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.
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.
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.
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.
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.
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.
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.