40 Amp Wire Size at 400 Feet (12V DC (Solar / Automotive / Marine))
Complete conductor sizing, Ohm's law voltage drop, $I^2R$ line power loss, and circuit breaker specification for a 40 A load over a 400 ft (121.9 m) one-way run.
Copper Wire (≤3% Drop)2× 1000 kcmil (Parallel)506.7 mm² · Drop: 1.72%
Aluminum Wire (≤3% Drop)1000 kcmilDrop: 0.68V (5.66%)
Voltage at Load End11.8 VLoss: 0.2 V (8.2 W)
Power & Breaker (125%)0.48 kW50A Breaker Recommended
All AWG & kcmil Wire Gauges for 40 Amps at 400 Feet (12V)
| Wire Gauge | Metric Area | NEC 75°C Cu Ampacity | Copper Voltage Drop | Load Voltage | Aluminum Drop | Line Loss ($I^2R$) | NEC Status |
|---|---|---|---|---|---|---|---|
| 14 AWG | 2.08 mm² | 15 A | 100.35 V (836.25%) | 0.0 V | 165.53 V (1379.42%) | 4014.0 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 63.06 V (525.5%) | 0.0 V | 104.02 V (866.82%) | 2522.4 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 39.68 V (330.68%) | 0.0 V | 65.46 V (545.47%) | 1587.3 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 24.94 V (207.81%) | 0.0 V | 41.14 V (342.79%) | 997.5 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 15.69 V (130.78%) | 0.0 V | 25.89 V (215.73%) | 627.8 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 9.87 V (82.24%) | 2.1 V | 16.28 V (135.66%) | 394.8 W | Drop/Amp High |
| 3 AWG | 26.67 mm² | 100 A | 7.83 V (65.22%) | 4.2 V | 12.91 V (107.58%) | 313.1 W | Drop/Amp High |
| 2 AWG | 33.62 mm² | 115 A | 6.21 V (51.74%) | 5.8 V | 10.24 V (85.34%) | 248.3 W | Drop/Amp High |
| 1 AWG | 42.41 mm² | 130 A | 4.92 V (41.01%) | 7.1 V | 8.12 V (67.65%) | 196.9 W | Drop/Amp High |
| 1/0 AWG | 53.49 mm² | 150 A | 3.9 V (32.52%) | 8.1 V | 6.44 V (53.64%) | 156.1 W | Drop/Amp High |
| 2/0 AWG | 67.43 mm² | 175 A | 3.1 V (25.8%) | 8.9 V | 5.11 V (42.55%) | 123.8 W | Drop/Amp High |
| 3/0 AWG | 85.01 mm² | 200 A | 2.46 V (20.46%) | 9.5 V | 4.05 V (33.75%) | 98.2 W | Drop/Amp High |
| 4/0 AWG | 107.2 mm² | 230 A | 1.95 V (16.23%) | 10.1 V | 3.21 V (26.76%) | 77.9 W | Drop/Amp High |
| 250 kcmil | 126.7 mm² | 255 A | 1.65 V (13.73%) | 10.4 V | 2.72 V (22.65%) | 65.9 W | Drop/Amp High |
| 300 kcmil | 152.0 mm² | 285 A | 1.37 V (11.44%) | 10.6 V | 2.27 V (18.88%) | 54.9 W | Drop/Amp High |
| 350 kcmil | 177.3 mm² | 310 A | 1.18 V (9.81%) | 10.8 V | 1.94 V (16.18%) | 47.1 W | Drop/Amp High |
| 400 kcmil | 202.7 mm² | 335 A | 1.03 V (8.58%) | 11.0 V | 1.7 V (14.15%) | 41.2 W | Drop/Amp High |
| 500 kcmil | 253.4 mm² | 380 A | 0.82 V (6.86%) | 11.2 V | 1.36 V (11.32%) | 32.9 W | Drop/Amp High |
| 600 kcmil | 304.0 mm² | 420 A | 0.69 V (5.72%) | 11.3 V | 1.13 V (9.44%) | 27.5 W | Drop/Amp High |
| 750 kcmil | 380.0 mm² | 475 A | 0.55 V (4.58%) | 11.5 V | 0.91 V (7.55%) | 22.0 W | ~ ≤5% |
| 1000 kcmil | 506.7 mm² | 545 A | 0.41 V (3.43%) | 11.6 V | 0.68 V (5.66%) | 16.5 W | ~ ≤5% |
Frequently Asked Engineering Questions
What size copper wire do I need for 40 amps at 400 feet (12V)?
To stay within the NEC recommended 3% maximum voltage drop for a 40A load over 400 feet one-way at 12V (DC), you need 2× 1000 kcmil (Parallel) copper wire. This produces a voltage drop of 0.2V (1.72%) and delivers 11.8V at the load.
What size aluminum wire is required for 40 amps at 400 feet?
Using aluminum conductors (75°C rating), you should use 1000 kcmil to maintain voltage drop at 0.68V (5.66%).
What breaker size is needed for a 40A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (40A × 1.25 = 50.0A), which corresponds to a standard 50A circuit breaker.
Compare Other Distances & Amperages
40 Amp Wire Size by Distance (12V)
40A at 20 ft (12V)40A at 30 ft (12V)40A at 40 ft (12V)40A at 50 ft (12V)40A at 60 ft (12V)40A at 75 ft (12V)40A at 80 ft (12V)40A at 100 ft (12V)40A at 120 ft (12V)40A at 125 ft (12V)40A at 150 ft (12V)40A at 175 ft (12V)