Journeyman Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial building has a 200-ampere, 120/208V, 3-phase, 4-wire service. The feeder supplying a panelboard has a calculated non-continuous load of 80 amperes and a continuous load of 100 amperes. What is the minimum ampere rating for the overcurrent protective device (OCPD) for this feeder?
- A180 amperes
- B225 amperes
- C200 amperes
- D250 amperes
Show answer & explanationAnswer & explanation
Correct answer: B. 225 amperes
For continuous loads, the overcurrent device must be sized at not less than 125% of the continuous load. Non-continuous loads are added at 100%. Therefore, (100 A * 1.25) + 80 A = 125 A + 80 A = 205 A. The next standard OCPD size from NEC 240.6(A) is 225 amperes.
Why the other options are wrong
- A. This only sums the continuous and non-continuous loads without applying the 125% rule for continuous loads.
- C. This is the service size, not the calculated OCPD size for the feeder, and it doesn't correctly apply the continuous load factor.
- D. This is an available standard size, but it is larger than necessary based on the calculation.
Continuous Load OCPD Sizing
Overcurrent protective devices (OCPDs) for circuits supplying continuous loads must be sized at a minimum of 125% of the continuous load plus 100% of non-continuous loads.
- Continuous load operates for 3 hours or more.
- OCPDs protect conductors and equipment.
- Must use next standard OCPD size if calculated value is not standard.
Memory trick: Continuous Current Calculates Carefully.