In the effort of saving money without having to buy expensive solar cables (and because I have a lot of extra coax cable lying around doing nothing), I would like to use a piece of coax cable (that looks like antenna cable), to transfer 54V and up to 5A of solar power into the house. The length would be no more than about 20 feet and can be made even shorter. This is a very small solar application to charge a small 48V battery bank. If I do not know the type of cable (such as RG-6) so that I cannot just look up the specs on it, generally speaking, is it fairly safe to assume just about ANY piece of coax normally used for cable TV or antennas can handle 54V and 5A? I think it is reasonable. Note that I would not use the end "F" connectors, I would use stripped ends to attach to the solar panel cables and somehow weatherproof them, and on the charger (indoor) side, also stripped ends.

I should mention that the near 5A flow would only be in certain conditions (like when the battery bank is in a mid state of charge and there is full sun directly over the panels). I would say on average, the current flow should be more like 2.5A to 3A.

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    Most cable has markings on the outside. Are you sure yours doesn't? Do you know what it was used for previously? It would be very helpful to determine the wire gauge of the center conductor. That could also be done by measuring it with calipers if you have some. Also, is it solid or stranded? – mkeith Dec 6 at 5:55
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    Signal cables are not meant to carry significant power, period. You need 250W, TV cables typically do like 2.5W (to power an amplifier). – Agent_L Dec 6 at 15:39
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    Twenty feet of the correct, safe wire would cost you about $20. Why are you even considering doing it wrong and risking an electrical fire when doing it right is a lot cheaper than dinner and a movie? – Eric Lippert Dec 6 at 17:26
up vote 20 down vote accepted

Your voltage is high enough that the electrical codes require you to use mains type power cable. This is made in mass quantity, and so, is cheap. However there is no need for silly "solar wire". They already make mains rated electrical wire for use outside.

14 AWG outdoor rated cable such as UF-B should suffice. Do not use NM.

You can also use common THWN-2 individual wires in Conduit, either EMT or outdoor rated electrical PVC. The PVC conduit will start to degrade from UV and turn white after a year or two in the sun. At that point, give it a quickie swiff sand, and paint it with Rustoleum white metal primer then a topcoat of your choice.

If you want to give yourself headroom for future expansion, use 12 AWG or 10 AWG.

  • Why not just use outdoor-rated armored cable? That 100-foot roll will incur a voltage drop of only 1.59V across its entire length at 5A. If you want overkill, use 14/3 and pair up your conductors instead; red+white = positive, black+green = negative. Voltage drop on that is 1.26V @ 5A across 100ft. – Doktor J Dec 6 at 18:09
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    Sure, any mains rated conductors would do the job. I don't feel confident enough on solar codes to advise paralleling or using safety earthing as a current return, both of which would be super verboten in AC mains. I say that because in most /3 cable, earth is bare and not insulated. – Harper Dec 6 at 19:07
  • in the armored cable I linked, earth is actually green-insulated, so you don't have to worry about the armor being "live" (though I'd suggest tying it to ground anyways, for a number of reasons). Apparently NFPA was actually updated so lower-size conductors are no longer permitted to be paralleled, so the 14/3 idea is out. Of course since they're literally in the same armored sheath and the wires and insulation are identical, there's nothing electrically wrong with paralleling them so long as the ends are tied together to make it obvious. But NFPA :) – Doktor J Dec 6 at 22:22
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    In my experience NFPA does nothing without a reason. Here‘s one: overcurrent protection. Paralleling is presumed to be for more ampacity. A single wire failure overloads the other wire. Unless you fuse every wire, but now you're fusing neutral; that has issues... So you need common trip with the hots. Bare minimum a 4 pole breaker! Now your idea still works if you can rearrange into two series strings of panels. – Harper Dec 6 at 23:11
  • Eh, since this is DC, a cheap fuse block with a fuse on each wire should be fine. Use automotive maxi fuses, and slap in the ones that glow when blown, and you've got a pretty sweet setup. – Doktor J Dec 7 at 3:17

Almost any coax or video cable center core would be able to carry 5A.

RG6 for example has an 18 AWG core, capable easily of in excess of 5A as a single cable. See the AWG current carrying chart here.

Notice in the chart that an 18AWG wire could carry up to 9.5A ...BUT.... you must realize that any temperature increase in the coax would be bad. The core is NOT protected by the normal PVC coating (good to over 100 degC), but using a soft low loss material. This is not as good at high temperatures and would deform easily.

I'd suggest if you are using the coax just because you have it readily at hand, you could use two coax runs and just use the outer shield as the conductor. This would have a much greater current rating. I would suggest the outer shield could probably carry 10-15A without any problems.

You could even connect the center core and the shield together and use as a single conductor ...this would easily meet your needs with two cables.

Update:
There's a lot of misinformation in the comments specifically about RG6 and RG59

Here is a link to a reasonable supplier of cable comparing the two. Notice that both have relatively heavy shields, easily capable of the current requirement.

Here is a link to a short form datasheet for RG59, RG6 and RG11

The screen is copper and looking at the Ohm/km it is easily classified at around 14-16AWG current capability for both RG59 and RG6.

