Which factors determine conductor ampacity?

• A. Conductor size
• B. Insulation type
• C. Conductor size, insulation type, temperature rating, and ambient conditions
• D. Number of conductors in a raceway

Answer: (C)

Ampacity is the maximum current a conductor can carry safely in a given environment, so several factors combine to set that value. The conductor size determines how much heat the conductor can shed; a larger cross-section can carry more current. The insulation type and its temperature rating specify how hot the insulation is allowed to get, which directly limits the current. The ambient conditions describe how hot the surroundings are and how heat dissipates around the conductor, which can raise or lower the allowable current. In addition, if there are many current-carrying conductors in a raceway or cable, derating rules further adjust the ampacity, but that is a subsequent adjustment to the base value. Therefore, the factors that determine conductor ampacity are size, insulation type, temperature rating, and ambient conditions.