Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 4 AWG70 A | 4 AWG85 A | 6 AWG75 A |
| Aluminum | 2 AWG75 A | 3 AWG75 A | 4 AWG75 A |
Equipment grounding conductor
A circuit protected at 70 A takes a 8 AWG copper or 6 AWG aluminum equipment grounding conductor, from the 100 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.
Longest run at 3% voltage drop
| Conductor | 120 V | 240 V |
|---|---|---|
| 4 AWG copper | 80 ft | 165 ft |
| 2 AWG aluminum | 80 ft | 160 ft |
NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A three-phase run reaches about 15% farther than these single-phase figures.
What a 70 A circuit feeds
- Subpanel and outbuilding feeders
- Tankless electric water heaters
Seventy amps is not a common panel size. It appears on appliances whose nameplate current falls between the standard ratings, such as a large instantaneous water heater or a commercial cooking appliance. 4 AWG copper reaches 70 A exactly at 60°C, and 6 AWG copper reaches it only in the 90°C column, which no ordinary termination lets you use.
A fixed storage water heater is a continuous load under 422.13, but an instantaneous unit falls outside that section and its current follows the draw. Manufacturers size the circuit from the nameplate instead, which is why 70 A usually appears with a specific appliance attached rather than as a general purpose feeder.
Equipment like this is installed to a listing and to the manufacturer's instructions, which 110.3(B) makes part of the code. Those instructions name the circuit size and the maximum overcurrent device, and for a water heater they also drive the disconnecting means required by 422.31. Read the nameplate before choosing the breaker, because a unit listed for a 70 A circuit does not always accept the device the ampacity table alone would suggest.
70 A wire size questions
- What size wire do I need for a 70 amp circuit?
- Use 4 AWG copper or 2 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 4 AWG copper or 3 AWG aluminum carries the circuit.
- What size ground wire for a 70 amp circuit?
- 8 AWG copper or 6 AWG aluminum, taken from the 100 A row of NEC Table 250.122.
- What size aluminum wire for 70 amps?
- 2 AWG aluminum at the 60°C column carries 75 A. Terminations on aluminum conductors must be identified for the material.
- Can I use 6 AWG copper for 70 A?
- Table 310.16 lists 6 AWG copper at 55 A at 60°C, 65 A at 75°C, and 75 A at 90°C. It falls short at 60°C, the column this circuit uses. The 90°C column does reach 70 A, and a higher column applies only where every termination is listed for it.