Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A branch circuit supplies a continuous commercial cooking load of 10kW at 208V, 3-phase. The circuit conductors are copper, 75°C rated. What is the minimum ampere rating required for the overcurrent protective device (OCPD) for this circuit?
- A60A
- B40A
- C50A
- D30A
Show answer & explanationAnswer & explanation
Correct answer: B. 40A
First, calculate the full-load current (FLC): I = 10,000 W / (208V * √3) = 10,000 / (208 * 1.732) = 10,000 / 360.256 ≈ 27.76A. For a continuous load, the OCPD must be rated at least 125% of the FLC (NEC 210.20(A)). So, 27.76A * 1.25 = 34.7A. The next standard OCPD size above 34.7A is 40A (NEC 240.6(A)).
Why the other options are wrong
- A. 60A is significantly larger than required, potentially allowing overload before tripping.
- C. 50A is larger than required, potentially allowing overload before tripping.
- D. 30A is too small; it does not account for the 125% continuous load factor.
Continuous Load OCPD Sizing
For continuous loads (expected to operate for 3 hours or more), the overcurrent protective device (OCPD) must be sized at not less than 125% of the continuous load current, as per NEC 210.20(A).
- Continuous load = 3 hours or more operation.
- OCPD must be 125% of the continuous load current.
- Calculated value rounded up to the next standard OCPD size (NEC 240.6(A)).
- Ensures OCPD does not trip prematurely and safely handles sustained current.
Memory trick: OCPD's Rule: Continuous loads get a 125% boost, then find the standard size.