Journeyman Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial occupancy requires a feeder to supply a continuous load of 75 amperes and a non-continuous load of 50 amperes. The conductors are 75°C rated. What is the minimum required ampacity for the ungrounded feeder conductors?
- A125 amperes
- B150 amperes
- C175 amperes
- D137.5 amperes
Show answer & explanationAnswer & explanation
Correct answer: B. 150 amperes
For continuous loads, the conductors must be sized to carry not less than 125% of the continuous load. Non-continuous loads are added at 100%. So, (75 A * 1.25) + 50 A = 93.75 A + 50 A = 143.75 A. The conductors must have an ampacity of at least 143.75 amperes. The next standard conductor size would be 150 amperes.
Why the other options are wrong
- A. This incorrectly sums the loads without applying the 125% factor to the continuous portion.
- C. This would be an oversized conductor and is not the minimum required ampacity.
- D. This incorrectly applies 125% to the total load, or other miscalculation.
Feeder Conductor Ampacity (Continuous/Noncontinuous)
Feeder conductors supplying continuous loads must have an ampacity not less than 125% of the continuous load, plus 100% of any non-continuous loads.
- Continuous load operates for 3 hours or more.
- Conductor ampacity must meet or exceed calculated value.
- Based on NEC 215.2(A)(1).
Memory trick: Feeder Continuous: One and a Quarter, Plus Non-Stop.