Wire size for 40 A over 15 ft on a 48 V system

Smallest wire that works

8 AWGat 1.94% drop

40 A over 15 ft one-way on a 48 V system

Circuit power40 A × 48 V	1,920W
Conductor length15 ft out and 15 ft back	30ft
Ampacity alone would allow1.94% drop — legal on heat, and fine on drop	8 AWG
Power lost in the cable1.95% of what you are sending	37.3W
— Voltage drop by size —
14 AWGnot rated for 40 A — 15 A max	—
12 AWGnot rated for 40 A — 20 A max	—
10 AWGnot rated for 40 A — 30 A max	—
8 AWG50 A rated	1.94%0.93 V
6 AWG65 A rated	1.23%0.59 V
4 AWG85 A rated	0.77%0.37 V
2 AWG115 A rated	0.48%0.23 V
1/0 AWG150 A rated	0.3%0.15 V
2/0 AWG175 A rated	0.24%0.12 V
4/0 AWG230 A rated	0.15%0.07 V

Current: 40 A
One-way distance: 15 ft
System voltage: 48 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 40 A over 15 ft at 48 V, ampacity alone would allow 8 AWG. Voltage drop says 8 AWG — 1.94%, or 0.93 V of the 48 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 30 ft, not 15 ft. Forgetting the return leg is how people arrive at exactly half the real drop.

Why it matters more than it sounds

1,920 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

At this length ampacity and voltage drop point at the same wire, which is unusual. Runs over about 20 ft at low DC voltage are normally decided by drop, not by heat.
3% is the NEC informational recommendation for a branch circuit, not a hard rule. On a 48 V DC system it is already generous: 1.44 V of the 48 V you started with.
Running at 48 V is doing a lot of work here. The same 1,920 W at 12 V would draw 160 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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