How the calculation works
Start from NEC Table 310.16 ampacities (60/75/90°C). Multiply by the ambient correction factor (Table 310.15(B)(1), 30°C base) and the adjustment factor for more than three current-carrying conductors (310.15(C)(1): 4–6 = 80%, 7–9 = 70%, 10–20 = 50%, 21–30 = 45%, 31–40 = 40%, 41+ = 35%). Continuous loads are sized at 125% (210.19(A)(1), 215.2(A)(1)). With 90°C conductors, the unadjusted 75°C ampacity must still cover the load for typical terminations (110.14(C)). Cu 14/12/10 AWG are capped at 15/20/30 A and Al 12/10 AWG at 15/25 A by 240.4(D). Overcurrent protection is rounded to the next standard size in 240.6(A).
Worked example
Example: a 40 A load and a 0.80 conductor-count adjustment gives a 40 A adjusted value from a 50 A table entry. This example alone does not establish final conductor or overcurrent-device compliance; check all termination and installation limits.
Common mistakes to avoid
- Using 90°C ampacity without checking the 60 or 75°C terminal rating.
- Forgetting the 125% factor for continuous loads.
- Miscounting current-carrying conductors in the raceway.
- Assuming 14, 12 or 10 AWG can use a larger breaker than 240.4(D) allows.
Frequently asked questions
Why does 90°C wire not allow 90°C ampacity?
Most terminals are rated 60 or 75°C. 90°C ampacity can be used for derating, but the final current is limited to the terminal rating.
CEC differences?
CEC Table 1–4 and Rule 4-004 differ slightly from NEC 310.16. Verify with the CEC for Canadian work.
Is 12 AWG copper always a 20 A circuit?
Under NEC 240.4(D), overcurrent protection for 12 AWG copper is generally limited to 20 A even though the 90°C column shows 30 A.
Can I use aluminum for branch circuits?
Yes, with terminations listed for aluminum. Aluminum needs a larger size than copper for the same load.
Read the guide
How to Size Wire for a 50 Amp Circuit (NEC) · 15 Amp vs 20 Amp Circuits: Wire Size, Receptacles and When to Use Each · Subpanel Wire Size: Feeder Conductors, Breakers and Grounds · How Many Outlets Can You Put on a Circuit? NEC Rules and Practical Limits
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