Wire size for 100 A over 20 ft on a 24 V system
| Circuit power100 A × 24 V | 2,400W |
|---|---|
| Conductor length20 ft out and 20 ft back | 40ft |
| Ampacity alone would allow3.23% drop — legal on heat, not acceptable on drop | 2 AWG |
| Power lost in the cable2.03% of what you are sending | 48.8W |
| — Voltage drop by size — | |
| 14 AWGnot rated for 100 A — 15 A max | — |
| 12 AWGnot rated for 100 A — 20 A max | — |
| 10 AWGnot rated for 100 A — 30 A max | — |
| 8 AWGnot rated for 100 A — 50 A max | — |
| 6 AWGnot rated for 100 A — 65 A max | — |
| 4 AWGnot rated for 100 A — 85 A max | — |
| 2 AWG115 A rated, drop too high | 3.23%0.78 V |
| 1/0 AWG150 A rated | 2.03%0.49 V |
| 2/0 AWG175 A rated | 1.61%0.39 V |
| 4/0 AWG230 A rated | 1.01%0.24 V |
- Current
- 100 A
- One-way distance
- 20 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 100 A over 20 ft at 24 V, ampacity alone would allow 2 AWG. Voltage drop says 1/0 AWG — 2.03%, or 0.49 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 40 ft, not 20 ft. Forgetting the return leg is how people arrive at exactly half the real drop.
Why it matters more than it sounds
2,400 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. 2 AWG carries 100 A without overheating, but it drops 3.23% 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 2,400 W at 12 V would draw 200 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 80 A over 20 ft on a 24 V system
- Wire size for 125 A over 20 ft on a 24 V system
- Wire size for 70 A over 20 ft on a 24 V system
- Wire size for 150 A over 20 ft on a 24 V system
- Wire size for 100 A over 15 ft on a 24 V system
- Wire size for 100 A over 25 ft on a 24 V system
- Wire size for 100 A over 10 ft on a 24 V system
- Wire size for 100 A over 30 ft on a 24 V system
- Wire size for 100 A over 20 ft on a 12 V system
- Wire size for 100 A over 20 ft on a 48 V system
Read more
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