Ampacity & wire size for amps
How much current a conductor can safely carry depends on far more than its gauge. This page explains the NEC Table 310.16 ratings and the four rules that actually decide the wire size for a given load.
Safety first. The figures here are an educational reference, not a code substitute. Wire sizing affects fire and shock safety; always verify against the current NEC and have circuits designed or reviewed by a licensed electrician for your jurisdiction.
Find a wire size for your load
Enter the current and conductor material to see the smallest gauge that can carry it once the 310.16 ampacity and the 240.4(D) breaker cap are both applied — with the reason flagged when the breaker cap is what sets the size.
A starting point from NEC Table 310.16 (before derating) with the 240.4(D) breaker cap applied — not a full load calculation. Verify against the current code and your installation.
NEC 310.16 ampacity tables
Base ampacities for copper and aluminum at each insulation temperature rating, alongside the 240.4(D) small-conductor breaker caps.
Copper — NEC Table 310.16
| AWG | 60 °C | 75 °C | 90 °C | 240.4(D) cap |
|---|---|---|---|---|
| 14 | 15 | 20 | 25 | 15 A |
| 12 | 20 | 25 | 30 | 20 A |
| 10 | 30 | 35 | 40 | 30 A |
| 8 | 40 | 50 | 55 | — |
| 6 | 55 | 65 | 75 | — |
| 4 | 70 | 85 | 95 | — |
| 3 | 85 | 100 | 115 | — |
| 2 | 95 | 115 | 130 | — |
| 1 | 110 | 130 | 145 | — |
| 1/0 | 125 | 150 | 170 | — |
| 2/0 | 145 | 175 | 195 | — |
| 3/0 | 165 | 200 | 225 | — |
| 4/0 | 195 | 230 | 260 | — |
Aluminum — NEC Table 310.16
| AWG | 60 °C | 75 °C | 90 °C | 240.4(D) cap |
|---|---|---|---|---|
| 12 | 15 | 20 | 25 | 15 A |
| 10 | 25 | 30 | 35 | 25 A |
| 8 | 35 | 40 | 45 | — |
| 6 | 40 | 50 | 55 | — |
| 4 | 55 | 65 | 75 | — |
| 3 | 65 | 75 | 85 | — |
| 2 | 75 | 90 | 100 | — |
| 1 | 85 | 100 | 115 | — |
| 1/0 | 100 | 120 | 135 | — |
| 2/0 | 115 | 135 | 150 | — |
| 3/0 | 130 | 155 | 175 | — |
| 4/0 | 150 | 180 | 205 | — |
Ampacities are amps (A) at a 30 °C ambient, before derating. The 240.4(D) column is the maximum overcurrent device for that small conductor — a separate limit from the ampacity to its left. Aluminum is not rated below 12 AWG.
The four rules that set the wire size
1. Start from NEC Table 310.16. It lists the base ampacity of each conductor by material and insulation temperature rating. These are the numbers most charts show — but they are the starting point, not the final answer.
2. Pick the right temperature column. Table 310.16 has 60 °C, 75 °C, and 90 °C columns for the insulation's rating. You may only use the higher columns if everything in the circuit is rated for that temperature, which is rarely the case end to end.
3. Respect the terminal rating (110.14(C)). Even with 90 °C wire, the breakers, lugs, and devices it lands on are usually rated 75 °C (or 60 °C on smaller residential gear). You must size to the lowest-rated component in the path — so most everyday work uses the 75 °C column regardless of the wire's own rating.
4. Apply derating, then check 240.4(D). Ambient temperature above 30 °C and bundling more than three current-carrying conductors both reduce the usable ampacity by correction factors. Separately, small conductors are capped by NEC 240.4(D): copper is limited to 15 A at 14 AWG, 20 A at 12 AWG, and 30 A at 10 AWG; aluminum to 15 A at 12 AWG and 25 A at 10 AWG — no matter what the raw ampacity table suggests.
Why "before derating" matters
Every ampacity number on this site is a before-derating value at the standard 30 °C ambient. Two identical conductors can have very different real-world ratings once you account for a hot attic, conduit packed with other circuits, or a long run where voltage drop becomes the limiting factor instead of heat. Treat the table value as a ceiling you adjust downward, never a guarantee.
Need the conductor's physical specs instead — diameter, area, or resistance? Use the main wire gauge chart, or convert a size with the AWG ↔ mm² converter.