8. Select and size the smaller electrical components.
Once you've picked your inverter and module brands, you'll be ready to select other components that will play supporting roles in your PV system. You and/or your contractor should by now have configured the array to have a set number of modules wired in series, parallel, or both.
This is where many requirements of the National Electrical Code (NEC) enter the equation. Specifically, residential grid-tied solar electric circuits must incorporate the following:
For a more detailed overview of all these products, please see Balance of System Items - Page 2.
This illustration of a PV circuit includes battery-back-up and a "DC loads" panel. The design of a normal grid-tied system (minus the batteries, charge controller, DC Breaker panel and battery fused disconnect) is a cakewalk compared to this set-up.
In order to determine the right size and features of the smaller components to install, you'll need the following information:
When shopping for components, check to see which brands of fuses or circuit breakers are compatible with each product. Compatibility is normally quite limited, so be sure at least one fuse or breaker model listed on the product spec sheet is easy to find and not too expensive.
Although most home PV systems are easily sized from among a handful of standard products on the market, it's still a good idea to understand the math used to quantify the volts, amps and watts pulsing through the circuit. Moreover, if you live in a place that gets really hot in the summer, or really cold in the winter, these calculations become critical in choosing components that can handle the extremes. A hot temperature increases heat inside wires and conduit (and across terminals), while a cold temperature can increase the voltage beyond what the array modules are rated to generate.
That's why building inspectors and utility companies carefully examine schematics and product spec sheets submitted with a solar PV permit application. During an onsite inspection, the inspector will likewise examine the ratings listed on the components themselves, confirming they're the same ones that you designated on your submittal.
Starting with the easy part of component sizing, the maximum voltage in a PV circuit (i.e. on the array side of the inverter) is calculated with the formula below:
Vmax = Vo.c. X #modules per string X low-temp voltage correction factor
If this equation looks familiar, it's the same one used in Step 6 to size the inverter. Again, incorporating the open-circuit voltage spec of 37.2 volts for Sharp residential module ND-235QCJ, configured with two array strings of ten modules, the math looks like this:
Vmax = 37.2 X 10 modules X 1.13, which is 420.36 volts.
The value used for "low temp voltage correction factor" was taken from a NEC Table 690.7, shown below. This is the easy way to handle an adjustment in voltage due to temperature. You simply look for your coldest local temp in the ranges provided in the table, then select the corresponding multiplier from the middle column. For Sacramento, that value is 1.13.
NEC Table 690.7
In the United States, the maximum voltage allowable in any residential circuit is 600 volts. Consequently, electrical components sold by suppliers are invariably rated for 600 volts. On the other hand, when choosing an overcurrent device on the DC side of the circuit, you'll generally need to use fuses, because circuit breakers are not able to withstand more than around 240 volts.
Selecting a Combiner or Junction Box
With voltage out of the way, the peskier calculation of current/amperage is the next challenge. The NEC uses the term ampacity rather than amperage when discussing component ratings and sizes. Ampacity is a measure of the ability of a conductor to handle current, and this measurement has a large safety margin built in just in case. The maximum current threshold is determined by a combination of math formulas, NEC tables that list the ampacity limits for wire, fuses, terminals and other electrical items, and in some cases the product specifications.
If you have more than one string of modules but don't want more than two conductors running downstream to your inverter, you would use a combiner. This might be the case, for instance, if you have limited space available to run wire through existing conduit. More often, however, home solar electric systems use a simple junction box and allow each set of conductors to pass through on their way to the inverter. Most inverters have input terminals (aka channels) that allow you to plug in 2 to 4 (and sometimes more) sets of conductors.
Whichever component you choose, the junction box or combiner should be placed near the array because you'll be switching to a less expensive wire type at that spot. The NEC requires that any wire transition take place inside an electrical enclosure. You can't just splice your connecting wires together, wrap them with electrical tape and leave them out in the elements.
The photo on the left features a Soladeck grid-tied PV combiner. Notice four sets of wires (positive and negative) entering at the bottom and marked with tape (red for ungrounded conductors, white for grounded conductors). Only one set of wire exits the top, along with the green ground wire. A grounding terminal in the lower right corner connects the green building wire with the bare copper ground coming in the bottom from the array.
