Solar Accessories: Cables and Connectors
In the previous articles in this series, we covered the essentials of cables and connectors for an off-grid solar power system. Be sure to read those articles, or go back to the first article in this series for a complete overview of how to design an off-grid energy system. In this article, we cover the cabling needs for each leg or connection in the system.
Remember, in solar installations the panels and batteries are always on, so make sure that all the breakers are off when making cabling connections. Even so, be careful when connecting batteries together.
The following list shows all the different kinds of connections, or legs, in which your solar system will require cabling:
• From solar panel strings to the combiner box
• From the combiner box to the charger
• From the charger to the battery array
• Within the battery array
• From the battery array to the inverter
• From the inverter to the devices or appliances you wish to power
PV wire may be used in all locations, but it must be used in the first leg as this is the one location required to be exposed to the elements. A typical installation will use THHN or similar cables, of various gauges, in all legs but the first.
The last leg involves normal AC household extension cords or wiring techniques, and will not be covered in detail in this guide.
Now that we have identified the legs in which cables will be applied, the types of cable and connectors available, it is now time to discuss what cable to use at each location.
Panels To Combiner
As mentioned previously, PV wire will be used exclusively for connecting panel strings to the combiner breakers. Because the combiner will be located outside with the panels, these runs will be relatively short. Also, each string of panels in series will only produce the amount of current of a single panel. As a result of both of these factors, a relatively small gauge wire can be used, compared to the other circuits in the system. For 60-cell panels, producing about 9 amps short-circuited in full sun regardless of specific power rating, 10 AWG PV wire is our choice for this location. This gauge is also about the smallest which can conveniently fit most PV connectors, such as MC4 connectors.
Combiner To Charger
For this leg, the current is a multiple of that in an individual string. While a single string may produce 9 amps maximum, a 3-string array will produce up to 27 amps, and a 6-string array will produce up to 54 amps. As a result, thicker gauge wire is required here. But, since this leg of the cable can be protected inside conduit, cheaper THHN can be used. There are many resources on the web for calculating heat losses for various currents and cable lengths. As examples, however, we would feel comfortable with 8 AWG THHN wire for short runs of a 3-string array, and 6 AWG THHN for long runs of a 3-string array or short runs of a 6-string array, and 4 AWG for long runs of a 6-string array.
Charger To Battery Array
For this leg, the current is probably as high as it will be anywhere else in the system, unless multiple chargers are used to feed a shared battery array. A 96 amp charger in bulk charge mode is producing current approximately seven times the current as that experienced by a 15 amp household circuit. As such, this leg should use 4 AWG for small capacity systems, and 2 AWG for larger capacity systems. Fortunately, this leg should remain completely indoors, and should involve very short cable lengths. As a result, less expensive THHN wire, and less of it, can be used.
Within The Battery Array
As with the above, these connections will be some of the highest currents in any system. However, in some cases the current will "skip over" the battery array and right out the inverter, so the actual currents experienced within the battery array may be lower than either the charger current or the inverter current. Regardless, just in case either the charger or inverter goes down during maximum operation of the other, the battery array cables should be either 4 AWG, or better, 2 AWG. As these are some of the smallest battery runs, again, this larger wire will not be very expensive overall. If multiple chargers feed a single battery array, then this wire size should be increased accordingly to 1 AWG, or even 1/0 AWG (also known as 0 AWG).
Battery Array To Inverter
To size these connections appropriately, divide the inverter power rating by the battery array voltage. For example, if an inverter can supply 1000 watts, and a 24 volt battery array is used, this wiring will need to handle 42 amps. Often, inverters will ship with the appropriately sized cables for their power and battery rating. If not, consult any of a number of web resources to calculate the appropriate size. The key is to keep these connections, as with the charger and battery connections, as short as possible. We would default to a 6 AWG wire for 1000 watts or less, a 4 AWG wire up to 2000 watts, a 1 AWG wire up to 3000 watts, and 1/0 AWG wire above 3000 watts.
Now that you understand the issues with cable selection for each leg of your solar system, you can more safely implement a high-power off-grid system. A convenient non-contact infrared thermometer will be helpful in verifying your cable selections. If the wire, on any leg, is noticeably warmer than ambient conditions at any time, perhaps by 25 degrees or more, the wire is probably too small. And, don't forget to check the temperature of those cable lugs; a bad connection here can result in a spectacular arcing electrical fire.
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