How this calculator works
A wire has to satisfy two tests. First, it must carry the current without overheating: each size has a rated ampacity, which depends on the wire's temperature rating, and for small wires the breaker is limited too. Second, a long run should not lose too much voltage: the voltage drop is twice the one-way distance times the current times the resistance per foot, and 3% is the usual limit. The wire to use is the larger of the two sizes.
Worked example: a 30 A load on a 120 V circuit 30 ft away needs 10 AWG copper for the current and 12 AWG for a 3% drop, so 10 AWG copper on a 30 breaker; the drop with that wire is 1.86%. At 100 ft the same load needs 6 AWG, and in aluminum 8 AWG. A 50 A load needs 8 AWG, a 20 A load 12 AWG, a 15 A load 14 AWG, and a 20 A load on 12 V DC at 15 ft needs 6 AWG.
Smallest copper wire for the current (75°C, not continuous)
Ampacity only, with the 15/20/30 amp limits for 14, 12 and 10 AWG. Long runs need larger wire for voltage drop.
| Load | Copper wire | Aluminum wire | Breaker |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 15 |
| 20 A | 12 AWG | 10 AWG | 20 |
| 30 A | 10 AWG | 8 AWG | 30 |
| 40 A | 8 AWG | 8 AWG | 40 |
| 50 A | 8 AWG | 6 AWG | 50 |
| 60 A | 6 AWG | 4 AWG | 60 |
| 70 A | 4 AWG | 3 AWG | 70 |
| 100 A | 3 AWG | 1 AWG | 100 |
Tips before you buy
- The common household sizes are 14 AWG for 15 amp circuits, 12 AWG for 20 amps, 10 AWG for 30 amps, 8 AWG for 40 amps (and for 50 amps in conduit at 75°C), and 6 AWG for 50 to 65 amps (the usual size for a 50 amp range or EV charger cable), in copper.
- A bigger wire has a smaller number. Each gauge number lower cuts the resistance by about 20%, so going from 12 to 10 AWG cuts it by about 37%.
- Aluminum needs about two sizes more than copper for the same current, and must use fittings rated for it.
- A long run needs a bigger wire to keep the voltage drop down, and low-voltage DC runs, such as solar and RV systems, are most affected.
- Most breakers and panels are rated for 75°C terminals, and some small equipment for 60°C. The wire must also match the terminal rating, so check the labels.
Tools & materials for this project
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Frequently asked questions
What size wire do I need for 30 amps?
10 AWG copper by ampacity at 75°C, on a 30 breaker. A long run needs more: 6 AWG for 100 ft at 120 V.
What size wire do I need for 50 amps?
8 AWG copper at 75°C (single wires in conduit) for a 50 amp circuit. Cable rated at 60°C, which is common for ranges and EV chargers, needs 6 AWG. A long run needs more.
What size wire do I need for 20 amps?
12 AWG copper, on a 20 amp breaker. For 15 amps it is 14 AWG.
Can I use aluminum wire?
Yes, for feeders and large circuits, but it must be larger: 10 AWG copper becomes 8 AWG aluminum for 30 amps. Use connectors rated for aluminum and follow your local code.
How far can I run a wire?
As far as the voltage drop allows. 10 AWG copper at 30 amps and 120 V drops 3% at about 48 ft one way. Enter your distance in the calculator for the size.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.