Am I using the right wire gauge for this low voltage application?

December 16, 2024 12:00 PM

I’m setting up a small hobby/garage solar installation: 600W of panels, 40A charge controller, and 12V/300Ah LiFePo4 battery. Based on charts like this one I’ve selected 10AWG wiring between the controller and the battery because it can carry 55A of current in an open-air “chassis wiring” configuration. I’m wondering about the much smaller 15A capacity in the power transmission column: should I be worried about that enormous spread? Is 10AWG wire enough for this setup?
posted by migurski to Home & Garden (8 answers total)

Best answer: The power transmission column is for when many wires are bundled together. In that case, each individual wire has lower ability to dissipate heat to the ambient environment, since it's surrounded by other wires. Open-air / chassis-wiring is for when you have few/one wire that is entirely surrounded by air and able to dissipate heat easily. Hence, it allows for significantly higher current conduction.

All that said, as a general comment, you'll rarely do wrong by using a lower gauge/thicker wire than is required.
posted by saeculorum at 12:05 PM on December 16, 2024 [1 favorite]

As another general comment, if you're cost-constrained (given this is a hobby project), it'll often be cheaper and easier to manage multiple conductors of a higher gauge/narrower wire than a single conductor of a lower gauge/thicker wire. For instance, 2 conductors of 12 gauge wire would be rated for 82A in a open-air chassis configuration, and would likely be cheaper and easier to bend/route than 1 conductor of 10 gauge wire.
posted by saeculorum at 12:12 PM on December 16, 2024 [1 favorite]

You might put some thought into how the panels are arranged in series or parallel. I have 20 100W panels for a total of 2kW, and I have them wired such that they produce ~10 amps at 210 volts. My charger is a Victron MPPT rated for 250 Volts, 60 Amps.

I used 10awg.
posted by Horkus at 1:42 PM on December 16, 2024

Malibu sells 10 awg zip wire that is outdoor/UV rated.
posted by hortense at 2:30 PM on December 16, 2024

Response by poster: Re: panel arrangement, I’m planning to wire the three 200W panels in series so they supply 36V; my understanding is the MPPT controller would convert this to 12V for the battery capped at its documented 40A output, so realistically I’d be seeing 480W from the three panels under optimal conditions.
posted by migurski at 2:30 PM on December 16, 2024

Best answer: You could use the NEC rules, which are thought to be very very conservative (in this case, conservatism means "don't burn down the garage" )

Doing that, you see that for 40A you probably want 8 or 6 gauge : link

Another thing to think about is future expansion - solar power is kind of addictive, so you will probably want more power later. The choice of 6, 8, or 10 gauge is usually pennies per foot, so might as well go bigger now?

True, thicker wires (lower gauge) are more expensive and a little more difficult to bend, but you won't likely notice an appreciable difference between 6 to 10 gauge on a short run.

Some MPPT controllers have a maximum gauge, so definitely read the specs.

Also, terminating wires is actually difficult to get right, so consider buying pre-terminated wires when possible. A site like Powerwerx.com can do these.
posted by soylent00FF00 at 5:54 PM on December 16, 2024 [1 favorite]

I see now that you're asking about the span from the controller to the battery. My apologies.

I don't know what your bus bar / circuit breaker / current paths look like, but my instincts tell me that 10awg is a bit small, I might go with 6. What wire gauge are you using between the batteries and the load? Surely not the same 10awg, right?

(Also, since batteries charge at higher than their nominal voltage, you'll probably get significantly more than 480 watts out of that arrangement. I bet you'll be pleasantly surprised.)
posted by Horkus at 7:42 PM on December 16, 2024 [1 favorite]

Response by poster: Powerwerx is a great resource, thanks for that! I’ve been finding pre-made cables online but this is the first place I’ve seen a way to configure every parameter instead of hoping for something to be in stock. I’m unsure of the gauge between the battery and load, but it’s a pair of conductors that came with the inverter and they are wide.

My controller supports a gauge of 8AWG. Based on all the excellent advice here it sounds like 10 might be sufficient given the open chassis design while 8 is safer, more future-proof, and the max I can do in any case.

Also, thanks for that hint about charging voltage, I see that it actually gets up near 14.6V which brings the charging wattage right near 600W.
posted by migurski at 9:52 PM on December 16, 2024 [2 favorites]