300 amp wire size
A 300 amp circuit needs 350 kcmil copper or 500 kcmil aluminum wire with 75 °C terminations. As the main service wire of a house, a 300 amp service may use 250 kcmil copper or 350 kcmil aluminum under the 83% rule: see 300 amp service wire size.
300 amp wire size by termination rating
| Termination rating | Copper | Aluminum |
|---|---|---|
| 60 °C | 500 kcmil | larger than 600 kcmil |
| 75 °C, THHN | 350 kcmil | 500 kcmil |
| Ground wire | 4 AWG | 2 AWG |
300 amp wire size by distance at 240 V
Sizes that keep the voltage drop within 3% on a 240 V circuit loaded to the full 300 A, 75 °C terminations. The copper ground wire grows with the hot wires on long runs.
| One-way distance | Copper | Drop | Copper ground | Aluminum | Drop |
|---|---|---|---|---|---|
| 25 ft | 350 kcmil | 0.2% | 4 AWG | 500 kcmil | 0.3% |
| 50 ft | 350 kcmil | 0.5% | 4 AWG | 500 kcmil | 0.5% |
| 100 ft | 350 kcmil | 0.9% | 4 AWG | 500 kcmil | 1.1% |
| 150 ft | 350 kcmil | 1.4% | 4 AWG | 500 kcmil | 1.6% |
| 200 ft | 350 kcmil | 1.8% | 4 AWG | 500 kcmil | 2.1% |
| 300 ft | 350 kcmil | 2.8% | 4 AWG | 600 kcmil | 2.7% |
- Copper
- Aluminum
Check your own 300 amp run
What are you sizing for?
A
ft
From the panel to the load, not there and back
Voltage
Conductor
Termination rating
Result
350 kcmil copper
Ampacity sets the size; the voltage drop is within 3%.
- Breaker
- 300 A
- Voltage drop
- 1.1 V (0.5%)
- Volts at the load
- 238.9 V
- Ground wire
- 4 AWG copper
How it is calculated
- Breaker: 300 A rounded up to a standard size = 300 A.
- Smallest copper conductor rated for 300 A at 75 °C: 350 kcmil.
- Drop = 2 × K × 300 A × 50 ft ÷ 350,000 cmil = 1.11 V, target 3% of 240 V.
What a 300 amp circuit is used for
- Service for a large house
- Feeder to a farm building or large shop