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Conductor Ampacity & Derating
Three rules work together here: the base ampacity (Table 310.16), the temperature column you're allowed to use at the terminals (110.14(C)), and any derating for heat (310.15). Miss one and your answer is wrong.
The 60 / 75 / 90°C rule (110.14(C))
- Circuits 100 A or less (or 14–1 AWG): use the 60°C column unless terminals are listed for 75°C.
- Over 100 A: use the 75°C column.
- The 90°C column is for derating math only — never the final ampacity.
Example 1 — Straight ampacity
A 60 A load on 75°C terminations. What copper conductor?
- Go to Table 310.16, 75°C copper column.
- 6 AWG = 65 A ≥ 60 A.
6 AWG Cu (65 A at 75°C)
Example 2 — Derating for bundling
Six current-carrying 10 AWG THHN conductors in one conduit, 30°C ambient, 75°C terminations.
- Start at 90°C (THHN) for the math: 10 AWG = 40 A.
- Adjust for 6 conductors (Table 310.15(C)(1) = 80%):
40 A × 0.80 = 32 A
- Check the termination limit (110.14(C)): 10 AWG at 75°C = 35 A. The derated 32 A is lower, so 32 A governs ampacity.
- Check the small-conductor rule (240.4(D)): 10 AWG copper is capped at a 30 A breaker.
Ampacity 32 A, but protect at 30 A max (240.4(D))
Example 3 — Ambient temperature correction
8 AWG THHN (90°C = 55 A) in a 40°C ambient.
- Ambient correction factor for 36–40°C at 90°C (Table 310.15(B)(1)) = 0.91.
55 A × 0.91 = 50.05 A
- Then apply the termination column limit as in Example 2.
Corrected ampacity ≈ 50 A (before termination check)
Order of operations: start at the conductor's insulation rating (often 90°C) → multiply by ambient correction → multiply by bundling adjustment → then compare against the 60/75°C termination limit and the OCPD rules. The smallest result wins.