Load & Conditions
AWG Sizes that Meet Your Load
| AWG Size | Base Ampacity | Derated Ampacity |
|---|---|---|
| Enter load amps above. | ||
Base ampacity: NEC Table 310.15(B)(16), 75°C column. Derating per 310.15(B)(1) and 310.15(C)(1). Verify against current code and local amendments.
How to Use This Ampacity Calculator
Enter your load current, conductor material, ambient temperature, and the number of current-carrying conductors in the raceway. The calculator looks up the 75°C column of NEC Table 310.15(B)(16) and applies NEC temperature correction and conductor count derating to show every AWG size that meets your load.
The top result is the smallest AWG that satisfies both the required current and the derated ampacity limit. Confirm the result against the actual breaker size, terminal temperature rating, and any local code amendments before installation.
Why Wire Ampacity Matters
Undersized conductors overheat under load. That excess heat damages insulation, accelerates conductor aging, and can cause fires. The NEC specifies ampacity limits precisely to prevent those outcomes — so using the right table and applying the correct derating factors is not optional for a code-compliant installation.
Two conditions lower a conductor's usable ampacity below the table value: higher ambient temperatures and bundling. Both reduce the conductor's ability to shed heat, and both require you to apply a derating factor before selecting wire size.
What Affects Wire Ampacity
Conductor material, AWG size, temperature rating, installation method, ambient temperature, and the number of current-carrying conductors in the raceway all affect ampacity. Copper carries more current than the same AWG aluminum. A 90°C-rated conductor has a higher table ampacity than a 75°C conductor, but the terminal temperature rating often limits the usable value to the 75°C column per NEC 110.14(C).
Common Ampacity Calculator Scenarios
- Sizing copper THHN wire for a 20-amp general-purpose branch circuit
- Checking ampacity after adding conductors to an existing conduit run (conductor count derating)
- Sizing aluminum conductors for a 200-amp service panel or large feeder
- Determining derated ampacity for conductors in a high-temperature attic or rooftop conduit
- Selecting the minimum wire size for a 40-amp or 50-amp appliance circuit
Frequently Asked Questions
Ampacity is the maximum continuous current a conductor can carry under specified conditions without exceeding its temperature rating. NEC Table 310.15(B)(16) lists ampacity for conductors in conduit at 86°F (30°C) ambient.
NEC Table 310.15(B)(16) is the primary reference for ampacity of conductors installed in conduit, cable, or raceway at up to three current-carrying conductors in a raceway.
Derating is required when ambient temperature exceeds 86°F (30°C) per NEC 310.15(B)(1), or when more than three current-carrying conductors share the same raceway per NEC 310.15(C)(1).
Higher ambient temperatures reduce the temperature differential between the conductor and the surrounding air, limiting the heat the conductor can shed. NEC Table 310.15(B)(1) provides correction factors for temperatures above and below the 30°C base.
Yes. Four or more current-carrying conductors bundled in the same raceway generate more collective heat. NEC 310.15(C)(1) requires derating to 80% for 4–6 conductors, 70% for 7–9, and 50% for 10–20.
No. Aluminum has higher resistance per given AWG size, so the same size aluminum conductor carries less current than copper. You typically need one to two AWG sizes larger in aluminum to match a copper conductor's ampacity.