The diagram on the right, which doesn't represent what you see in the photo, shows how the wiring of two strings runs from the array through a combiner box. Most grid-tied inverters don't use a battery charge controller, so the thick red and black wires (positive and negative) will run downstream to a central inverter instead. (If microconverters are used in your system, the combiner will combine wires carrying AC current and may pass through circuit breakers instead of fuses.) At any rate, the fuses inside a combiner box provide overcurrent protection, while the lightning arrestor provides surge protection, which may or may not be required in your city. The green line represents the ground connection. Notice that all the physical equipment (the modules, box enclosure, etc.) are tied to ground. This is an NEC requirement. Photo: SolaDeck ---- Diagram: HomePower.com
Overcurrent protection (either fuses or breakers) must be included in a PV source or output circuit only if you have three or more array strings. Fuses are usually placed inside a combiner box (if you use one) or inside the DC disconnect (if you don't).
Most O.C. devices are rated for a maximum operating temperature of 40°C (or 104°F). This is fine for everday household wiring. On the other hand, because of their location outdoors or inside attics, PV components may be subjected to a lot more heat than that. Thus, if you plan to place any fuses or breakers in high heat, you should refer to the product spec sheets for temperature adjustment factors. Otherwise, the circuit may experience nuisance trips or blown fuses in hot weather.
To determine the normal O.C. device rating (i.e. the fuse or breaker size), start with this equation:
Circuit ampacity = Imax X 1.56
On the DC side of the circuit, the short-circuit current (Isc) is used for this calculation. If your fuse will be placed inside a combiner or junction box, for example, then Isc will equal the short-circuit current spec for the modules. For our sample array of Sharp modules, the calculation is thus:
8.60 amps (the short circuit current) X 1.56 = 13.42 amps.
Since fuses are sold in standard sizes (6, 8, 10, 15, 20, 25, 30 amps, etc.), the NEC states that you must select the closest size at or just above the ampacity value. For 13.42 amps, that means a 15-amp fuse.
For PV circuits incorporating a normal inverter with a transformer, only one of the two wires in a pair -- the ungrounded, or hot wire -- is fused. However, if you have a transformerless inverters, both wires in the pair must be fused.
Also, in case you're wondering, the 1.56 multiplier in the ampacity calculation is a shortcut that incorporates two NEC formulas that apply to PV circuits. The first formula is Imax X 1.25, which equals what the NEC calls the continuous current of a circuit. The second formula is Continuous Current X 1.25, which provides a cushion above the first value in order to avoid nuisance trips from minor current fluctuations. Now, if you take 1.25 X 1.25 (or 1.25 squared), you get 1.56.
For our sample grid-tied system with a normal inverter, a two array strings, and a voltage (measured earlier) of 420.36 volts, the junction or combiner box we purchase must be rated for 600-DC volts (i.e. the standard size), accommodate the positive and negative conductor for at least two strings, and have a minimum 30-amp rating. (You can still insert a 15-amp fuses, but the standard rating for components in this range is 30 amps.)
At left, a Soladeck AC/DC 3R Pass-Through enclosure works as a junction box for roof-mounted PV systems. It comes with flashing so it can be mounted flat on composite shingle tiles. In this photo, three sets of wire (for three module strings) and a ground appear to be exiting in the direction of the attic. Most junction boxes are mounted in a vertical position, however, and preferably in a shady spot out of the sun. Notice the terminals provided in this product for connecting the positive and negative conductors, and the ground wire (bare copper to green). This is the best way to make wire connections, although a basic electrical enclosure with no terminals is much cheaper to buy. At right, a polaris wire connector would be used to connect wires in the inexpensive junction box with no terminals. Common wire nuts are not rated to withstand high heat and may melt, causing a short, so they should never be used for a rooftop solar array.
Incidentally, some combiner models come pre-wired on the inside, saving you installation time. Here's the product list for Midnite Solar, a company that sells both pre-wired and traditional combiners for residential and commercial PV applications. Junction boxes and combiners should ideally be rated for PV applications, since these products are designed to handle high heat. You'll also want your box to have a NEMA rating of 3R or 4, if it will be placed outdoors. Moreover, any box you buy should should have plenty of space inside to make wire connections (including the equipment ground wire) easy and comfortable. Wires crammed together in a tiny space will naturally generate more heat and pose a higher risk of a short-circuit or disconnection from a terminal. Your job of wiring becomes much easier when bus bars or terminal strips and blocks are provided in the enclosure.
Selecting a DC Disconnect
If you decide not to use a combiner, you'll most likely have two or more sets of conductors flowing downstream into the DC disconnect. A disconnect is a manual on/off switch placed in the circuit to provide humans with a way to quickly power off one section of the PV circuit. For a small grid-tied PV system, you should ask your building inspector and utility if the DC disconnect already installed on your inverter will fulfill the requirement. If so, you'll save both on installation time and money by skipping the extra component.
