enters the box from the array, and a less expensive building wire (usually THWN-2) exits inside the conduit.

A combiner box provides the added function of "combining" wires from multiple module strings into one beefier pair of positive and negative wires. This reduces the time and expense of running the multiple sets down to the inverter in a larger diameter of conduit. While this sounds advantageous, in reality the separate circuits may be transporting energy at different voltages. Thus, combining the currents before they reach the inverter could result in less power generation. This subject is discussed in more detail in the Steps to Going Solar section.

A combiner should include overcurrent protection (i.e. fuses) if you have three or more array strings. (For AC and low DC voltages, circuit breakers are used instead.) Not every fuse or breaker brand fits every electrical component, so be sure to find out what types and models are compatible when choosing products.

Midnite Solar MNPV2-MC4 Combiner Box
This combiner comes pre-wired (some parts of it, anyway) which saves on installation time. Two MC4 connectors at the bottom, outside the enclosure, receive the snap-in connectors of the array input wires for each of two module strings. The red wire represents the ungrounded conductor that passes through an OCPD (i.e. fuse) at the top. The white wire represents the grounded. The combiner box has a NEMA 3R rating, which means you can use it outdoors, but only in a verticalposition.

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The junction box on the left receives wiring from a ground-based array. A cover plate will be screwed onto the enclosure as the last step, to make the enclosure water-resistant. Although the photo shows how wires and a bare copper ground are connected, the wire nuts you see here won't work for a roof-mounted junction box. Instead, you'll need to use polaris connectors (right) because they're built to handle high heat. Left photo:

Any electrical enclosure should have a NEMA 3R rating if it will be used outdoors. If you plan to mount the box horizontally, you'll need one with a NEMA 4 rating. Always study the spec sheet for an enclosure or electrical component carefully to verify it's the right product for the scenario in which it will be used.

A Soladeck AC/DC 3R Pass-Through enclosure with flashing. This product is mounted flat on roof tiles and carry wire down through the attic (see hole at the top). Most junction boxes are mounted vertically, with conduit running down the side of the house, but the attic solution may cut down on wire distance, and therefore voltage drop in the circuit.

If you decide to purchase a combiner box, be sure to select one specifically designed for PV applications. For most home systems, a 600-volt, 30 amp combiner is the correct size. Depending on the size and number of modules, the correct fuse to install in the box will be either 15, 25 or 30 amps. Correct sizing for electrical components and wire is discussed in our Steps to Going Solar guide.

Switchgear and Other Circuit Safety Devices

Small electrical devices that connect or disconnect your PV system, or transfer electricity to another circuit are known collectively as switchgear. These devices may be scattered throughout the circuit, and the quantity you'll need depends on the type of PV system (e.g. grid-tied or standalonel), the complexity and size of the circuit, building code requirements where you live, and your personal desire to add optional surge protection.

Disconnects are manually operated on/off switches in PV systems. The NEC requires an easily accessible DC Disconnect to be placed between the PV array combiner (or junction box), and the inverter. An AC Disconnect must be placed between the inverter and main electrical panel. In both cases, the goal is to provide two easy-to-access power on/off switches in the case of an emergency or to conduct maintenance work and testing on part of the circuit.

Square-D DC Disconnect

A ground fault circuit interrupter (GFCI) is also mandatory in PV systems. Fortunately, most inverters include an internal GFCI (or GFI), so you won't need to make a separate purchase. A ground fault indicates that somewhere in your system, a conductor may have short-circuited or otherwise come into contact with metal. A significant shock hazard may result when this happens.

To monitor the risk, a GFCI compares the amount of current in the ungrounded (hot) conductor with the amount of current in the neutral conductor. If the amperaage in the neutral conductor doesn't match the current in the hot conductor, the component will be triggered. The missing current is taking some path other than the intended path, such as through the racking of a PV array, or an electrical box.

Inverters will usually alert you to the ground fault with a sound alarm and/or LED indicator or message on the display. The user's manual provides instructions for how to proceed, and a licensed electrician or PV technician may have to be called in to troubleshoot the problem.

Here's a schematic that shows how a PV system might be wired:

Electrical scheme for a grid-tied inverter. You probably won't need toinstall a ground fault interrupter and "MOV" (metal oxide varistor), as these protections are typically provided by the inverter. Inside the inverter, the two facing wire coils signify a transformer, which steps down the array voltage to 240V. For a longer explanation of how grid-tied circuits are wired, visit the website.

Other items sold as switchgear include transfer switches (which shut down one circuit and energize another), metal oxide varistors (MOV's), DC current shunts, grounding hardware and lightning arrestors..

