Journeyman Electrician Exam (NEC)Equipment and DevicesMedium
A new 480-volt, 75 kVA, 3-phase transformer is being installed to supply a 208Y/120-volt panelboard. The primary overcurrent protection is provided by fuses. What is the maximum rating of the primary fuses permitted if the transformer's full-load primary current is 90.2 amperes?
- A125 amperes
- B100 amperes
- C110 amperes
- D150 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 110 amperes
According to NEC 450.3(B), for transformers with a rated primary current of 9 amperes or more, the maximum primary protection for fuses is 125% of the primary full-load current (PFLC). So, 90.2 A * 1.25 = 112.75 A. Since this calculated value is not a standard fuse size, the next higher standard rating is permitted, which is 110 A (NEC 240.6(A)).
Why the other options are wrong
- A. This is 125% of the PFLC, but 110A is a standard size and the next one up.
- B. This is below the calculated maximum and not the next standard size.
- D. This is too high; it exceeds the 125% rule and the next standard size.
Transformer Primary Overcurrent Protection (Fuses)
For transformers with a primary full-load current of 9 amperes or more, the maximum primary overcurrent protection with fuses is 125% of the primary full-load current. If the calculated value does not correspond to a standard fuse size, the next higher standard size is permitted.
- NEC 450.3(B) governs transformer overcurrent protection.
- For PFLC ≥ 9A, max fuse is 125% of PFLC.
- Next higher standard size is allowed if calculated value is not standard (NEC 240.4(B)).
Memory trick: Transformer's first defense: Current times factor, then standard up.