Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial building requires a 120/208V, 3-phase, 4-wire service. The calculated demand load for the ungrounded conductors is 250A. What is the minimum continuous ampacity required for the service entrance conductors, assuming they serve continuous loads?
- A250 Amperes
- B275 Amperes
- C350 Amperes
- D312.5 Amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 312.5 Amperes
For continuous loads, the service entrance conductors must have an ampacity not less than 125% of the continuous load, per NEC 215.2(A)(1) and 230.42(A)(1). Therefore, 250A * 1.25 = 312.5A. The conductors must be selected to have at least this ampacity after any applicable derating.
Why the other options are wrong
- A. This is the calculated demand load, not the required ampacity for continuous loads.
- B. This is an incorrect calculation.
- C. This exceeds the minimum required ampacity.
Service Conductor Sizing (Continuous Load)
Service entrance conductors supply power from the utility to the service equipment and must be sized to safely carry the connected load, including continuous loads.
- For continuous loads, conductors must be sized at 125% of the load.
- Applies to both branch circuits and feeders/services.
- Refer to NEC 230.42(A)(1) for service conductors.
Memory trick: Service conductors: 125% for continuous, 100% for non-continuous.