300 Amp Wire Size at 125 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 300 A load over a 125 ft (38.1 m) one-way run.
Copper Wire (≤3% Drop)350 kcmil177.3 mm² · Drop: 1.15%
Aluminum Wire (≤3% Drop)500 kcmilDrop: 2.76V (1.33%)
Voltage at Load End205.6 VLoss: 2.39 V (1241.6 W)
Power & Breaker (125%)108.08 kW400A Breaker Recommended
All AWG & kcmil Wire Gauges for 300 Amps at 125 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 | 203.68 V (97.93%) | 4.3 V | 335.98 V (161.53%) | 105837.4 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 127.99 V (61.54%) | 80.0 V | 211.13 V (101.51%) | 66508.1 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 80.54 V (38.72%) | 127.5 V | 132.86 V (63.88%) | 41852.1 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 50.62 V (24.34%) | 157.4 V | 83.49 V (40.14%) | 26301.3 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 31.85 V (15.31%) | 176.1 V | 52.54 V (25.26%) | 16552.0 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 20.03 V (9.63%) | 188.0 V | 33.04 V (15.89%) | 10408.6 W | Drop/Amp High |
| 3 AWG | 26.67 mm² | 100 A | 15.89 V (7.64%) | 192.1 V | 26.2 V (12.6%) | 8254.3 W | Drop/Amp High |
| 2 AWG | 33.62 mm² | 115 A | 12.6 V (6.06%) | 195.4 V | 20.79 V (9.99%) | 6547.9 W | Drop/Amp High |
| 1 AWG | 42.41 mm² | 130 A | 9.99 V (4.8%) | 198.0 V | 16.48 V (7.92%) | 5190.8 W | Drop/Amp High |
| 1/0 AWG | 53.49 mm² | 150 A | 7.92 V (3.81%) | 200.1 V | 13.06 V (6.28%) | 4115.6 W | Drop/Amp High |
| 2/0 AWG | 67.43 mm² | 175 A | 6.28 V (3.02%) | 201.7 V | 10.36 V (4.98%) | 3264.7 W | Drop/Amp High |
| 3/0 AWG | 85.01 mm² | 200 A | 4.98 V (2.4%) | 203.0 V | 8.22 V (3.95%) | 2589.6 W | Drop/Amp High |
| 4/0 AWG | 107.2 mm² | 230 A | 3.95 V (1.9%) | 204.0 V | 6.52 V (3.13%) | 2053.6 W | Drop/Amp High |
| 250 kcmil | 126.7 mm² | 255 A | 3.34 V (1.61%) | 204.7 V | 5.52 V (2.65%) | 1737.5 W | Drop/Amp High |
| 300 kcmil | 152.0 mm² | 285 A | 2.79 V (1.34%) | 205.2 V | 4.6 V (2.21%) | 1448.3 W | Drop/Amp High |
| 350 kcmil | 177.3 mm² | 310 A | 2.39 V (1.15%) | 205.6 V | 3.94 V (1.9%) | 1241.6 W | ✓ PASS (≤3%) |
| 400 kcmil | 202.7 mm² | 335 A | 2.09 V (1.0%) | 205.9 V | 3.45 V (1.66%) | 1086.0 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 1.67 V (0.8%) | 206.3 V | 2.76 V (1.33%) | 868.8 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 1.39 V (0.67%) | 206.6 V | 2.3 V (1.11%) | 724.2 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 1.11 V (0.54%) | 206.9 V | 1.84 V (0.88%) | 579.3 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.84 V (0.4%) | 207.2 V | 1.38 V (0.66%) | 434.5 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 300 amps at 125 feet (208V)?
To stay within the NEC recommended 3% maximum voltage drop for a 300A load over 125 feet one-way at 208V (3-Phase), you need 350 kcmil copper wire. This produces a voltage drop of 2.39V (1.15%) and delivers 205.6V at the load.
What size aluminum wire is required for 300 amps at 125 feet?
Using aluminum conductors (75°C rating), you should use 500 kcmil to maintain voltage drop at 2.76V (1.33%).
What breaker size is needed for a 300A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (300A × 1.25 = 375.0A), which corresponds to a standard 400A circuit breaker.
Compare Other Distances & Amperages
300 Amp Wire Size by Distance (208V)
300A at 20 ft (208V)300A at 30 ft (208V)300A at 40 ft (208V)300A at 50 ft (208V)300A at 60 ft (208V)300A at 75 ft (208V)300A at 80 ft (208V)300A at 100 ft (208V)300A at 120 ft (208V)300A at 150 ft (208V)300A at 175 ft (208V)