125 Amp Wire Size at 40 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 125 A load over a 40 ft (12.2 m) one-way run.
Copper Wire (≤3% Drop)400 kcmil202.7 mm² · Drop: 2.68%
Aluminum Wire (≤3% Drop)600 kcmilDrop: 0.35V (2.95%)
Voltage at Load End11.7 VLoss: 0.32 V (40.2 W)
Power & Breaker (125%)1.5 kW160A Breaker Recommended
All AWG & kcmil Wire Gauges for 125 Amps at 40 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 | 31.36 V (261.33%) | 0.0 V | 51.73 V (431.07%) | 3919.9 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 19.71 V (164.22%) | 0.0 V | 32.51 V (270.88%) | 2463.3 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 12.4 V (103.34%) | 0.0 V | 20.46 V (170.46%) | 1550.1 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 7.79 V (64.94%) | 4.2 V | 12.85 V (107.12%) | 974.1 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 4.9 V (40.87%) | 7.1 V | 8.09 V (67.42%) | 613.0 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 3.08 V (25.7%) | 8.9 V | 5.09 V (42.39%) | 385.5 W | Drop/Amp High |
| 3 AWG | 26.67 mm² | 100 A | 2.45 V (20.38%) | 9.6 V | 4.03 V (33.62%) | 305.7 W | Drop/Amp High |
| 2 AWG | 33.62 mm² | 115 A | 1.94 V (16.17%) | 10.1 V | 3.2 V (26.67%) | 242.5 W | Drop/Amp High |
| 1 AWG | 42.41 mm² | 130 A | 1.54 V (12.82%) | 10.5 V | 2.54 V (21.14%) | 192.3 W | Drop/Amp High |
| 1/0 AWG | 53.49 mm² | 150 A | 1.22 V (10.16%) | 10.8 V | 2.01 V (16.76%) | 152.4 W | Drop/Amp High |
| 2/0 AWG | 67.43 mm² | 175 A | 0.97 V (8.06%) | 11.0 V | 1.6 V (13.3%) | 120.9 W | Drop/Amp High |
| 3/0 AWG | 85.01 mm² | 200 A | 0.77 V (6.39%) | 11.2 V | 1.27 V (10.55%) | 95.9 W | Drop/Amp High |
| 4/0 AWG | 107.2 mm² | 230 A | 0.61 V (5.07%) | 11.4 V | 1.0 V (8.36%) | 76.1 W | Drop/Amp High |
| 250 kcmil | 126.7 mm² | 255 A | 0.51 V (4.29%) | 11.5 V | 0.85 V (7.08%) | 64.4 W | ~ ≤5% |
| 300 kcmil | 152.0 mm² | 285 A | 0.43 V (3.58%) | 11.6 V | 0.71 V (5.9%) | 53.6 W | ~ ≤5% |
| 350 kcmil | 177.3 mm² | 310 A | 0.37 V (3.07%) | 11.6 V | 0.61 V (5.06%) | 46.0 W | ~ ≤5% |
| 400 kcmil | 202.7 mm² | 335 A | 0.32 V (2.68%) | 11.7 V | 0.53 V (4.42%) | 40.2 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.26 V (2.15%) | 11.7 V | 0.42 V (3.54%) | 32.2 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.21 V (1.79%) | 11.8 V | 0.35 V (2.95%) | 26.8 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.17 V (1.43%) | 11.8 V | 0.28 V (2.36%) | 21.5 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.13 V (1.07%) | 11.9 V | 0.21 V (1.77%) | 16.1 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 125 amps at 40 feet (12V)?
To stay within the NEC recommended 3% maximum voltage drop for a 125A load over 40 feet one-way at 12V (DC), you need 400 kcmil copper wire. This produces a voltage drop of 0.32V (2.68%) and delivers 11.7V at the load.
What size aluminum wire is required for 125 amps at 40 feet?
Using aluminum conductors (75°C rating), you should use 600 kcmil to maintain voltage drop at 0.35V (2.95%).
What breaker size is needed for a 125A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (125A × 1.25 = 156.2A), which corresponds to a standard 160A circuit breaker.
Compare Other Distances & Amperages
125 Amp Wire Size by Distance (12V)
125A at 20 ft (12V)125A at 30 ft (12V)125A at 50 ft (12V)125A at 60 ft (12V)125A at 75 ft (12V)125A at 80 ft (12V)125A at 100 ft (12V)125A at 120 ft (12V)125A at 125 ft (12V)125A at 150 ft (12V)125A at 175 ft (12V)