70 Amp Wire Size at 120 Feet (208V Three-Phase (US Commercial Wye))
Complete conductor sizing, Ohm's law voltage drop, $I^2R$ line power loss, and circuit breaker specification for a 70 A load over a 120 ft (36.6 m) one-way run.
Copper Wire (≤3% Drop)4 AWG21.15 mm² · Drop: 2.16%
Aluminum Wire (≤3% Drop)3 AWGDrop: 5.87V (2.82%)
Voltage at Load End203.5 VLoss: 4.49 V (544.0 W)
Power & Breaker (125%)25.22 kW90A Breaker Recommended
All AWG & kcmil Wire Gauges for 70 Amps at 120 Feet (208V)
| 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 | 45.63 V (21.94%) | 162.4 V | 75.26 V (36.18%) | 5531.8 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 28.67 V (13.78%) | 179.3 V | 47.29 V (22.74%) | 3476.2 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 18.04 V (8.67%) | 190.0 V | 29.76 V (14.31%) | 2187.5 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 11.34 V (5.45%) | 196.7 V | 18.7 V (8.99%) | 1374.7 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 7.14 V (3.43%) | 200.9 V | 11.77 V (5.66%) | 865.1 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 4.49 V (2.16%) | 203.5 V | 7.4 V (3.56%) | 544.0 W | ✓ PASS (≤3%) |
| 3 AWG | 26.67 mm² | 100 A | 3.56 V (1.71%) | 204.4 V | 5.87 V (2.82%) | 431.4 W | ✓ PASS (≤3%) |
| 2 AWG | 33.62 mm² | 115 A | 2.82 V (1.36%) | 205.2 V | 4.66 V (2.24%) | 342.2 W | ✓ PASS (≤3%) |
| 1 AWG | 42.41 mm² | 130 A | 2.24 V (1.08%) | 205.8 V | 3.69 V (1.77%) | 271.3 W | ✓ PASS (≤3%) |
| 1/0 AWG | 53.49 mm² | 150 A | 1.77 V (0.85%) | 206.2 V | 2.93 V (1.41%) | 215.1 W | ✓ PASS (≤3%) |
| 2/0 AWG | 67.43 mm² | 175 A | 1.41 V (0.68%) | 206.6 V | 2.32 V (1.12%) | 170.6 W | ✓ PASS (≤3%) |
| 3/0 AWG | 85.01 mm² | 200 A | 1.12 V (0.54%) | 206.9 V | 1.84 V (0.89%) | 135.3 W | ✓ PASS (≤3%) |
| 4/0 AWG | 107.2 mm² | 230 A | 0.89 V (0.43%) | 207.1 V | 1.46 V (0.7%) | 107.3 W | ✓ PASS (≤3%) |
| 250 kcmil | 126.7 mm² | 255 A | 0.75 V (0.36%) | 207.3 V | 1.24 V (0.59%) | 90.8 W | ✓ PASS (≤3%) |
| 300 kcmil | 152.0 mm² | 285 A | 0.62 V (0.3%) | 207.4 V | 1.03 V (0.5%) | 75.7 W | ✓ PASS (≤3%) |
| 350 kcmil | 177.3 mm² | 310 A | 0.54 V (0.26%) | 207.5 V | 0.88 V (0.42%) | 64.9 W | ✓ PASS (≤3%) |
| 400 kcmil | 202.7 mm² | 335 A | 0.47 V (0.23%) | 207.5 V | 0.77 V (0.37%) | 56.8 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.37 V (0.18%) | 207.6 V | 0.62 V (0.3%) | 45.4 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.31 V (0.15%) | 207.7 V | 0.51 V (0.25%) | 37.8 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.25 V (0.12%) | 207.8 V | 0.41 V (0.2%) | 30.3 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.19 V (0.09%) | 207.8 V | 0.31 V (0.15%) | 22.7 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 70 amps at 120 feet (208V)?
To stay within the NEC recommended 3% maximum voltage drop for a 70A load over 120 feet one-way at 208V (3-Phase), you need 4 AWG copper wire. This produces a voltage drop of 4.49V (2.16%) and delivers 203.5V at the load.
What size aluminum wire is required for 70 amps at 120 feet?
Using aluminum conductors (75°C rating), you should use 3 AWG to maintain voltage drop at 5.87V (2.82%).
What breaker size is needed for a 70A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (70A × 1.25 = 87.5A), which corresponds to a standard 90A circuit breaker.
Compare Other Distances & Amperages
70 Amp Wire Size by Distance (208V)
70A at 20 ft (208V)70A at 30 ft (208V)70A at 40 ft (208V)70A at 50 ft (208V)70A at 60 ft (208V)70A at 75 ft (208V)70A at 80 ft (208V)70A at 100 ft (208V)70A at 125 ft (208V)70A at 150 ft (208V)70A at 175 ft (208V)