Cables, Connectors and Wire

It's nowadays standard for solar modules to be equipped with two cables 3-5 feet long, with MC4 or Tyco connectors at the end. One connector has a male shaft (to carry DC negative electricity) and the other a female tube (to carry DC positive). These male and female connectors are snapped together along a row to form one or more strings (aka panels). The connectors also have a rubber watertight seal that should never be removed.

The wiring (i.e. the cables) attached to modules are made of PV wire. This is double sheathed sturdy wire that's sunlight-heat-moisture resistant. It's outdoor wire, in other words, and should not be placed inside conduit because there's no fire-retardant applied to the sheath. PV wire is an expensive commodity, and you'll most likely have to purchase more of it (with the right type of connectors), so you can extend the PV source circuit to the combiner, junction box or enclosure described above.

Early versions of solar cables were known as MC3 or Solarline 1 cables. MC stands for "Multi Contact". Some modules sold today still use the MC3's, but most will require either the newer MC4 cables (aka Solarline 2), or Tyco locking connectors. At any rate, be careful when handling the cables with the modules exposed to sunlight, since they are transmitting a live current. And you should never perform any wiring when the circuit is connected downstream. Arcing may occur, which can melt the contacts in the connectors and damage the cables. An installer may also sustain a shock injury or worse.

For parallel wiring of an array, use MC4 branch connectors (or couplers), which are Y-shaped.

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MC4 (left) and Tyco cables (middle). On the left is a homemade bus bar, mounted in an attic. Using bus bars can help reduce power losses during the trip downstream to a combiner or inverter.

Cables are generally sold in 6, 30, 50 and 100 foot lengths, with a wire gauge size of AWG 10 (30-amp capacity) or AWG 12 (25 amps). They're also typically rated to handle either 600 or 1,000 volts. (Europe uses the 1,000-volt rating. In the United States, the maximum voltage in a PV circuit is 600 volts.)

It's recommended that you buy beefier wiring than you need, since that will reduce line losses and voltage drops. But the larger the gauge (i.e. diameter), the more it cost per foot. Generally speaking, home installations use 10-gauge PV wire between the array and combiner, although 12-amp might be OK for smaller strings.

Once the circuit wiring leaves the combiner (or other enclosure), you'll most likely be using a typical building wire known as THWN-2 . The "-2" means it's rated for 90 degrees celsius (194 degrees farenheit). If you live in an environment that's relatively cool (like the high country), you may be able to go with wire that has a 60-degree celsius rating, so ask your AHJ.

The gauge of the building wire is determined by a number of ampacity calculations. See Step 7 of Going Solar, for the math. As for the colors of the wire, you'll be running three wires from the combiner (if you use one) to the inverter:

Note: Some types of inverters require that the negative conductor be the nongrounded one. The product literature should spell this out.

Of the three wires, the first two are referred to as conductors in the circuit. The grounding wire is not. Even though it's made of the same building wire, this green wire is not part of the power circuit. It simply tags along downstream, touching base (so to speak) with all the different metal equipment the other two wires pass through. That's how the PV system components get "grounded".

If the copper inside one of the two conductors were to accidentally make contact with the metal housing of an electrical box or the array racking, the ground wire is there to bleed off the current so no one gets electrocuted. This scenario is known as a ground fault.

Once the conductors exit out the other side of the inverter, they'll be transporting AC current rather than DC. At this point, the voltage in the circuit becomes the standard 120 volt, 60-cycle variety, just like your grid electricity. So now you'll need wires that are:

Since the inverter itself produces 240 volts of AC current, you'll have to run two hot wires to split the sum in half. (Just as your utility delivers two 120-volt wires into your home instead of one with 240 volts, so too must the inverter.) After passing through an AC Disconnect and a Net Meter, these conductors are connected to the Main Panel by way of a double-pole (or DP) breaker. This is another electrical component you'll have to purchase as part of your solar power system. The breaker you buy must allow for backfeeding, so your PV electricity can flow into the grid. Some breakers only allow one direction, and will be labeled with "line" and "load"locations on the front. This you don't want.

Although AC and DC wire can never be interconnected, you can use the same THWN-2 or other building wire for both purposes. You can also use the same colors, and (in most cases), the same gauge. To know how much total wire you'll need, you'll want to measure the distance your two conductors and ground wire will travel, beginning with the array combiner or junction box, and ending at the Main Panel.

System Grounding

Most people are unaware that circuit grounding involves more than just the third wire and neutral grounded conductor mentioned above. While those wires connected to a grounding terminal at the main panel, more wiring may be necessary to protect your PV system. In addition to a ground fault, a few other hazards are possible, including:

To truly protect electrical equipmens, appliances and home electronics, the NEC and building inspectors may require you to do the following:

For more info on this subject, check Article 690.45 of the NEC. If your building inspection agency doesn't mention it on the instructions and permit application, don't assume there are no extra grounding requirements. Be sure to ask someone at the agency. If you'd like read up on grounding, try Photovoltaic Design and Installation for Dummies, and maybe a basic guide to home electrical wiring.


