A new commercial retail space requires a 120/208V, 3-phase, 4-wire service. The calculated demand load for the service, including continuous and noncontinuous loads, is 150A. All conductors are copper with 75°C insulation. What is the minimum ampere rating required for the service entrance conductors?
- A125A
- B200A
- C150A
- D187.5A
Show answer & explanationAnswer & explanation
Correct answer: D. 187.5A
According to NEC 215.2(A)(1) and 230.42(A)(1), feeders and service conductors must have an ampacity not less than the sum of the noncontinuous loads plus 125% of the continuous loads. If the total calculated demand load includes both, and we must ensure continuous loads are accounted for, we apply the 125% factor to the continuous portion. If the 150A already represents the *total* demand, we must consider if a portion of it is continuous. In the absence of specific breakdown, a conservative approach for service conductors is to verify that the overcurrent device and conductors are rated for 125% of the continuous load plus 100% of the noncontinuous load. Here, if the 150A is the *total* calculated load, we need to ensure the conductors are sized for 125% of the continuous portion. Assuming a worst-case where the entire 150A could be continuous, or a significant portion, the conductor ampacity must be at least 150A * 1.25 = 187.5A. Therefore, the conductors must be rated for at least 187.5A.
Why the other options are wrong
- A. 125A is too small, as it does not account for the 125% rule for continuous loads.
- B. 200A would be permissible, but 187.5A is the calculated minimum, making 200A an over-sizing if 187.5A is the exact requirement.
- C. 150A is the calculated demand load, but service conductors must be rated for 125% of continuous loads, making this insufficient if any part is continuous.
Service Conductor Sizing (Continuous Loads)
Service entrance conductors supplying continuous loads (expected to operate for 3 hours or more) must have an ampacity of not less than the noncontinuous load plus 125% of the continuous load, as per NEC 215.2(A)(1) and 230.42(A)(1).
- Continuous load is 3 hours or more of operation.
- Conductor ampacity must be at least 125% of the continuous load.
- Applies to service and feeder conductors, and their overcurrent protection.
Memory trick: Size the service for the long haul, especially continuous current.