Square-D 600V 30-amp fusible DC Disconnect
If you include a standalone DC disconnect in your circuit, you'll have to size it in the same manner as the junction or combiner box. Most of the time, the right-sized model for your circuil will be rated for 600 volts DC. You'll likewise have the choice of buying either a fusible or non-fusible product. In the case of a fusible disconnect, the size you pick for your fuses depends on how much current each set of conductors is carrying from the array through the disconnect, and whether or not a combiner is placed in the circuit ahead of the disconnect.
If you're not combining current in your PV circuit, the same formulas used above applies here:
Circuit ampacity = Imax X 1.56
If a combiner box is used, then:
O.C. ampacity = Imax X #Module Strings in the array X 1.56
For our sample array with a combiner box, the math would be 8.60 amps X 2 strings X 1.56, which is 26.84 amps. The closest-sized fuse at or above this amount would is a 30-amp fuse.
To learn more about DC Disconnects and their ratings, check out the popular Square-D Model HU361RB. The "U" in the model number stands for unfused. Even when you're not shopping for a fusible model, you would still need to compute an ampacity rating for the product. So the math above is still relevant, and the product you buy should be rated for 30 amps.
Selecting an AC Disconnect
This disconnect is placed between the inverter and the home's main service panel. Significantly, the electricity an AC disconnect sees bears little resemblance to the PV array electricity on the DC side of your system. In particular, you'll have two "hot" conductors (in addition to a neutral) running from the inverter to the main service panel that will pass through this disconnect. Each will carry half of the 240 volts generated by the inverter.
The NEC ampacity formula also changes on the AC side of the circuit. Instead of 1.56, the multiplier is 1.25. And in place of the short-circuit current, you must use the maximum or continuous output current listed on the inverter spec sheet. So the ampacity calculation looks like this:
Circuit Ampacity = Inverter AC output current X 1.25
The Fronius IG 4000, for instance, lists an output current of 16.7 amps. Thus, 16.7 X 1.25, which is 20.88 amps. So the right breaker or fuse in the circuit (or inside the inverter, on the AC output side) should be rated at 25 amps.
For the AC disconnect itself, you would choose a 30-amp model, 2-pole product. If your inverter is transformerless, and you decide to buy a fusible AC disconnect, you'll need to get a 3-pole model in order fuse the neutral conductor in the circuit, since it won't be grounded.
For a longer discussion of how to size overcurrent protection in a PV system, here's an article from NEC expertJohn Wiles.
Selecting a DP Circuit Breaker
When you run your wiring from the AC disconnect to the main panel, you'll need to install a new two-pole (aka DP) circuit breaker into the panel as part of that connection. The breaker must be of the backfed type, since current must be able flow backwards into the utility grid. Each pole will handle one of the two hot 120-volt conductors coming from the inverter.
"Two-pole" means the circuit breaker has two trip switches, although it takes up the same amount of space as a single-pole breaker. When you shop for this component, be sure to check your Main Panel first to see what brands of circuit breakers are compatible with it.
The same math performed for the AC Disconnect can be used here:
Circuit Ampacity = Inverter AC output current X 1.25
Again, 16.7 X 1.25 = 20.88 amps, which means a 25-amp breaker is appropriate for each hot conductor. Furthermore, the NEC stipulates that the PV breaker be placed at the opposite end on the panel from the "main" breakers. This provides a physical barrier between the two power sources (the utility grid and the inverter), which reduces the chance of any arcing, short-circuiting or other accidental collision of titans.
Note: If your main service panel has a bus bar capacity of 100 amps, the maximum breaker size you can add is 20% of 100, which is 20 amps. That means you can't use any inverter higher than 3800 watts without getting a main panel upgrade or a "line side tap". The highest inverter output current acceptable for 20-amp breakers would be 16 amps, since 16 X 1.25 equal 20. Alternatively, you could reduce your "main" breaker size on the service panel from 100 to 80 amps, allowing you to use a larger breaker size. However, this may result in frequent breaker trips whenever you use multiple appliances around the home. If your main panel bus bar is rated for 200 amps, you can use a PV system breaker up to 20% X 200, or 40 amps.
Selecting a Net Meter
If it's required, a net meter enclosure and socket must be installed between your inverter and the main service panel. The instructions you receive from your utility company should include specs identifying the type of component that will fulfill this purpose. Check with a company representative if you're not sure what product to buy.
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