Master Electrician Exam (NEC)Overcurrent ProtectionMedium
An electrician is installing a 200-ampere feeder for a commercial building. The feeder protection is provided by a circuit breaker. What is the maximum continuous load permitted on this feeder if the overcurrent protective device is not specifically listed for 100% continuous operation?
- A120 amperes
- B250 amperes
- C200 amperes
- D160 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 160 amperes
According to NEC 210.20(A) and 215.2(A)(1), the overcurrent device for a continuous load shall be sized at not less than 125% of the continuous load. Conversely, if the OCPD is rated for 200A and not 100% rated, it can only carry 80% of its rating continuously (200A * 0.80 = 160A).
Why the other options are wrong
- A. This represents 60% of the OCPD rating, which is too low.
- B. This exceeds the rating of the overcurrent device.
- C. This would only be permissible if the overcurrent device was specifically listed for 100% continuous operation.
Continuous Load Protection
For continuous loads (operating for 3 hours or more), the overcurrent device must be sized at no less than 125% of the continuous load, or the continuous load must not exceed 80% of the overcurrent device rating, unless the device is 100% rated.
- Continuous load: 3 hours or more.
- OCPD must be 125% of continuous load.
- Continuous load must be 80% of OCPD rating (if not 100% rated).
Memory trick: Continuous loads need 'time' and 'extra' protection, like a 'long' journey.