40 Amp Wire Size at 20 Feet (120V Single-Phase (US Standard Branch))
Complete conductor sizing, Ohm's law voltage drop, $I^2R$ line power loss, and circuit breaker specification for a 40 A load over a 20 ft (6.1 m) one-way run.
Copper Wire (≤3% Drop)8 AWG8.37 mm² · Drop: 1.04%
Aluminum Wire (≤3% Drop)8 AWGDrop: 2.06V (1.71%)
Voltage at Load End118.8 VLoss: 1.25 V (49.9 W)
Power & Breaker (125%)4.8 kW50A Breaker Recommended
All AWG & kcmil Wire Gauges for 40 Amps at 20 Feet (120V)
| 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 (4.18%) | 115.0 V | 8.28 V (6.9%) | 200.7 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 3.15 V (2.63%) | 116.8 V | 5.2 V (4.33%) | 126.1 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 1.98 V (1.65%) | 118.0 V | 3.27 V (2.73%) | 79.4 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 1.25 V (1.04%) | 118.8 V | 2.06 V (1.71%) | 49.9 W | ✓ PASS (≤3%) |
| 6 AWG | 13.3 mm² | 65 A | 0.78 V (0.65%) | 119.2 V | 1.29 V (1.08%) | 31.4 W | ✓ PASS (≤3%) |
| 4 AWG | 21.15 mm² | 85 A | 0.49 V (0.41%) | 119.5 V | 0.81 V (0.68%) | 19.7 W | ✓ PASS (≤3%) |
| 3 AWG | 26.67 mm² | 100 A | 0.39 V (0.33%) | 119.6 V | 0.65 V (0.54%) | 15.7 W | ✓ PASS (≤3%) |
| 2 AWG | 33.62 mm² | 115 A | 0.31 V (0.26%) | 119.7 V | 0.51 V (0.43%) | 12.4 W | ✓ PASS (≤3%) |
| 1 AWG | 42.41 mm² | 130 A | 0.25 V (0.21%) | 119.8 V | 0.41 V (0.34%) | 9.8 W | ✓ PASS (≤3%) |
| 1/0 AWG | 53.49 mm² | 150 A | 0.2 V (0.16%) | 119.8 V | 0.32 V (0.27%) | 7.8 W | ✓ PASS (≤3%) |
| 2/0 AWG | 67.43 mm² | 175 A | 0.15 V (0.13%) | 119.8 V | 0.26 V (0.21%) | 6.2 W | ✓ PASS (≤3%) |
| 3/0 AWG | 85.01 mm² | 200 A | 0.12 V (0.1%) | 119.9 V | 0.2 V (0.17%) | 4.9 W | ✓ PASS (≤3%) |
| 4/0 AWG | 107.2 mm² | 230 A | 0.1 V (0.08%) | 119.9 V | 0.16 V (0.13%) | 3.9 W | ✓ PASS (≤3%) |
| 250 kcmil | 126.7 mm² | 255 A | 0.08 V (0.07%) | 119.9 V | 0.14 V (0.11%) | 3.3 W | ✓ PASS (≤3%) |
| 300 kcmil | 152.0 mm² | 285 A | 0.07 V (0.06%) | 119.9 V | 0.11 V (0.09%) | 2.7 W | ✓ PASS (≤3%) |
| 350 kcmil | 177.3 mm² | 310 A | 0.06 V (0.05%) | 119.9 V | 0.1 V (0.08%) | 2.4 W | ✓ PASS (≤3%) |
| 400 kcmil | 202.7 mm² | 335 A | 0.05 V (0.04%) | 119.9 V | 0.08 V (0.07%) | 2.1 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.04 V (0.03%) | 120.0 V | 0.07 V (0.06%) | 1.6 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.03 V (0.03%) | 120.0 V | 0.06 V (0.05%) | 1.4 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.03 V (0.02%) | 120.0 V | 0.05 V (0.04%) | 1.1 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.02 V (0.02%) | 120.0 V | 0.03 V (0.03%) | 0.8 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 40 amps at 20 feet (120V)?
To stay within the NEC recommended 3% maximum voltage drop for a 40A load over 20 feet one-way at 120V (1-Phase), you need 8 AWG copper wire. This produces a voltage drop of 1.25V (1.04%) and delivers 118.8V at the load.
What size aluminum wire is required for 40 amps at 20 feet?
Using aluminum conductors (75°C rating), you should use 8 AWG to maintain voltage drop at 2.06V (1.71%).
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 (120V)
40A at 30 ft (120V)40A at 40 ft (120V)40A at 50 ft (120V)40A at 60 ft (120V)40A at 75 ft (120V)40A at 80 ft (120V)40A at 100 ft (120V)40A at 120 ft (120V)40A at 125 ft (120V)40A at 150 ft (120V)40A at 175 ft (120V)