Conductors, Cables, and Ampacities
Ampacity is a chain of limits: the conductor's insulation rating, the termination temperature, ambient temperature, and the number of current-carrying conductors bundled together. The lowest limit governs.
The method
Start with the ampacity table for the conductor type and size. Apply the ambient temperature correction factor and the conductor-bundling (derating) factor. Then compare the result with the termination rating of the device or equipment-commonly 60 °C or 75 °C. A 90 °C insulated conductor can be derated from the 90 °C column, but the final ampacity may not exceed the termination limit, so a 90 °C conductor on a 75 °C breaker is effectively a 75 °C conductor. Neutral conductors count as current-carrying when they carry harmonic current or are part of a multiwire circuit with non-linear loads; grounding conductors never count for derating.
Traps
- Applying the 90 °C ampacity to a termination rated 75 °C.
- Forgetting to apply both ambient and bundling factors, or applying one twice.
- Using the wrong table for the conductor or cable type, or ignoring the minimum size rule for the circuit.
- Sizing conductors from the breaker rather than the calculated load, or ignoring the 125 percent continuous-load factor.
Worked example
A 40 A continuous load requires 50 A of conductor and protection. Three current-carrying conductors in a raceway at 40 °C ambient: take the 75 °C ampacity, apply the 40 °C correction and the 80 percent bundling factor, and confirm the result still exceeds 50 A. If not, move up one size.