CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A single-phase motor has a nameplate FLC of 40 amperes. The installer plans to use an inverse-time circuit breaker for branch-circuit short-circuit and ground-fault protection. Per NEC Table 430.52, what is the maximum standard breaker size permitted without invoking any exception?
- A50 amperes
- B80 amperes
- C100 amperes
- D125 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 100 amperes
Table 430.52 permits inverse-time circuit breakers sized up to 250% of motor FLC for typical motors. 40A × 2.50 = 100A, which is a standard breaker size, so 100A is the maximum without needing the oversizing exception.
Why the other options are wrong
- A. 50A is far below the 250% allowance and unnecessarily restrictive.
- B. 80A corresponds to 200%, which is used for instantaneous-trip breakers, not standard inverse-time breakers.
- D. 125A exceeds the calculated 100A maximum and would require invoking the 430.52(C)(1) Exception for next standard size up.
Motor Short-Circuit Protection Sizing
NEC Table 430.52 sets maximum percentages of motor FLC for sizing branch-circuit short-circuit and ground-fault protective devices, varying by device type.
- Inverse-time breaker: max 250% of FLC (typical motors)
- Non-time delay fuse: max 300% of FLC
- Instantaneous-trip breaker: max 800% (or higher with exceptions)
- Exception allows next standard size up if calculated value isn't standard
Memory trick: Inverse-time breakers get two-and-a-half times the current.