Wire size for a 70 amp breaker
| Wire | Condition | Minimum size for 70 A |
|---|---|---|
| Copper | NM-B (Romex) cable or 60°C terminals | 4 AWG |
| Copper | THHN/THWN in conduit, 75°C terminals | 4 AWG |
| Aluminum | 75°C terminals | 3 AWG |
| Aluminum | 60°C terminals | 2 AWG |
4 AWG copper is rated 70 A in the 60°C column, 85 A at 75°C and 95 A at 90°C. The size above is the smallest whose ampacity is at least 70 A. Most modern breakers and panels have 75°C terminals, but a 70 A circuit is presumed 60°C unless the equipment is marked otherwise (NEC 110.14(C)).
Voltage drop: how far can 4 AWG run at 70 A?
The 4 AWG above carries 70 A safely but loses voltage over distance. At the full 70 A and 240 V, 4 AWG copper stays within the NEC's 3% recommendation for about 166 ft one way. 3 AWG stretches that to 210 ft.
| One-way distance | 4 AWG copper | 3 AWG copper | 2 AWG copper |
|---|---|---|---|
| 25 ft | 0.5% | 0.4% | 0.3% |
| 50 ft | 0.9% | 0.7% | 0.6% |
| 100 ft | 1.8% | 1.4% | 1.1% |
| 150 ft | 2.7% | 2.1% | 1.7% |
| 200 ft | 3.6% | 2.9% | 2.3% |
Percent drop at 70 A and 240 V.
On 120 V circuits, the 70 A percentages are twice these values.
Check your own 70 A run with the voltage drop calculator, or let the wire size calculator choose the size for you.
What a 70 amp circuit is used for
- Sub panels feeding a workshop or an addition
- Large heat pumps and electric heating
- Welders and shop equipment on a dedicated circuit
- A charger drawing 56 A continuous (56 × 1.25 = 70 A)
Cable type matters
4/2 and 4/3 NM-B (Romex) is rated 70 A at 60°C, an exact match for a 70 A breaker. In conduit, 4 AWG THHN is rated 85 A at 75°C, so it has some spare room.
Continuous loads and common mistakes
On a 70 A breaker, a continuous load running three hours or more should not exceed 80% of the rating. That is 56 A, or about 13,440 W at 240 V. A 70 A breaker is a standard size, but not every panel has a slot or a bus rating that suits it. Check what the panel is listed for before you buy anything.
For a 70 A breaker the ground wire comes from NEC Table 250.122. Use the ground wire size calculator. For other breaker sizes, see the breaker to wire size chart, or jump to 20 amps, 30 amps, 40 amps, 50 amps, 60 amps, 100 amps.
Frequently asked questions
What size wire for a 70 amp breaker?
4 AWG copper at 75°C terminals. In aluminum use 3 AWG.
How far can 4 AWG wire run on 70 amps?
About 166 feet one way at 240 V before the voltage drop passes the NEC's 3% recommendation. Past that distance, step up to 3 AWG.
What size wire for a 70 amp breaker?
4 AWG copper at 60 or 75°C. For aluminum use 3 AWG at 75°C.
What size wire for a 70 amp sub panel?
4 AWG copper in conduit or as NM-B cable. Check voltage drop if the panel is far from the main.