20 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 20 A load over a 40 ft (12.2 m) one-way run.
Copper Wire (≤3% Drop)2 AWG33.62 mm² · Drop: 2.59%
Aluminum Wire (≤3% Drop)1/0 AWGDrop: 0.32V (2.68%)
Voltage at Load End11.7 VLoss: 0.31 V (6.2 W)
Power & Breaker (125%)0.24 kW25A Breaker Recommended
All AWG & kcmil Wire Gauges for 20 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 | 5.02 V (41.81%) | 7.0 V | 8.28 V (68.97%) | 100.3 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 3.15 V (26.27%) | 8.8 V | 5.2 V (43.34%) | 63.1 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 1.98 V (16.53%) | 10.0 V | 3.27 V (27.27%) | 39.7 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 1.25 V (10.39%) | 10.8 V | 2.06 V (17.14%) | 24.9 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 0.78 V (6.54%) | 11.2 V | 1.29 V (10.79%) | 15.7 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 0.49 V (4.11%) | 11.5 V | 0.81 V (6.78%) | 9.9 W | ~ ≤5% |
| 3 AWG | 26.67 mm² | 100 A | 0.39 V (3.26%) | 11.6 V | 0.65 V (5.38%) | 7.8 W | ~ ≤5% |
| 2 AWG | 33.62 mm² | 115 A | 0.31 V (2.59%) | 11.7 V | 0.51 V (4.27%) | 6.2 W | ✓ PASS (≤3%) |
| 1 AWG | 42.41 mm² | 130 A | 0.25 V (2.05%) | 11.8 V | 0.41 V (3.38%) | 4.9 W | ✓ PASS (≤3%) |
| 1/0 AWG | 53.49 mm² | 150 A | 0.2 V (1.63%) | 11.8 V | 0.32 V (2.68%) | 3.9 W | ✓ PASS (≤3%) |
| 2/0 AWG | 67.43 mm² | 175 A | 0.15 V (1.29%) | 11.8 V | 0.26 V (2.13%) | 3.1 W | ✓ PASS (≤3%) |
| 3/0 AWG | 85.01 mm² | 200 A | 0.12 V (1.02%) | 11.9 V | 0.2 V (1.69%) | 2.5 W | ✓ PASS (≤3%) |
| 4/0 AWG | 107.2 mm² | 230 A | 0.1 V (0.81%) | 11.9 V | 0.16 V (1.34%) | 1.9 W | ✓ PASS (≤3%) |
| 250 kcmil | 126.7 mm² | 255 A | 0.08 V (0.69%) | 11.9 V | 0.14 V (1.13%) | 1.6 W | ✓ PASS (≤3%) |
| 300 kcmil | 152.0 mm² | 285 A | 0.07 V (0.57%) | 11.9 V | 0.11 V (0.94%) | 1.4 W | ✓ PASS (≤3%) |
| 350 kcmil | 177.3 mm² | 310 A | 0.06 V (0.49%) | 11.9 V | 0.1 V (0.81%) | 1.2 W | ✓ PASS (≤3%) |
| 400 kcmil | 202.7 mm² | 335 A | 0.05 V (0.43%) | 11.9 V | 0.08 V (0.71%) | 1.0 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.04 V (0.34%) | 12.0 V | 0.07 V (0.57%) | 0.8 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.03 V (0.29%) | 12.0 V | 0.06 V (0.47%) | 0.7 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.03 V (0.23%) | 12.0 V | 0.05 V (0.38%) | 0.5 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.02 V (0.17%) | 12.0 V | 0.03 V (0.28%) | 0.4 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 20 amps at 40 feet (12V)?
To stay within the NEC recommended 3% maximum voltage drop for a 20A load over 40 feet one-way at 12V (DC), you need 2 AWG copper wire. This produces a voltage drop of 0.31V (2.59%) and delivers 11.7V at the load.
What size aluminum wire is required for 20 amps at 40 feet?
Using aluminum conductors (75°C rating), you should use 1/0 AWG to maintain voltage drop at 0.32V (2.68%).
What breaker size is needed for a 20A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (20A × 1.25 = 25.0A), which corresponds to a standard 25A circuit breaker.
Compare Other Distances & Amperages
20 Amp Wire Size by Distance (12V)
20A at 20 ft (12V)20A at 30 ft (12V)20A at 50 ft (12V)20A at 60 ft (12V)20A at 75 ft (12V)20A at 80 ft (12V)20A at 100 ft (12V)20A at 120 ft (12V)20A at 125 ft (12V)20A at 150 ft (12V)20A at 175 ft (12V)