How the calculation works
Ambient correction: NEC Table 310.15(B)(1) (30°C base). Adjustment for >3 current-carrying conductors: 310.15(C)(1) (4–6: 80%, 7–9: 70%, 10–20: 50%, 21–30: 45%, 31–40: 40%, 41+: 35%). Multiplier = ambient × bundling. CEC Tables 5A/5C are comparable.
Worked example
40°C, 75°C wire, 8 conductors: 0.88 × 0.70 = 0.616; 100 A → 61.6 A.
Common mistakes to avoid
- Using the wrong insulation temperature column for the ambient correction.
- Applying the adjustment to conductors that are not current-carrying.
- Rounding the breaker up before applying derating.
Frequently asked questions
Neutrals count?
Only when carrying unbalanced/harmonic current (310.15(E)).
Does derating apply to short conduit nipples?
The NEC exempts nipples of 24 in. or less from the conductor-count adjustment under 310.15(C)(1) exceptions.
Are ambient and bundling factors combined?
Yes, they are multiplied together and applied to the table ampacity.
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