Table of Contents
Ampacity and Wire Size
Ampacity is the maximum current a conductor can continuously carry under specified conditions without exceeding its temperature rating. The National Electrical Code (NEC) Table 310.16 provides standard ampacity values for various wire sizes, materials, and insulation temperature ratings. Selecting the correct wire size ensures safety by preventing overheating.
Wire sizing considers three factors: the continuous current the wire must carry, the ambient temperature, and the number of current-carrying conductors bundled together. Higher temperature-rated insulation allows more current for the same wire size, while bundling multiple conductors requires derating because heat cannot dissipate as efficiently.
NEC Ampacity Tables
The NEC requires that the wire ampacity be at least equal to the continuous load. For continuous loads (running 3+ hours), the circuit must be rated at 125% of the load current. Always round up to the next standard wire size when the exact current falls between ratings.
Copper Wire Ampacity (75C)
| AWG | Copper 75C | Aluminum 75C |
|---|---|---|
| 14 | 15A | -- |
| 12 | 20A | 15A |
| 10 | 30A | 25A |
| 8 | 50A | 35A |
| 6 | 65A | 45A |
| 4 | 85A | 60A |
| 2 | 115A | 85A |
| 1/0 | 150A | 120A |
FAQ
Why is aluminum wire larger than copper for the same amps?
Aluminum has higher electrical resistivity than copper (about 61% of copper conductivity), so it requires a larger cross-section to carry the same current safely. However, aluminum is lighter and cheaper, making it economical for larger feeders and utility lines.
What is the 80% rule?
For standard circuit breakers, continuous loads should not exceed 80% of the breaker rating. A 20A breaker on a continuous load should carry no more than 16A. This ensures the breaker and wiring do not overheat during extended operation.
When should I use 90C wire?
90C-rated wire like THWN-2 is commonly used because it provides more ampacity headroom and is required for derating calculations. However, most terminations (breakers, outlets) are rated for 60C or 75C, so the actual allowed current is limited by the termination rating, not the wire rating.