45 Amp Wire Size at 120 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 45 A load over a 120 ft (36.6 m) one-way run.
Copper Wire (≤3% Drop)400 kcmil202.7 mm² · Drop: 2.9%
Aluminum Wire (≤3% Drop)750 kcmilDrop: 0.31V (2.55%)
Voltage at Load End11.7 VLoss: 0.35 V (15.6 W)
Power & Breaker (125%)0.54 kW60A Breaker Recommended
All AWG & kcmil Wire Gauges for 45 Amps at 120 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 | 33.87 V (282.23%) | 0.0 V | 55.87 V (465.55%) | 1524.1 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 21.28 V (177.35%) | 0.0 V | 35.11 V (292.55%) | 957.7 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 13.39 V (111.61%) | 0.0 V | 22.09 V (184.1%) | 602.7 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 8.42 V (70.14%) | 3.6 V | 13.88 V (115.69%) | 378.7 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 5.3 V (44.14%) | 6.7 V | 8.74 V (72.81%) | 238.3 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 3.33 V (27.76%) | 8.7 V | 5.49 V (45.78%) | 149.9 W | Drop/Amp High |
| 3 AWG | 26.67 mm² | 100 A | 2.64 V (22.01%) | 9.4 V | 4.36 V (36.31%) | 118.9 W | Drop/Amp High |
| 2 AWG | 33.62 mm² | 115 A | 2.1 V (17.46%) | 9.9 V | 3.46 V (28.8%) | 94.3 W | Drop/Amp High |
| 1 AWG | 42.41 mm² | 130 A | 1.66 V (13.84%) | 10.3 V | 2.74 V (22.83%) | 74.7 W | Drop/Amp High |
| 1/0 AWG | 53.49 mm² | 150 A | 1.32 V (10.97%) | 10.7 V | 2.17 V (18.1%) | 59.3 W | Drop/Amp High |
| 2/0 AWG | 67.43 mm² | 175 A | 1.04 V (8.71%) | 11.0 V | 1.72 V (14.36%) | 47.0 W | Drop/Amp High |
| 3/0 AWG | 85.01 mm² | 200 A | 0.83 V (6.91%) | 11.2 V | 1.37 V (11.39%) | 37.3 W | Drop/Amp High |
| 4/0 AWG | 107.2 mm² | 230 A | 0.66 V (5.48%) | 11.3 V | 1.08 V (9.03%) | 29.6 W | Drop/Amp High |
| 250 kcmil | 126.7 mm² | 255 A | 0.56 V (4.63%) | 11.4 V | 0.92 V (7.64%) | 25.0 W | ~ ≤5% |
| 300 kcmil | 152.0 mm² | 285 A | 0.46 V (3.86%) | 11.5 V | 0.76 V (6.37%) | 20.9 W | ~ ≤5% |
| 350 kcmil | 177.3 mm² | 310 A | 0.4 V (3.31%) | 11.6 V | 0.66 V (5.46%) | 17.9 W | ~ ≤5% |
| 400 kcmil | 202.7 mm² | 335 A | 0.35 V (2.9%) | 11.7 V | 0.57 V (4.78%) | 15.6 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.28 V (2.32%) | 11.7 V | 0.46 V (3.82%) | 12.5 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.23 V (1.93%) | 11.8 V | 0.38 V (3.19%) | 10.4 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.19 V (1.54%) | 11.8 V | 0.31 V (2.55%) | 8.3 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.14 V (1.16%) | 11.9 V | 0.23 V (1.91%) | 6.3 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 45 amps at 120 feet (12V)?
To stay within the NEC recommended 3% maximum voltage drop for a 45A load over 120 feet one-way at 12V (DC), you need 400 kcmil copper wire. This produces a voltage drop of 0.35V (2.9%) and delivers 11.7V at the load.
What size aluminum wire is required for 45 amps at 120 feet?
Using aluminum conductors (75°C rating), you should use 750 kcmil to maintain voltage drop at 0.31V (2.55%).
What breaker size is needed for a 45A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (45A × 1.25 = 56.2A), which corresponds to a standard 60A circuit breaker.
Compare Other Distances & Amperages
45 Amp Wire Size by Distance (12V)
45A at 20 ft (12V)45A at 30 ft (12V)45A at 40 ft (12V)45A at 50 ft (12V)45A at 60 ft (12V)45A at 75 ft (12V)45A at 80 ft (12V)45A at 100 ft (12V)45A at 125 ft (12V)45A at 150 ft (12V)45A at 175 ft (12V)