Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A residential dwelling unit is being constructed with a 200A, 120/240V single-phase service. The service entrance conductors are 250 kcmil aluminum. What is the minimum continuous current rating required for the main service overcurrent device?
- A150A
- B225A
- C175A
- D200A
Show answer & explanationAnswer & explanation
Correct answer: D. 200A
For a 200A service, the main overcurrent device must have a continuous current rating of at least 200A. The ampacity of the service entrance conductors, 250 kcmil aluminum, is 200A (NEC 310.16, 75°C column for service conductors), which matches the required service rating.
Why the other options are wrong
- A. 150A is too low for a 200A service.
- B. 225A is higher than the minimum required rating, though permissible if the conductors support it.
- C. 175A is too low for a 200A service.
Service Overcurrent Device Rating
The main service overcurrent device must have a continuous current rating not less than the noncontinuous load plus 125% of the continuous load, and not less than the ampacity of the ungrounded service-entrance conductors, or the rating of the service.
- Must protect the service entrance conductors.
- Must be rated for the service size (e.g., 200A service = 200A OCPD).
- Continuous loads require 125% factor (NEC 210.20(A)).
- Can be rounded up to the next standard size if conditions are met (NEC 240.4(B)).
Memory trick: Overcurrent Protection: Match the service, protect the wires, handle the heat.