Wire Ampacity Calculator
Get allowable ampacity from NEC Table 310.16, with the 310.15(C)(1) adjustment for more than three current-carrying conductors applied automatically — base value, derating, and the worked steps.
Adjusted ampacity
50.0A
Terminations usually limit you to the 75°C column (110.14(C)).
How to find conductor ampacity (NEC)
- 1
Look up the base ampacity
Find your conductor size in NEC Table 310.16 under the correct temperature column (60°C, 75°C, or 90°C). Copper #8 at 75°C, for example, is 50 A.
- 2
Apply the conductor-count adjustment
When more than three current-carrying conductors share a raceway, derate per 310.15(C)(1): 4–6 conductors × 80%, 7–9 × 70%, 10–20 × 50%, 21–30 × 45%, 31–40 × 40%.
- 3
Respect the termination temperature
Even if you adjust from the 90°C column, the final usable ampacity is limited by the lowest-rated termination — usually 60°C or 75°C per 110.14(C).
Ampacity — frequently asked questions
What is conductor ampacity?
Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. For most building wire, you read it from NEC Table 310.16 by conductor size and termination temperature column.
When do you derate conductor ampacity?
Per NEC 310.15(C)(1), when more than three current-carrying conductors are bundled in a raceway or cable, you multiply the base ampacity by an adjustment factor (80% for 4–6, 70% for 7–9, 50% for 10–20, and so on). Ambient temperatures other than 30°C add a separate correction (Table 310.15(B)).
Which temperature column should I use?
Start at the column matching your conductor's insulation rating (often 90°C for THHN) for adjustment math, but the final ampacity is limited to the rating of the terminations — typically the 60°C or 75°C column under 110.14(C).
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