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At left, conduit is used to transport solar power system wiring down from the roof to electrical devices, switchgear and the main panel. A hole saw is used to create the right size openings. At right, a conduit bender is used on half-inch EMT.

Metal or plastic conduit is used to contain and protect PV circuit wiring as it moves downstream from the array to the main electrical panel. The NEC mandates that all indoor wiring other than Romex be run through metal conduit or raceway. For outside wire runs, either PVC or metal conduit may be used.

Like electrical enclosures, conduit prevents people and pets from coming into contact with potential exposed metal conductors. It likewise affords protection from the elements, UV rays, rodents, birds and insects. Conduit turns should be wide enough to preclude the possiblity of wires being crimped or becoming frayed. A conduit bender is commonly used in construction to create smooth turns that will fit tight clearances.

Here are the types commonly used in PV systems:

Electrical Metallic Tubing (EMT) is a sturdy conduit with galvanized steel construction that shields against magnetic fields and is impact-resistant.

Flexible Metallic Tubing (FMT) bends and twists easily, allowing turns at corners without the need to insert elbow joints. It's frequently used at the point of contact with an electrical device, allowing a hard right (or left) turn.

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From left to right: EMT, FMT, PVC and LFNC.

Rigid Nonmetallic Conduit (PVC) is lighter and cheaper than metal conduit but not quite as durable for the outdoors. PVC stands for rigid polyvinyl chloride. Its sections can be glued together with PVC cement, making connections watertight. Fittings and elbows are also available. Always include a grounding wire when using PVC conduit.

Liquid-tight Flexible Nonmetallic Conduit (LFNC) is used instead of the rigid PVC for tight turns and is waterproof, flame resistant, and non-corrosive. It also weighs less than flexible metallic tubing.

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On the left, the TallSmall PipeGuard is designed to support smaller-diameter piping and conduit up to 6 inches above the roof surface and doesn't require any special tools, clamps or backing sheet. On the right, the OlyFlow PipeGuard uses "strut" clamps to secure the conduit.

Wiring inside conduit tends to heat up faster than wiring left in the open air, so extra calculations are required to account for the higher temperature exposure. A high ambient temperature generally reduces voltage but also resistance in wire. Current may therefore increase beyond the short-circuit limit. In recent years, new language has been added to the National Electric Code (NEC) to more carefully address this scenario in PV systems.

Most PV systems use wire rated for 90 degrees celsius (194 degrees fahrenheit), so the conduit enclosure isn't too much of concern. However, you do have to calculate the minimum diameter necessary for your conduit to fit whatever number of wires will run through it. There are many online calculators available that do this math for you. See Calculators, Tools and Apps for links.

Net Meter Socket

A net meter socket made by Millbank. (A cover plate is included.)

Where net metering is a part of the utilitly interconnection agreement, you may be required to furnish and install the enclosure and socket that hold the meter. (Some utilities use a dual metering system, which requires a different component. Check with your company to see which device is appropriate.)

Typically, the net meter is installed between the inverter and AC disconnect. The utility worker will attach the meter's face during the site inspection.

Specifications for the enclosure should be included on the application or instructions you receive from your utility company regarding your grid-tied system installation. For example, here's what the Sacramento Municipal Utility District (SMUD) says in its program handbook:

"Installations must include a Meter Socket PV 4-Jaw 125 Amp 1PH (B-line 011 or Circle A&W 011) that will accommodate the PV Production Meter installed by SMUD to measure the energy generated by the customer. (This meter is in addition to the standard revenue meter used for net metering.)"

Connecting the PV System to the Main Panel

Connecting your PV system to the main service panel is accomplished with a dual-pole backfeed circuit breaker. "Backfeed" means electricity is allowed to flow in either direction. "Dual pole" means you can connect two hot wires to the main panel with one circuit breaker. This is essential, since there are alwasy two hot lines of 120 volts running to the panel from a central inverter.The breaker size you choose depends on the amount of current pumped out by the inverter. Common sizes are 20 and 30 amps.

When shopping for circuit breakers, keep in mind that different brands of main panels (or other electical components, for that matter) only accommodate specific types of breakers. Therefore, you'll have to check your panel specs to see which brands and models will fit. Also, if your main breaker is installed in the middle of the panel, or the panel is already full, your contractor or licensed electrician will need to rewire the unit. The NEC requires that the main breaker and PV system breaker be placed at opposite ends.

Something to keep in mind when sizing your PV systems is that a main panel is limited in the amount of total current that can pass through their bus bars. 100 amp and 200 amp panels are standard in residential settings. So if you have the smaller size, you may be limited to an inverter rated at 3,800 watts or less. This issue is discussed in more detail in the electrical section of our Steps to Going Solar guide.

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