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Wire Ampacity Calculator

Look up the allowable ampacity for copper and aluminum wire sizes per NEC Table 310.16, with derating for ambient temperature and conductor fill.

How to Calculate Wire Ampacity

What Is Wire Ampacity?

Ampacity is the maximum current (in amperes) that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Exceeding ampacity causes insulation to degrade, potentially leading to short circuits, arc faults, and fire. Every wire size has a rated ampacity listed in NEC Table 310.16, but the actual ampacity under real-world conditions may be lower due to temperature and bundling derating factors.

NEC Table 310.16 Explained

Adjusted Ampacity = Base Ampacity × Temp Derating × Conductor Fill Derating

Both derating factors multiply together. If either applies, the adjusted ampacity will be lower than the table value.

NEC Table 310.16 provides base ampacities at an ambient temperature of 30°C (86°F) for copper and aluminum conductors at 60°C, 75°C, and 90°C insulation ratings. Most new installations use the 75°C column.

Temperature Derating (NEC 310.15(B)(2)(a))

When ambient temperature exceeds 30°C, the wire cannot dissipate heat as effectively, so its ampacity must be reduced. For example, at 40°C ambient, a 75°C-rated conductor has a derating factor of 0.88 — a wire rated for 25A now only carries 22A.

Conductor Fill Adjustment (NEC 310.15(B)(3)(a))

When more than 3 current-carrying conductors are bundled in a conduit or cable, mutual heating reduces each conductor's capacity. The adjustment factors are: 4–6 conductors → 80%, 7–9 → 70%, 10–20 → 50%, 21–30 → 45%, 31–40 → 40%.

Worked Example

Scenario: 6 AWG copper (75°C base = 65A), ambient 40°C, 6 current-carrying conductors in conduit.

Base ampacity: 65A
Temperature derating at 40°C: 0.88
Conductor fill derating (6 conductors): 0.80
Adjusted ampacity = 65 × 0.88 × 0.80 = 45.76A

Even though the wire is rated for 65A, under these conditions you can only load it to ~46A.

Practical Tips

Both derating factors apply cumulatively — a wire in a hot attic with many conductors in conduit can lose 50%+ of its rated ampacity.
Neutral conductors carrying only unbalanced current from a 3-phase system are not counted as current-carrying. However, a neutral in a 2-wire circuit is counted.
Equipment grounding conductors are not counted for derating when there's only one in the raceway.
When derating reduces ampacity below the load, you must upsize the wire — no exceptions.

Code References

NEC 310.16, NEC 310.15(B)(2)(a), NEC 310.15(B)(3)(a)

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Frequently Asked Questions

What is ampacity and why does it matter?

Ampacity is the maximum current (in amps) that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Exceeding ampacity causes the wire to overheat, which can degrade insulation and create fire hazards. NEC Article 310 provides ampacity tables for various wire sizes, materials, and temperature ratings.

When do I need to derate wire ampacity?

You must derate (reduce) ampacity in two situations: (1) When ambient temperature exceeds 30°C (86°F), apply the temperature correction factor from NEC Table 310.15(B)(2)(a). (2) When there are more than 3 current-carrying conductors in a raceway or cable, apply the adjustment factor from NEC Table 310.15(B)(3)(a). Both derating factors multiply together with the base ampacity.

Why is the small conductor rule (NEC 240.4(D)) important?

NEC 240.4(D) limits the overcurrent protection for small conductors regardless of their ampacity: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A. Even though 12 AWG copper has an ampacity of 25A at 75°C, you cannot protect it with a breaker larger than 20A for general circuits. There are exceptions for motor circuits and other specific applications.