Wire & Breaker Ampacity Calculator
NEC Table 310.16 allowable ampacity, corrected for ambient temperature and conductor bundling, with a suggested standard breaker size.
How this is calculated
NEC Table 310.16 lists the base allowable ampacity for insulated copper and aluminum conductors under standard conditions (30°C ambient, no more than 3 current-carrying conductors in a raceway or cable). When real conditions differ, NEC 310.15 requires applying correction factors:
- Ambient temperature correction (Table 310.15(B)(1)) — hotter environments reduce allowable ampacity.
- Conductor bundling adjustment (Table 310.15(C)(1)) — more current-carrying conductors sharing a raceway reduce each conductor's heat dissipation, so ampacity is derated.
Adjusted ampacity = base ampacity × ambient factor × bundling factor
The 60°C/75°C terminal rating rule
Even if a conductor is rated 90°C (like most THHN/THWN-2), NEC 110.14(C) generally requires sizing based on the temperature rating of the terminals it lands on — commonly 60°C for equipment rated 100A or less, and 75°C above that, unless equipment is specifically listed for a higher rating. A 90°C conductor is often chosen for its higher ampacity headroom under derating, but the final circuit is still limited by the weakest-rated terminal in the circuit. This calculator shows the ampacity for the column you select — always cross-check against actual equipment terminal ratings.
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How this calculator works
NEC Table 310.16 (60 / 75 / 90 °C columns, copper and aluminum), ambient correction from 310.15(B)(1) and bundling adjustment from 310.15(C)(1).
Worked example
10 AWG copper THHN in the 90°C column, in a 10–15°C ambient, with no more than 3 current-carrying conductors.
Base ampacity from Table 310.16 is 40 A. The cool-ambient correction factor is 1.15, so the adjusted ampacity is 40 × 1.15 = 46 A, and the next standard breaker at or below that is 45 A.
Remember the terminal rule: if the breaker and device terminals are rated 60°C, the same 10 AWG wire is limited to its 60°C value of 30 A, no matter how cool the room is.
Frequently asked questions
How many amps can 12 AWG copper carry?
Table 310.16 lists 20 A at 60°C, 25 A at 75°C and 30 A at 90°C. In practice 12 AWG copper is protected at 20 A for general branch circuits because of the small-conductor rule in NEC 240.4(D).
Why use the 90°C column if terminals are only 60°C or 75°C?
The 90°C rating gives you a higher starting number to apply derating to (for heat or bundling). After derating, the result still cannot exceed the ampacity of the terminal temperature column — that is how the 90°C headroom gets used.
When do I have to derate for bundling?
When there are more than three current-carrying conductors in a raceway or cable. 4–6 conductors use 80%, 7–9 use 70%, and 10–20 use 50% of the base ampacity, per Table 310.15(C)(1).
Does this include the 125% continuous-load rule?
No. Continuous loads (3 hours or more) must be sized at 125% of the load before you compare against ampacity. Apply that first, then use this table to pick the conductor.
Common mistakes
- Using the 90 °C column for the final answer. The 90 °C column is a starting point for derating. The result may not exceed the ampacity of the lowest-rated termination, usually 60 °C or 75 °C (NEC 110.14(C)).
- Forgetting the small-conductor limits. NEC 240.4(D) caps the overcurrent protection of 14, 12 and 10 AWG copper at 15, 20 and 30 A in most cases, whatever Table 310.16 says.
- Counting grounds as current-carrying. Equipment grounding conductors count for conduit fill but not for the bundling adjustment. A neutral that carries only the unbalanced current of a 3-wire circuit is not counted either.
- Skipping the continuous-load step. A load that runs three hours or more is sized at 125% before you compare it with the conductor ampacity.
What it does not cover
The small-conductor breaker limits of NEC 240.4(D) and other overcurrent rules are flagged on the page, not fully modelled.
Sources
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