9. Select and Size Wires
Once you've selected the brand and model of modules you plan to use, then sized and configured your array, picked out an inverter model, and chosen all your smaller electrical devices (i.e. combiner or junction box, disconnects, etc.), you'll be ready to size wire, overcurrent devices (fuses and breakers), and conduit for the circuit. For residential solar electric systems, always use copper wire. Aluminum wire is less expensive but breaks easily and corrodes. It's also less efficient in conducting electricity.
Wire sizing calculations typically require the following information, all of which are discussed in this tutorial:
Selecting wire also involves choosing a correct gauge, or diameter. As the image above illustrates, wire gauges correspond to the amount of current intensity you expect to flow through your circuit. Basically, the fatter the metal conductor inside the wire jacket, the more current it can conduct without generating a lot of friction and heat.
To understand this better, imagine electrical wire as if it were a hose carrying water. The more volume inside the hose for the water to flow throught, the less pressure there is on the hose to deliver it. Wire gauges are designated by the American Wire Gauge (AWG) system. Although it seems counter-intuitive, the smaller the number, the fatter the gauge. Wire may also take either a solid or stranded form. Stranding provides more flexibility for beefier gauges that would otherwise be hard to bend.
Besides the gauge, wire is sold by color and by the foot or spool size. A spool may be 50 feet, 500 feet, or something in between. Generally, you'll need three colors: black for an ungrounded conductor, white for the second conductor that's grounded, and green for the equipment grounding wire. (If you live in Europe or are using a transformerless conductor, then both conductors are ungrounded, or "hot". In place of the white wire you would use the color red. That's let utility workers and electricians know they're dealing with an ungrounded circuit.
Before purchasing wire for any electrical installation, you or your contractor must become proficient in using the wire tables and adhering to the rules laid out in the National Electric Code (NEC). Your local building inspector will never approve any installation that doesn't meet NEC standards. Unfortunately, the code is revised every couple years, so different inspection agencies invariably use different editions (e.g. 2008, 2011, 2012). You should find out early in your project which version your local authorities plan to enforce.
When it comes to wire, the NEC generally dictates the minimum wire gauge you need to use. Once that's calculated, you can always go bigger. Most home PV is wired with AWG 10 on the DC side (AWG 6 or 8 if you're using a combiner box), and AWG 10 or 8 on the AC side. The bare copper equipment ground for the array is usually AWG 6, which is sturdy enough to withstand the elements.
Even though standard sizes are used, you will still have to explain to the inspector the amount of amperage and volts you expect to be pumping through your system. Moreover, your wire gauges will need to be rated for as much or more ampacity as any fuse or breaker installed to regulate them. As discussed in the previous step, the terminals inside the junction box, disconnects and any other electrical enclosures are likewise rated to withstand a maximum amount of heat or current. And you'll have to post a safety signs on your system identifying specific voltages and amperage.
For these reasons, it's recommended that you consult an expert (or hire a licensed contractor) before designing and installing a solar electric system. If you decide to do it yourself, you'll be doing some more math.
Wire Characteristics and Their Strange Names
Generally, either PV wire or USE-2 wire is used for the array, and is not run inside conduit. USE stands for "underground service entrance". It's not, however, restricted to subterranean applications. Both USE-2 and PV wire can handle high ambient heat. Their jackets will not degrade from ultraviolet exposure and they're both moisture resistant. PV wire has an extra layer of insulation.
Most installers then switch to a less expensive building wire, typically THWN-2 Copper, on the other side of the junction or combiner box. Since this wire is run through conduit, it does not need to be UV resistant. However, it must be able to withstand high heat and wet conditions that result from condensation. All wire used in most home PV systems is rated for 90-degrees Celsius worth of high ambient heat. That's equivalent to 194 degrees Fahrenheit. Most indoor wire is rated for 75 degrees Celsius.
THWN-2 wire can be run all the way to the Main Service Panel. It's good both for DC circuits and AC circuits, although you may need to switch to a different gauge once your wiring exits the inverter. That's because the inverter will change the amps and volts of the solar electricity once it becomes alternating current.
At any rate, here's what the letters (and number) stand for in THWN-2:
T - thermoplastic insulation around the copper
H - rated for 75 degrees Celsius
W - suitable for wet conditions
N - Nylon jacket
-2 - rated for 90 degrees C (canceling out the H coding above.)
