Wire Ampacity Calculator
Determine the maximum current a conductor can safely carry per NEC 310.16. Select wire gauge, insulation type, temperature rating, and conductor count for real-time adjusted ampacity.
How Ampacity Is Calculated
NEC 310.16 provides base ampacities; adjustment factors from other tables refine the value for real-world conditions.
The Ampacity Formula
Adjusted Ampacity = Base Ampacity × Temp Factor × Bundling Factor × Insulation Factor
- Base Ampacity — From NEC Table 310.16, 75°C column, copper conductors at 30°C ambient, ≤ 3 conductors
- Temperature Correction — NEC Table 310.15(B)(2)(a): derates ambient temps above 30°C for 75°C insulation
- Bundling Adjustment — NEC Table 310.15(B)(3)(a): more conductors = less heat dissipation
- Insulation Rating — 60°C columns are 0.71× of 75°C base; 90°C columns are 1.15× of 75°C base
NEC 310.16 Ampacity Table
Copper conductor base ampacities at 30°C ambient, ≤ 3 current-carrying conductors (75°C column).
| AWG/kcmil | 60°C | 75°C | 90°C |
|---|---|---|---|
| 18 AWG | 10 | 14 | 16 |
| 16 AWG | 13 | 18 | 21 |
| 14 AWG | 18 | 25 | 29 |
| 12 AWG | 21 | 30 | 35 |
| 10 AWG | 28 | 40 | 46 |
| 8 AWG | 46 | 65 | 75 |
| 6 AWG | 67 | 95 | 109 |
| 4 AWG | 89 | 125 | 144 |
| 3 AWG | 103 | 145 | 167 |
| 2 AWG | 121 | 170 | 196 |
| 1 AWG | 138 | 195 | 224 |
| 1/0 AWG | 163 | 230 | 265 |
| 2/0 AWG | 188 | 265 | 305 |
| 3/0 AWG | 220 | 310 | 357 |
| 4/0 AWG | 256 | 360 | 414 |
| 250 kcmil | 288 | 405 | 466 |
| 300 kcmil | 316 | 445 | 512 |
| 350 kcmil | 359 | 505 | 581 |
| 400 kcmil | 387 | 545 | 627 |
| 500 kcmil | 440 | 620 | 713 |
Adjustment Factors Reference
NEC tables used for derating ampacity based on temperature and conductor bundling.
Ambient Temp Correction
NEC Table 310.15(B)(2)(a) for 75°C insulation:
Bundling Adjustment
NEC Table 310.15(B)(3)(a):
Frequently Asked Questions
Common questions about wire ampacity and NEC 310.16.
Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. Per NEC 310.16, ampacity depends on wire size, insulation type, temperature rating, number of current-carrying conductors, and ambient temperature.
Higher ambient temperatures reduce a wire's ampacity because the insulation has less capacity to dissipate heat. For 75°C rated wire, ambient temperatures above 30°C require derating: at 46-50°C multiply by 0.75, at 56-60°C multiply by 0.58. Lower temperatures allow higher current capacity.
When multiple current-carrying conductors are bundled together in a raceway or cable, heat builds up and reduces each conductor's ampacity. NEC Table 310.15(B)(3)(a) specifies: 4-6 conductors reduce to 80%, 7-9 to 70%, 10-20 to 50%, 21-30 to 45%, 31-40 to 40%, and 41+ to 35% of base ampacity.