  • Also to consider - coax, by its nature has a very thick insulation layer, which will also act as thermal insulation. I would be very leery of using almost any common coax for this purpose, especially if it has any bends. Especially, consider that standard RG-6 does not have a copper center conductor: Normally the center is copper plated steel. Given the greater resistance of steel I'd be even less enthusiastic. – WhatRoughBeast Dec 6 at 4:59
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    I've seen copper plated steel in larger coax....but never in the small stuff. Still I offered what I think is the right alternative ….use two wires and only use the shield. – Jack Creasey Dec 6 at 5:38
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    The cable is probably RG59, not RG6, unless it came from something other than home installs. So this probably does not apply. And it sounds like a bad idea anyway. – Chris Stratton Dec 6 at 14:32
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    @aconcernedcitizen In cheap antenna cables, the shield is alu foil with just few copper strands added for continuity, so I disagree with the statement that shield must have better current carrying capability than the core. I'm betting that OP doesn't have the TV cable at hand, they're just cargo-culting coaxial "solar cable" – Agent_L Dec 6 at 15:48
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    @Mazura Of course you would not expect the specs for these cable to talk about voltage or current rating, they are designed for RF applications. – Jack Creasey Dec 6 at 23:40

Despite the "try it" suggestion above, I would say do not use coax cable for carrying 5A. The outer braid and centre conductor are optimised for carrying RF signals on the outer surfaces of the wire strands, and the strands are too thin to support that kind of current, even if the centre one looks thick: look at any 5A lighting cable.

Check the voltage from the solar panels (I think the ones I have are around 70V) - it will probably be well within the insulation rating of standard mains cable (120V or 240V).

Choose main cable that gives a margin of error, say 20%, over the rated maximum current output from the solar panels, rather than one giving the exact rating.

You want cable that loses least due to resistance as well, so consider that higher-rated cable most probably has a lower end-to-end resistance for a given length, and hence will be more efficient.

Ensure that contacts at each end are low-resistance, too.

I strongly suggest you consult either with a local qualified electrical tradesman, or at least check websites about wiring and safety - which includes fire risks.

eg for UK standards: https://www.diydoctor.org.uk/projects/cablesizes.htm

Most coax has a type or part number laser-markes onto the outer jacket at periodic intervals. If not, measure the diameter of the inner conductor and convert to an equivalent AWG. You could also pass 5A through a sample piece in an environment representative of your worst-case thermal situation (e.g. when it's going through a thermally insulating feedthrough) for ~30 mins and see if there's an appreciable temperature rise.

No, you can "not" assume that any "random" coaxial cable can "reliably" carry 5A. Determine what kind of cable you have, then look up its specifications.

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    What then would be a "safe" way to determine if some unmarked piece of coax cable can handle a continuous (many hours) of 5A load? Is there some industry standard test or should I just try it and feel the cable with my fingers every few minutes? 5A is really not that much. For example, I have seen very tiny wires used on some multi-testers yet they are rated at up to 5A (since there is a 5A setting on the tester). The conductors in an "average" piece of "TV" cable seem significantly thicker than those. – David Dec 6 at 3:07
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    @David if you don't want to void your home insurance and potentially be terribly poor ig something bad happens, then you don't use random unmarked coax. You use something (provably) code compliant. Modern electrical codes were mostly written after a bunch of people died or a lot of money burned to a crisp, so you should usually listen to them. – mbrig Dec 6 at 6:56
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    Coax is not made for current-carrying, it's made for low RF loss. – MikeW Dec 6 at 14:42

I think the 1/4 inch (or so) coax cables (RG59, RG6 etc.) would have too small a center conductor to safely carry 5 Amp, The 0.4 inch cables (RG8 or RG11) should be OK for 5 Amp.

Keep in mind that you could have excessive voltage drop, especially in the center conductor. There is usually a very thin layer of copper (less than .002" thick) over the high-resistance steel core. Buy the 20' of cheap UF cable as suggested.

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    Yes after researching this more that is exactly what I will do. I am using older panels which have MC3 connectors so I can simply get 20 foot extensions for those and be good to go. It was however an interesting experience posting this question to see all the answers. – David Dec 7 at 18:05
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    @David, please accept one of the answers OR write your own and accept it. From a "book-keeping" standpoint it is nice to have questions be answered (officially). – mkeith Dec 7 at 22:28

My solution would be just to try it, and check for heat using my fingers. If all is well, then I am done. If I feel excessive heat then perhaps I can run a second cable and twist both conductors together on each cable, effectively making each become a wire instead, and increasing current carrying capability. From a casual observer, it/they will appear to be coax antenna cable(s). Another advantage of twisting the outer jacket with the center conductor is it may better match the hole in the solar panel connector and will provide a better (tighter) fit, which can then be weather protected by various means (silicone rubber, electrical tape, weather stripping...). I think it will be fun to try.

Another way to test this is to take a laboratory power supply unit (lab PSU for short), and run 5A thru the cable, feeling it often for excessive heat.

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    This is a comment not an answer, add it as a comment to your question or better: update the question. – Bimpelrekkie Dec 6 at 9:29
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    @Bimpelrekkie I disagree. It is a possible answer, and as such, even if provided by the OP, perfectly valid. If the answer is good or bad, however, is another question. – glglgl Dec 6 at 13:41
  • No. If you can feel heat, it's because of severe voltage drop. – Mike Waters Dec 7 at 0:12
  • I am surprised that nobody mentioned fitment to various different solar connectors which is important. 5A is not much current at all and I measured the panels today in the weaker winter sun and it was only about 3.5A. Voltage drop is not really an issue cuz solar panels are already way above the battery bank voltage they are rated for. For example, a 24V panel outputting about 36V (about 50% over the nominal bank voltage). So who cares if it drops 1 or 2 volts down to 34-35V? – David Dec 7 at 0:32
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    Amazing how I clearly said 20 feet or shorter in the original question and people are talking about 100 foot runs and low resistance in their answers. It is like they are trying to answer some other question that was not asked. – David Dec 7 at 10:45

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