Need the Numbers?
View the full NEC Table 310.15(B)(16) values in our interactive ampacity chart, or use the calculator to apply temperature and conductor count derating automatically.
What Is Wire Ampacity?
Ampacity is the maximum continuous current (in amperes) a conductor can carry under specified installation conditions without exceeding its temperature rating. It is not a fixed number for a given wire gauge — it depends on:
- Conductor size (AWG or kcmil): Larger conductors have more surface area and lower resistance, so they can carry more current
- Conductor material: Copper has lower resistance than aluminum, so #8 AWG copper has higher ampacity than #8 AWG aluminum
- Insulation temperature rating: Higher-rated insulation can withstand more heat, allowing a higher base ampacity
- Ambient temperature: Hotter surroundings reduce the wire's ability to shed heat, lowering usable ampacity
- Number of conductors bundled together: More conductors in a raceway means more cumulative heat generation, requiring derating
How to Read NEC Table 310.15(B)(16)
The table is organized by AWG size (from small to large) in rows, and three temperature rating columns (60°C, 75°C, 90°C) for both copper and aluminum conductors. The values are base ampacities assuming:
- No more than 3 current-carrying conductors in the raceway
- Ambient temperature of 30°C (86°F)
- Conductors in conduit, cable, or earth
Example: #10 AWG copper at 75°C = 35A. #10 AWG aluminum at 75°C = 30A. This means #10 AWG copper can continuously carry 35A under standard conditions before breaching NEC requirements.
Which Temperature Column to Use
Per NEC 110.14(C), the usable ampacity is limited to the lowest temperature rating in the circuit — typically the termination rating. Most residential equipment (breaker lugs, terminals, outlets) is rated 60°C or 75°C:
- 60°C column: Used only when terminations are limited to 60°C. Applies to some older equipment and specific fixtures.
- 75°C column: Use this for virtually all modern residential and commercial circuits. Most breakers and panel lugs are 75°C-rated when conductors are #1 AWG or smaller, or 60/75°C for conductors #1/0 AWG and larger.
- 90°C column: Used only as a correction factor baseline when derating — never as the direct usable ampacity unless all terminations are 90°C-rated (rare in residential work).
When and How to Derate Ampacity
1. Temperature Derating (NEC 310.15(B)(1))
When ambient temperature exceeds 30°C, multiply the table ampacity by the correction factor from NEC Table 310.15(B)(1). Common factors at 75°C rating:
If #10 AWG copper at 75°C has a base of 35A, at 40°C ambient it derated to 35 × 0.88 = 30.8A. You might still protect it with a 30A breaker, but a 35A breaker would be code violation at this ambient.
2. Conductor Count Derating (NEC 310.15(C)(1))
When 4 or more current-carrying conductors share the same raceway, apply a bundling derating factor:
Neutral conductors that only carry imbalanced current in a balanced 3-wire circuit are typically not counted. Equipment grounding conductors are never counted.
Copper vs. Aluminum Ampacity — Common Comparison
| AWG | Copper 75°C (A) | Aluminum 75°C (A) | Typical Use |
|---|---|---|---|
| #14 | 20A | — | 15A branch circuit (Cu only) |
| #12 | 25A | 20A | 20A receptacle circuits |
| #10 | 35A | 30A | 30A dryer, A/C |
| #8 | 50A | 40A | 40A HVAC, 32A EV charger |
| #6 | 65A | 50A | 50A range, 48A EV charger |
| #4 | 85A | 65A | 60A+ feeders |
| #2 | 115A | 90A | 100A subpanel feeders |
| #1/0 | 150A | 120A | 100–125A service |
| #4/0 | 230A | 180A | 200A service (Cu); 200A Al needs 350 kcmil |
Values from NEC Table 310.15(B)(16), 75°C column, ≤3 conductors in raceway, 30°C ambient. See the full NEC Ampacity Chart for all AWG sizes.
Frequently Asked Questions
Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating, under specified installation conditions. It depends on conductor size, material, insulation rating, ambient temperature, and number of bundled conductors.
The primary NEC table listing allowable ampacity for conductors in conduit, cable, or earth — for copper and aluminum at 60°C, 75°C, and 90°C ratings, assuming ≤3 current-carrying conductors and 30°C ambient temperature.
Use the 75°C column for most residential and commercial circuits. Per NEC 110.14(C), the circuit's usable ampacity is limited to the lowest-rated termination in the circuit, which is usually 75°C for modern equipment.
When ambient temperature exceeds 30°C (apply NEC Table 310.15(B)(1) correction factors), or when four or more current-carrying conductors share the same raceway (apply NEC 310.15(C)(1) bundling derating: 80%, 70%, or 50% depending on count).
Yes. Aluminum is standard for service entrances, feeders, and branch circuits ≥ 30A. It requires anti-oxidant compound and aluminum-rated connectors. For 15A/20A branch circuits, CO/ALR-rated devices are required.