Conductor Ampacity and Temperature Correction

Start from the table value for the insulation type, then correct for ambient temperature and derate for more than three current-carrying conductors in a raceway. The final number is your real ampacity.

People forget the terminal temperature limit. Your 90-degree wire is still tied to a 75-degree lug, and the connection is the weakest link in the chain.

When it applies

This governs how much current a conductor can safely carry, and it applies to essentially every feeder and branch circuit you size. Insulation temperature rating, ambient heat and the number of current-carrying conductors bundled together all pull the usable ampacity down from the headline number.

Common mistakes

• Sizing to the 90 degree column when the terminations are only rated 75 degrees — the lowest-rated part of the path governs.
• Ignoring ambient correction in a hot boiler room or a rooftop conduit baking in the sun.
• Forgetting to derate once more than three current-carrying conductors share a raceway.

In the field

In an industrial run I treat the table value as a ceiling, never a starting point. I stack the temperature correction and the fill adjustment first, then compare against my terminal rating and take the smallest number. Precision here is the difference between a conductor that runs cool for decades and one that cooks its insulation.