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Wire size for 5 A over 300 ft on a 24 V system

Smallest wire that works
2 AWGat 2.42% drop
5 A over 300 ft one-way on a 24 V system
Circuit power5 A × 24 V120W
Conductor length300 ft out and 300 ft back600ft
Ampacity alone would allow39.25% drop — legal on heat, not acceptable on drop14 AWG
Power lost in the cable2.42% of what you are sending2.9W
— Voltage drop by size —
14 AWG15 A rated, drop too high39.25%9.42 V
12 AWG20 A rated, drop too high24.75%5.94 V
10 AWG30 A rated, drop too high15.5%3.72 V
8 AWG50 A rated, drop too high9.72%2.33 V
6 AWG65 A rated, drop too high6.14%1.47 V
4 AWG85 A rated, drop too high3.85%0.92 V
2 AWG115 A rated2.42%0.58 V
1/0 AWG150 A rated1.52%0.37 V
2/0 AWG175 A rated1.21%0.29 V
4/0 AWG230 A rated0.76%0.18 V
Current
5 A
One-way distance
300 ft
System voltage
24 V

Ampacity and voltage drop are two different questions

Ampacity asks whether the wire gets hot enough to be dangerous. Voltage drop asks whether enough of the power arrives at the other end. On a low-voltage DC run they almost never give the same answer, and the bigger one wins.

For 5 A over 300 ft at 24 V, ampacity alone would allow 14 AWG. Voltage drop says 2 AWG — 2.42%, or 0.58 V of the 24 V you started with.

Why the distance doubles

Current goes out on one conductor and comes back on the other. The copper in the circuit is 600 ft, not 300 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

120 W leaves the source. Undersized cable does not just waste a few percent — on a solar charge controller it means the battery never sees absorption voltage, never finishes a charge cycle, and sulphates its way to an early death while every meter in the system reads normal.

What is not in the table

Copper at 75 °C. Aluminium conducts about 60% as well — go up two sizes. Conduit fill, ambient temperature above 30 °C and bundled conductors all derate ampacity further.

Notes

  • Two different answers, and the bigger one wins. 14 AWG carries 5 A without overheating, but it drops 39.25% over this distance. You need 2 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 120 W at 12 V would draw 10 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.

Nearby sizes

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