Typical rule for breaker sizing relative to conductor ampacity?

• A. The overcurrent device must not exceed the ampacity of the conductor and must protect it from overheating.
• B. Breaker must be twice the conductor ampacity.
• C. Breaker sizing is independent of conductor ampacity.
• D. Breaker must be at least equal to conductor ampacity.

Answer: (A)

Ampacity is the maximum current a conductor can carry safely without overheating, considering insulation, insulation temperature rating, and ambient conditions. The overcurrent protective device (the breaker) must be sized to interrupt before the conductor overheats, which means its rating must not exceed the conductor’s ampacity. In practice, you select a breaker that is equal to or less than the conductor’s ampacity so normal operation stays within the conductor’s thermal limits, but a fault will trip the breaker to protect the conductor. If the breaker were larger than the conductor’s ampacity, the conductor could overheat before the protection acts. The idea that breaker sizing is independent of conductor ampacity or that it must be at least equal to the ampacity would fail to protect the wiring from overheating.