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DC Wire Sizing

Wire size for low-voltage DC runs, decided by voltage drop rather than ampacity — which is the answer that actually matters once the run is longer than a few feet.

Wire size has two answers, and the bigger one wins.

Ampacity asks whether the conductor gets hot enough to be dangerous. Voltage drop asks whether enough of the power arrives at the far end. On a low-voltage DC run these almost never agree, and only one of them is a code requirement.

At 40 A over 50 ft on a 12 V system, ampacity alone allows 8 AWG. Voltage drop says 4/0. That is not a small difference in cable cost, and picking the wrong one is why so many solar installations underperform quietly.

The distance doubles

Current goes out on one conductor and comes back on the other. A 50 ft run is 100 ft of copper. Forgetting the return leg gives exactly half the real drop, and it is the single most common arithmetic error in this calculation.

Why it matters more than a few percent

On a solar charge controller, undersized cable means the battery never sees absorption voltage. It never finishes a charge cycle, sulfates, and dies early — while every meter in the system reads normal. The failure is invisible until the bank is finished.

Three percent is a recommendation, not a rule

NEC 210.19 informational note 4 suggests 3% for a branch circuit. On a 12 V system that is 0.36 volts, which is already generous rather than strict. Nothing in the code stops you exceeding it; physics just charges you for it.

What the tables do not include

Figures here are copper at 75 °C. Aluminum conducts about 60% as well — go up two sizes. Conduit fill, ambient temperature above 30 °C, and bundled conductors all derate ampacity further, and none of that is in the numbers.

The cheapest fix for an expensive cable run is almost always a higher system voltage.

Wire size for 50 A over 50 ft on a 24 V system

Smallest wire that works
1/0 AWGat 2.54% drop
50 A over 50 ft one-way on a 24 V system
Voltage drop at 50 A over 50 ft86421/02.5%2/02.0%4/01.3%3%1/0 AWG is the smallest that stays under 3%
Circuit power50 A × 24 V1,200W
Conductor length50 ft out and 50 ft back100ft
Ampacity alone would allow16.21% drop — legal on heat, not acceptable on drop8 AWG
Power lost in the cable2.54% of what you are sending30.5W
— Voltage drop by size —
14 AWGnot rated for 50 A — 15 A max—
12 AWGnot rated for 50 A — 20 A max—
10 AWGnot rated for 50 A — 30 A max—
8 AWG50 A rated, drop too high16.21%3.89 V
6 AWG65 A rated, drop too high10.23%2.46 V
4 AWG85 A rated, drop too high6.42%1.54 V
2 AWG115 A rated, drop too high4.04%0.97 V
1/0 AWG150 A rated2.54%0.61 V
2/0 AWG175 A rated2.01%0.48 V
4/0 AWG230 A rated1.27%0.3 V

Notes

  • Two different answers, and the bigger one wins. 8 AWG carries 50 A without overheating, but it drops 16.21% over this distance. You need 1/0 AWG — sized by voltage drop, not by heat.
  • 3% is the NEC informational recommendation for a branch circuit, not a hard rule. On a 24 V DC system it is already generous: 0.72 V of the 24 V you started with.
  • Running at 24 V is doing a lot of work here. The same 1,200 W at 12 V would draw 100 A and need a far heavier cable.
  • Figures are copper at 75 °C. Aluminium runs about 60% of copper conductivity — go up two sizes. Conduit fill, ambient temperature above 30 °C and bundling all derate ampacity further, and none of that is in this table.

DC wire size and voltage drop

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