CSLB C-10 Electrical ContractorEquipment and InstallationHard
A single-phase, dry-type transformer rated 4.5 kVA has a 480-volt primary. The calculated primary full-load current is 9.4 amperes. Per NEC Table 450.3(B), since the primary current exceeds 9 amperes, the maximum primary overcurrent protection allowed is 125% of FLC. What is the maximum standard primary overcurrent device rating permitted?
- A12 amperes
- B20 amperes
- C10 amperes
- D15 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 15 amperes
125% x 9.4A = 11.75A. Since this does not correspond to a standard overcurrent device size, NEC 450.3(B) and 240.6(A) permit rounding up to the next higher standard rating, which is 15 amperes.
Why the other options are wrong
- A. 12A is not a standard overcurrent device size recognized by 240.6(A).
- B. 20A skips over the correct next standard size and exceeds what the rule allows.
- C. 10A is below the calculated 11.75A minimum requirement.
Transformer Primary Protection (450.3(B))
For transformers 600V or less with primary current 9A or greater, primary-only overcurrent protection may be sized up to 125% of primary FLC, rounded up to the next standard device size if needed.
- If primary current is less than 9A, up to 167% is permitted
- If primary current is less than 2A, up to 300% is permitted
- Rounding to next standard size is allowed per 240.6(A) when 125% doesn't match a standard rating
Memory trick: Above 9 amps, 125% is your cap — round up to fill the gap.