How to size a wire
A wire must pass two separate checks. First, its ampacity (how much current it can carry without overheating) must be at least the load current, or 125% of it for continuous loads. Second, the voltage drop over the run should stay under about 3% so motors, chargers and lights get enough voltage. On short runs ampacity decides the size; on long runs voltage drop usually does.
Formula
Example: 40 A at 240 V, 150 ft, copper
8 AWG copper is rated 50 A, so it passes ampacity. Its voltage drop is 2 × 150 × 40 × 0.764 ÷ 1000 = 9.2 V, or 3.8%, which is too high. 6 AWG gives 5.9 V (2.5%), so 6 AWG is the answer.
Wire size chart (NEC 310.16, 75°C)
| Size | Copper | Aluminum | Typical use |
|---|---|---|---|
| 14 AWG | 15 A* | – | Lighting circuits |
| 12 AWG | 20 A* | 15 A* | Receptacles, kitchen |
| 10 AWG | 30 A* | 25 A* | Water heater, dryer, A/C |
| 8 AWG | 50 A | 40 A | Range, small subpanel |
| 6 AWG | 65 A | 50 A | EV charger, subpanel |
| 4 AWG | 85 A | 65 A | Subpanel feeder |
| 2 AWG | 115 A | 90 A | 90 A feeder |
| 1/0 AWG | 150 A | 120 A | Feeders |
| 2/0 AWG | 175 A | 135 A | Feeders |
| 4/0 AWG | 230 A | 180 A | 200 A service (Al, per 310.12) |
* Limited by NEC 240.4(D) small-conductor overcurrent rules.
Frequently asked questions
What size wire do I need for 50 amps?
6 AWG copper (65 A at 75°C) is the usual choice for a 50 A circuit such as an EV charger or range. Long runs may need 4 AWG to keep voltage drop under 3%.
What size wire for a 100 amp subpanel?
Typically 3 AWG copper or 1 AWG aluminum at 75°C. Runs over about 100 ft often need one size larger for voltage drop.
How far can I run 12 gauge wire on a 20 amp circuit?
At 120 V and a full 16 A continuous load, 12 AWG copper stays under 3% drop up to about 58 ft one way. At lighter loads it can go farther.
Is aluminum wire safe?
Modern AA-8000 aluminum feeders are code-approved and common for services and feeders. Use connectors rated AL/CU and an anti-oxidant compound.