CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A 25-horsepower, 230-volt, 3-phase motor has a Table 430.250 full-load current of 68 amperes. What is the minimum ampere rating required for the motor's disconnecting means switch?
- A136 amperes
- B78.2 amperes
- C68 amperes
- D85 amperes rounded from 115%
Show answer & explanationAnswer & explanation
Correct answer: B. 78.2 amperes
NEC 430.110(A) requires the disconnecting means for a motor circuit to have an ampere rating of at least 115% of the motor's full-load current: 68A x 1.15 = 78.2A. A disconnect switch rated at least 80A (next standard switch size) would be selected in practice, but the calculated minimum is 78.2A.
Why the other options are wrong
- A. 200% of FLC applies to short-circuit protection sizing, not the disconnect switch rating.
- C. This is the FLC itself, without the required 115% safety margin.
- D. 85A is not the correct calculated value; it misapplies the percentage.
Motor Disconnect Sizing (430.110)
The motor disconnecting means must be rated at least 115% of the motor's full-load current for motors rated 600V or less.
- Applies to the disconnect switch, not the branch-circuit protection
- Use Table 430.250 (3-phase) or 430.248 (1-phase) for FLC values
- Distinct from 250% short-circuit protection sizing under 430.52
Memory trick: 115% keeps the switch safely ahead of the motor's pull.