As a rule, any -2 wire (pronounced "Tack 2") indicates that two functions are served: wet conditions and high heat. You will find another, cheaper wire on the market, THHN/THWN, that serves an "either/or" situation, rather than both. So if you have wet condtions but hot high heat, then it will work. If you have high heat only, then it will work, because the HH in THHN indicates a 90 degrees C rating. (In installer parlance, HH means "Hella Hot!").
If a supplier tells you THHN/THWN is the same as THWN-2, this is simply not true. A building inspector will fail your installation if you try to use it in conduit or in a high heat environment. So always be sure the wire you buy is stamped "THWN-2", because you need both a high heat rating and water resistance at the same time.
Also remember that THWN-2 is not UV- resistant. That's why it must always run in conduit when used outdoors.
For a closer look at residential wire and cable types, here's an overview.
Incidentally, having to use conduit for some (and maybe much of) your solar electric system wiring is a good thing. Conduit protects wire from getting blown around, eaten by rodents, or tugged on by small children. On the other hand, wire can get awfully hot inside conduit, which is a bad thing. That's why determining the minimally allowable gauge you can use is a big deal for building inspectors. Solar designers must conduct ampacity calculations that take into account the extra heat of a conduit enclosure before choosing a wire gauge.
And one more thing to note about conduit... The NEC requires metallic conduit for wire runs inside a home or office. Outside, you can use cheaper plastic conduit, better known as PVC. The down side of PVC is that it degrades much more quickly over time than metal. Electrical Metal Tubing (EMT) is the best choice for most home PV installations. Although it's more expensive, it will leave a much better impression on potential homebuyers 10 or 20 years down the road.
Dividing the Circuit Wiring into Sections
To help you visualize how a home grid-tied PV system is wired, here's a diagram posted online by a do-it-yourselfer:
This circuit includes DC and AC disconnects, and a grounding wire ("GND 6 AWG Bare"). The schematic identifies fuses, wire sizes, voltages, amps and the array size ("2760 WATT DC PV"). Two locations, the "Main Roof" and "Basement", are indicated on the drawing, and refer to component locations. Also notice the wire that runs from the last module all the way back to the combiner box. In wiring terms, this is called the "home run". The lightning arrestors, incidentally, are usually optional. For a system using microinverters, check out the schematic provided by Enphase. Diagram: wind-sun.com
This is called a three-line drawing. It traces the circuit from the array to the utility meter and includes the path of the three principle wires - the nongrounded conductor, the grounded conductor, and the equipment grounding wire. (In other words, two conductors and a ground. ) To help get your bearings, picture the array modules sitting on the roof (top of diagram). There's a junction box (far left). The wires exiting it are most likely traveling down the side of the house.
Notice that a dashed line encloses the rest of the circuit. Judging by the earth ground symbol in the bottom righthand corner, it's safe to assume that this part of the circuit is at ground level. Notice the Sunny Boy inverter, which has a built-in DC disconnect attached at the input (i.e. array) side. Wires travel from the other (output) side of the inverter to the main service panel in the home. From there, the wire proceeds to the utility meter.
Here's a quick rundown of Electricity 101: Like all electrical circuits, you need one conductor (wire) running from the power source (the array) to the load center (main service panel), and a different wire that theoretically carries the electrons back from whence they came. Hence, the "circuit". The third ground wire, technically known as the equipment grounding conductor (EGC), is not part of the PV circuit. It's a safety feature that connects all the metal boxes and other electrical hardware to earth ground. Only when a live (hot) wire shorts into a metal component will electricity "conduct" through the EGC and into the earth. Thus, an equiipment grounds plays a critical role in every circuit without ever being part of its regular operation.
Sizing Conductors and Identifying Wire Types
To determine the wire you need -- including types, colors (green for ground, white for neutral, etc.), gauges and lengths -- designers first divide the system circuit into sections. Then they address the electrical characteristics and other variables of each section in turn.
A simple grid-tied solar electric system includes three sections:
In many electrical guides the PV Output and Inverter Input circuits are separate sections, with the DC Disconnect in between. However, since home grid-tied systems normally possess the same electrical characteristics on either side of the disconnect, this tutorial will mesh the two sections into one.)
PV Source Circuit
Your solar modules come with four or five-foot long cables already attached, including a positive and a negative lead. In most cases, the wire type on module leads is PV Wire. The two leads should include snap-in connectors, male or female, which makes connecting the modules together a straightforward process. Be sure to note the type of connector on the module you select for your array -- for example MC4 -- since you may need to buy a few more of these connectors to wire the circuit.
