1

Is it safe to push 6 amps at 12v over 100ft of 18 gauge wire? If not, what’s the max amperage for 100 feet of 18 gauge wire at 12v

4
  • 1
    Per the answer, does it need to be 18 gauge? Larger gauge would be better, 16 or even 14 gauge.
    – crip659
    Aug 24, 2023 at 10:46
  • 1
    This calculator is quite handy; this tab goes to smaller gauges including 18 AWG. calculator.net/…
    – Ecnerwal
    Aug 24, 2023 at 11:25
  • If the wire is too thin and long it will add resistance which will reduce both voltage and current at the far end. If the device at the far end is safety-critical and expected to work, that would be unsafe. But otherwise, the wire's current rating is independent of its length.
    – jay613
    Aug 24, 2023 at 12:14
  • Do you mean safe for the equipment being powered, or safe as in life and fire safety, or safe in some other sense?
    – Greg Hill
    Aug 24, 2023 at 13:58

3 Answers 3

6

All that really matters, concerning wire diameter, is the current; the insulation determines voltage [well, skin effect too, which can be ignored in thin wire at mains frequencies].

18 AWG (American Wire Gauge) is rated for 16 amperes for chassis wiring, but for only 2.3 A for power transmission. 100 ft. certainly seems like power transmission, and further, it would have a resistance of ~0.6 ohms for each wire. If you're using twin lead, resistance would be ~1.2 Ω.

Using Ohm's Law, V = I x R, 6 A x 1.2 Ω = 7.2 V. This is the voltage dropped across 100 ft. (~30 m), i.e., though you put 12 V into the wire, only 4.8 V is available at the output. The rest is lost in the wire, wasting ~43 W as heat.

The issue is not so much safety, since that 43 W is distributed over the length of wire, and would not be likely to melt the insulation or start a fire, but that you will not get anywhere near 12 V out the far end.

2

It's safe, it's just not going to work. At all... People get into the mindset of "low voltage takes small diameter wires" no, to go any distance it takes larger. Much larger.

This is the death of many a solar power system.

Just crunching the numbers with 14 AWG Romex wire, I see 27% voltage drop. That's not going to work.

With 12 AWG wire, I see 17% drop, still not viable.

Really the wire is just too big for copper. You'll be up at #6 or #4 XHHW aluminum to really transmit it without severe voltage drop. Use neutral bars intended for subpanels as Al-Cu splices.

If you can step it up to 36V for transmission then step down to 12V at the end, that would be better. Now 3% drop happens at 14 AWG or probably 7% drop at 18 AWG, and the DC/DC can make up for that.

2
  • 1
    "What on earth would make you think that would work?" If there was a thought this would definitely work then there would be no thought to ask. It is the uncertainty that leads people to ask such things.
    – MacGuffin
    Aug 24, 2023 at 21:26
  • @MacGuffin fair enough. Edited Aug 25, 2023 at 6:36
0

If you have 100 feet of 18 AWG wire already in place, and you need 12V at the far end of it: If your thing at the far end always draws 6A @ 12V you can feed the 18AWG wire 19.66V and waste the 7.66V dropped as heat in the wire. If the load sometimes draws less, that may provide too much voltage to the load, though.

This sort of hack depends on the range of acceptable voltage at the load end of the wire, and the variation (or lack thereof) of the load current.

If the the load is "nominal 12V" but will happily take 12-20V, the current can vary from 0-6A without harm to the end device. If the 12V device dies at 15V, then if the load ever draws less than 3.7A it will get more than 15V and die.

If you haven't got wire that size in place, buy bigger wire.

7 Amps is evidently a good maximum value for 18 AWG two-conductor cable (PVC insulated and 30°C environment)

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.