Wire size for 125 A over 300 ft on a 12 V system
| Circuit power125 A × 12 V | 1,500W |
|---|---|
| Conductor length300 ft out and 300 ft back | 600ft |
| Ampacity alone would allow76.25% drop — legal on heat, not acceptable on drop | 1/0 AWG |
| — Voltage drop by size — | |
| 14 AWGnot rated for 125 A — 15 A max | — |
| 12 AWGnot rated for 125 A — 20 A max | — |
| 10 AWGnot rated for 125 A — 30 A max | — |
| 8 AWGnot rated for 125 A — 50 A max | — |
| 6 AWGnot rated for 125 A — 65 A max | — |
| 4 AWGnot rated for 125 A — 85 A max | — |
| 2 AWGnot rated for 125 A — 115 A max | — |
| 1/0 AWG150 A rated, drop too high | 76.25%9.15 V |
| 2/0 AWG175 A rated, drop too high | 60.44%7.25 V |
| 4/0 AWG230 A rated, drop too high | 38%4.56 V |
- Current
- 125 A
- One-way distance
- 300 ft
- System voltage
- 12 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 125 A over 300 ft at 12 V, ampacity alone would allow 1/0 AWG. Voltage drop says larger than 4/0 — %, or V of the 12 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
1,500 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 12 V DC system it is already generous: 0.36 V of the 12 V you started with.
- 12 V is what makes this expensive. The same 1,500 W at 24 V halves the current and quarters the loss — the cheapest way out of a big cable bill is almost always a higher system voltage.
- 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 100 A over 300 ft on a 12 V system
- Wire size for 150 A over 300 ft on a 12 V system
- Wire size for 80 A over 300 ft on a 12 V system
- Wire size for 175 A over 300 ft on a 12 V system
- Wire size for 125 A over 250 ft on a 12 V system
- Wire size for 125 A over 200 ft on a 12 V system
- Wire size for 125 A over 300 ft on a 24 V system
- Wire size for 125 A over 300 ft on a 48 V system
Read more
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