Wire Size for 70 Amp Circuit
Find the copper or aluminum wire for a 70 A subpanel, spa or shop feeder and check the voltage drop for your run.
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Assumes NEC 2023 Table 310.16: copper or aluminum, up to 3 current-carrying conductors in a raceway or cable, 30 °C ambient, 60 °C terminals for a 70 A circuit (110.14(C)), 3 % drop target; the 83 % dwelling service rule (310.12) does not apply below 100 A; ground wire not included. Runs in your browser. Verify with your local code and a licensed electrician before you install.
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How to use the 70 Amp Wire Size Calculator
- Keep 70 in “Breaker rating (A)”, or type the rating of your breaker.
- Enter the nameplate or calculated current in “Load current (A)”. The starting load is 56 A, which is 80 % of 70 A. Leave it empty to size for the full 70 A.
- Tick “Continuous load (3 hours or more)” for a spa heater, a charger or another long-running load. The design current becomes 125 % of the load and must stay at or under 70 A.
- Pick the “Supply type” and enter “Voltage (V)”: 240 for a subpanel or spa feeder. For three phase, use the line-to-line voltage.
- Type the “One-way length” in ft or m, measured along the cable or conduit route from the main panel to the subpanel or load.
- Choose Copper or Aluminum under “Conductor” and the “Insulation temperature rating” printed on the cable (the terminal setting still caps the answer at the 60 or 75 °C column).
- Leave “Allowed voltage drop (%)” at 3 unless your project sets another limit.
- Open the advanced settings only when they apply: “Terminal temperature rating” (set 75 °C only when the breaker and the subpanel lugs are listed for it), “Ambient temperature” and the number of current-carrying conductors.
- Read “Recommended wire size”, “Deciding limit” and “Longest run of the recommended size”, then the steps and warnings.
The example buttons load a copper subpanel at 56 A and 60 ft, a 70 A feeder at 30 ft, a 40 ft run, an aluminum 150 ft run and a copper run with 60 °C terminals.
What size wire do I need for a 70 amp breaker?
A 70 A breaker needs a conductor whose allowed ampacity reaches 70 A. With Auto terminals (60 °C for circuits up to 100 A, NEC 110.14(C)(1)) that is 4 AWG copper (70 A in the 60 °C column) or 2 AWG aluminum. If the breaker and the lugs are listed for 75 °C, copper stays at 4 AWG (85 A in the 75 °C column, and no smaller size reaches 70 A) and aluminum drops to 3 AWG (75 A). Select “75 °C (listed equipment)” in the advanced terminal setting to see the 75 °C result.
| Settings (70 A breaker, 56 A load, 240 V, 60 ft) | Copper | Aluminum |
|---|---|---|
| Terminals Auto (60 °C) | 4 AWG | 2 AWG |
| Terminals listed for 75 °C | 4 AWG | 3 AWG |
A 56 A load is 80 % of 70 A, the most a continuous load can be on a 70 A breaker, because the 125 % rule sets the design current at 70 A.
The ground wire is separate. For a 70 A breaker the equipment grounding conductor is 8 AWG copper or 6 AWG aluminum (NEC Table 250.122, 60 to 100 A), found with the equipment grounding conductor calculator.
How far can you run 70 amp wire?
With a 56 A load on 240 V, 4 AWG copper reaches about 209 ft one way before the drop passes 3 %. At a full 70 A load it reaches about 167 ft. Beyond that the calculator moves up a size.
| Copper unless noted, 240 V | One-way length | Result |
|---|---|---|
| 56 A load | 60 ft | 4 AWG, 0.86 % drop |
| 56 A load | 100 ft | 4 AWG, 1.44 % drop |
| 56 A load | 200 ft | 4 AWG, 2.88 % drop |
| 70 A load | 150 ft | 4 AWG, 2.70 % drop |
| 70 A load | 250 ft | 2 AWG, 2.83 % drop |
| 56 A load, aluminum | 100 ft | 2 AWG, 1.49 % drop |
Does the 83 % dwelling service rule apply at 70 amps?
No. The dwelling service size under NEC 310.12 is for 100 to 400 A services. A 70 A subpanel, spa or shop feeder uses the breaker-rated size above.
Worked examples
| Input | Result |
|---|---|
| 70 A breaker, 56 A load, 240 V, 60 ft, copper 75 °C | 4 AWG (0.863 %) |
| 70 A breaker, 70 A load, 240 V, 30 ft, copper 75 °C | 4 AWG (0.539 %) |
| 70 A breaker, 56 A load, 240 V, 40 ft, copper 75 °C | 4 AWG (0.575 %) |
| 70 A breaker, 56 A load, 240 V, 150 ft, aluminum 75 °C | 2 AWG (2.23 %) |
| 70 A breaker, 56 A load, 240 V, 40 ft, copper 60 °C | 4 AWG (0.575 %) |
Calculated by the tool above; open it with the same values to see each step.
Questions, briefly answered
What size wire for a 70 amp breaker?
4 AWG copper or 2 AWG aluminum with Auto terminals (60 °C column). With the breaker and lugs listed for 75 °C, copper stays at 4 AWG and aluminum is 3 AWG.
What size wire for a 70 amp subpanel?
The same breaker-rated size: 4 AWG copper or 2 AWG aluminum. At a 56 A load (80 % of 70 A) and 60 ft, 4 AWG copper gives a 0.86 % drop.
Is 4 gauge wire good for 70 amps?
Yes, for a 70 A breaker with Auto terminals: 4 AWG copper is rated 70 A in the 60 °C column. A 56 A load over 200 ft still stays under 3 % (2.88 %).
What size aluminum wire for 70 amps?
2 AWG aluminum with Auto terminals, or 3 AWG aluminum when the equipment is listed for 75 °C. At a 56 A load over 100 ft, 2 AWG aluminum gives a 1.49 % drop and reaches about 201 ft before the drop passes 3 %. Aluminum needs lugs marked AL/CU.
How far can 4 AWG copper run at 70 amps?
About 209 ft one way at a 56 A load, or about 167 ft at a full 70 A load, both at a 3 % target. For 250 ft at a 70 A load, the answer is 2 AWG (2.83 %).
Does the dwelling service rule change a 70 amp feeder?
No. NEC 310.12 covers 100 to 400 A dwelling services, so a 70 A feeder is sized from the breaker and its terminal rating.
What size ground wire for a 70 amp breaker?
8 AWG copper or 6 AWG aluminum under NEC Table 250.122 (60 to 100 A). It is not included in the wire size above.