Wire size by amperage
Minimum conductor size for not more than three current-carrying conductors at 30 °C (86 °F) ambient.
| Circuit | Copper, 60 °C (NM cable) | Copper, 75 °C (THHN, 75 °C terminals) | Aluminum, 75 °C |
|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 6 AWG | 8 AWG | 6 AWG |
| 60 A | 4 AWG | 6 AWG | 4 AWG |
| 70 A | 4 AWG | 4 AWG | 3 AWG |
| 80 A | 3 AWG | 4 AWG | 2 AWG |
| 90 A | 2 AWG | 3 AWG | 2 AWG |
| 100 A | 1 AWG | 3 AWG | 1 AWG |
| 125 A | 1/0 AWG | 1 AWG | 2/0 AWG |
| 150 A | 3/0 AWG | 1/0 AWG | 3/0 AWG |
| 200 A | 250 kcmil | 3/0 AWG | 250 kcmil |
15, 20 and 30 A rows follow NEC 240.4(D): 14, 12 and 10 AWG copper are limited to 15, 20 and 30 A overcurrent protection no matter which column you use. Aluminum 12 and 10 AWG are limited to 15 and 25 A. Full values are in the wire ampacity chart.
Which temperature column applies?
- NM cable (Romex) is limited to the 60 °C column by NEC 334.80, even though its conductors are 90 °C rated.
- Equipment terminals set the limit under NEC 110.14(C). Circuits of 100 A or less, or 14 to 1 AWG, use 60 °C unless the equipment is marked 75 °C. Most modern breakers and panels are marked 60/75 °C.
- The 90 °C column is only a starting point for derating (ambient temperature or more than three conductors). The result still cannot exceed the terminal rating.
Continuous loads: size for 125%
A load that runs three hours or more, such as an EV charger, water heater or lighting, needs conductors and overcurrent protection sized at 125% of the load (NEC 210.19(A) and 210.20(A)). A 40 A EV charger needs a 50 A circuit, so 6 AWG copper NM or 8 AWG THHN with 75 °C terminals.
Dwelling services and feeders: the 83% rule
For a 120/240 V single-phase dwelling service or main feeder, NEC 310.12 lets the conductor ampacity be 83% of the service rating. That is why 100 A services commonly use 4 AWG copper or 2 AWG aluminum, and 200 A services use 2/0 copper or 4/0 aluminum. It does not apply to branch circuits or to feeders that serve only part of the dwelling load.
Long runs: check voltage drop
The chart only covers heat. A 20 A, 120 V circuit on 12 AWG copper drops about 3% at 47 ft of one-way length with a full 20 A load. Past that, upsize. The voltage drop formula guide shows the math and the voltage drop calculator does it for you.
In Canada
CEC Tables 2 and 4 give the same ampacities for these sizes as NEC 310.16. Rule 8-102 makes the 3% and 5% voltage drop limits mandatory. See the Canadian voltage drop calculator.
This chart is a quick reference. Always confirm against the code edition your jurisdiction has adopted, the equipment markings and your inspector.