Wire size for 15 A over 250 ft on a 48 V system
| Circuit power15 A × 48 V | 720W |
|---|---|
| Conductor length250 ft out and 250 ft back | 500ft |
| Ampacity alone would allow49.06% drop — legal on heat, not acceptable on drop | 14 AWG |
| Power lost in the cable1.91% of what you are sending | 13.7W |
| — Voltage drop by size — | |
| 14 AWG15 A rated, drop too high | 49.06%23.55 V |
| 12 AWG20 A rated, drop too high | 30.94%14.85 V |
| 10 AWG30 A rated, drop too high | 19.38%9.3 V |
| 8 AWG50 A rated, drop too high | 12.16%5.83 V |
| 6 AWG65 A rated, drop too high | 7.67%3.68 V |
| 4 AWG85 A rated, drop too high | 4.81%2.31 V |
| 2 AWG115 A rated, drop too high | 3.03%1.46 V |
| 1/0 AWG150 A rated | 1.91%0.92 V |
| 2/0 AWG175 A rated | 1.51%0.73 V |
| 4/0 AWG230 A rated | 0.95%0.46 V |
- Current
- 15 A
- One-way distance
- 250 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 15 A over 250 ft at 48 V, ampacity alone would allow 14 AWG. Voltage drop says 1/0 AWG — 1.91%, or 0.92 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 500 ft, not 250 ft. Forgetting the return leg is how people arrive at exactly half the real drop.
Why it matters more than it sounds
720 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 15 A without overheating, but it drops 49.06% 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 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 720 W at 12 V would draw 60 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
- Wire size for 10 A over 250 ft on a 48 V system
- Wire size for 20 A over 250 ft on a 48 V system
- Wire size for 5 A over 250 ft on a 48 V system
- Wire size for 25 A over 250 ft on a 48 V system
- Wire size for 15 A over 200 ft on a 48 V system
- Wire size for 15 A over 300 ft on a 48 V system
- Wire size for 15 A over 150 ft on a 48 V system
- Wire size for 15 A over 250 ft on a 24 V system
- Wire size for 15 A over 250 ft on a 12 V system
Read more
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