Back in the design phase of this tutorial, you determined the number of modules to be placed in series and/or parallel. Our sample array consists of two parallel strings of 10 modules. So the wiring diagram and all subsequent calculations will take this into account. For now, the diagram above shows one string of 12 modules. The negative polarity conductor is indicated on the nearest end of the module row to the combiner box. Meanwhile, the positive conductor runs from the opposite end of the string, across the top, and then into the box. In the parlance of installers, whichever conductor is at the far end (i.e. farthest from the junction or combiner box) represents the "home run". It can be either the negative or positive wire, depending how you choose to interconnect your modules. Here, the home run is positive.
As you can see, some extra PV wire must be added to the array in order to complete the source circuit. You'll also have to decide on how you want to ground all the modules and racks together. There are two options:
1. Run an equipment grounding conductor (EGC) from module to module, rack to rack, and onward to the junction box. Traditionally, a sturdy bare copper wire of AWG 6 solid (not stranded) is run from lug to lug. AWG 6 is pretty sturdy, so it can stand up to the elements (or a rodent gnawing on it) without breaking. If you prefer to use AWG 8 or a thinner PV/USE-2 wire, it must be encased in a PV wire (or USE-2) jackett. But bare copper is the usual choice.
2. Bond the racks and modules with grounding clips and jumpers. Instead of connecting a ground to every module and rail section, you can accomplish the same result by slipping in little grounding clips (they look like washers) when you clamp your modules onto the rails. You'll also insert electrical jumper cables on either side of spliced rails in your racking to keep the equipment grounding path uninterrupted. See photos below. Once that's done, you'll only need to connect your bare copper ECG at one lug attached to each row, then run the same wire to the junction box or combiner.
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On the left, a Wiley WEEB grounding clip is used with mounting harware to create a solid connection between module and rail. The clip is the flat plate with the two little dimples. At right, a WEEB jumper cable is connected across a rail splice. Equipment bonded together in this manner insures that a stray current will move efficiently in the desired direction - to ground. For more on how the clips and jumpers work, watch this video.
A bare copper ground wire of AWG 6 is typically used with lugs to ground each row of modules. When used with WEEBS and jumpers (see above), you need only to ground each row at one location, then run the bare copper wire to the junction box. This securely grounds a roof array in accordance with NEC rules.
Many installers prefer to USE-2 over PV wire for the home run because it's cheaper than PV wire. Both types have the same electrical characteristics as far as their ratings and the NEC. Both types are UV and moisture resistant, and rated for 90 degrees Celsius. However, PV wire is double insulated, so it should enjoy a longer life than USE-2. It's also now required for systems that include a transformerless inverter. As far as picking a gauge, since the module manufacturer has already sized the leads (either AWG 10 or 12), sizing for other wire in the PV source circuit can be considered pretty much done for you.
One very important exception to the rules for wiring the PV Source Circuit is an array that uses microinverters or newer AC modules. Wiring in these scenarios is a totally different animal. In particular, you'll have 240 volts of AC by the time the electricity leaves each microinverter or module. If you decide to go either route, you must follow the wiring instructions in the product installation guides, and in some cases install special mult-wire cables sold by the manufacturer. For more on this subject, see the Enphase installation manual.
When ordering PV or USE-2 wire, always pick the color black. You can use black for both the positive and negative lead by simply adding a colored piece of tape on either end to identify the grounded and nongrounded conductors (or positive and negative). Wire jacket colors other than black contain less carbon and deteriorate faster in the sunlight, so you should avoid them on the roof.
Solar equipment suppliers usually offer a single product for roof array wiring -- AWG 10 PV Wire, sold in 20-foot rolls. If you measure the distance of the home run from the end of each string to the junction or combiner box, then you'll know how many feet you'll need. Most of the time, you can connect the module lead nearest the box directly to it. However, if the distance is too far, you'll to add an extension, which might be referred to as the "near end run".
When buying PV wire, you also have the option of having one or both ends equipped with the snap-in connectors. This will cost more money but can save you time, so think about how your modules and home/near runs will connect together and to the junction/combiner box. To reiterate, different brands of connectors don't interconnect, so you'll need to select the same type that's used on your module leads. Usually, but not always, it's an MC4.
For more info on equipment and system grounding, check the basic overview in the BOS Components section of our tutorial. It's easy to confuse EGC and GEC, so try to remember that the first is for equipment, the other is for a direct path to earth via a buried rod.
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