Wire Ampacity Calculator (NEC 310.16)
Find the safe current-carrying capacity of a conductor after temperature and bundling derating, or work backward to the minimum wire size for a given load. Based on NEC Table 310.16 with 310.15 correction and adjustment factors.
How to use it
- Choose copper or aluminum, the insulation temperature column (60/75/90°C) and wire size.
- Enter the ambient temperature and the number of current-carrying conductors.
- Read the derated, usable ampacity (limited by the termination rating per 110.14(C)).
Final ampacity = base ampacity (Table 310.16) × ambient correction factor (Table 310.15(B)(1)) × bundling adjustment factor (Table 310.15(C)(1)), then capped at the ampacity of the termination temperature rating per 110.14(C).
Frequently asked questions
How do you calculate derated ampacity?
Start with the base ampacity from NEC Table 310.16, multiply by the ambient temperature correction factor (Table 310.15(B)(1)) and the adjustment factor for more than three current-carrying conductors (Table 310.15(C)(1)), then limit the result to the termination temperature rating per 110.14(C).
What is the ampacity of 12 AWG copper wire?
12 AWG copper is 20 A at 60°C, 25 A at 75°C, and 30 A at 90°C per NEC Table 310.16, before any derating. Terminations commonly limit it to the 60°C or 75°C column.
When do you derate for number of conductors?
When more than three current-carrying conductors share a raceway or cable, NEC 310.15(C)(1) requires adjustment: 80% for 4–6, 70% for 7–9, 50% for 10–20, and so on.
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