NEC generally permits a circuit breaker to carry what percent of its rating when supplying a load not operated more than 3 hours?

NEC generally permits a circuit breaker to carry what percent of its rating when supplying a load not operated more than 3 hours?

• A. 100
• B. 80
• C. 50
• D. 125

Answer: (A)

This question is about the difference between continuous and noncontinuous loads under NEC. If a load will not operate for three hours or more, it’s considered noncontinuous. For noncontinuous loads, the protection device can carry up to its full rated current, so the circuit breaker can operate at 100% of its rating. This keeps things straightforward for short operations without unnecessary derating. If the load were continuous (three hours or more), you’d size protections differently—typically ensuring the continuous load doesn’t exceed about 80% of the breaker's rating, which translates to using a breaker larger than the continuous current. For example, a continuous load of 80 A would require about a 100 A breaker (since 80 A is 80% of 100 A). So the correct concept is that noncontinuous loads allow the breaker to carry up to 100% of its rating. The other percentages don’t apply to this specific scenario.

This question is about the difference between continuous and noncontinuous loads under NEC. If a load will not operate for three hours or more, it’s considered noncontinuous. For noncontinuous loads, the protection device can carry up to its full rated current, so the circuit breaker can operate at 100% of its rating. This keeps things straightforward for short operations without unnecessary derating.

If the load were continuous (three hours or more), you’d size protections differently—typically ensuring the continuous load doesn’t exceed about 80% of the breaker's rating, which translates to using a breaker larger than the continuous current. For example, a continuous load of 80 A would require about a 100 A breaker (since 80 A is 80% of 100 A).

So the correct concept is that noncontinuous loads allow the breaker to carry up to 100% of its rating. The other percentages don’t apply to this specific scenario.