CSLB C-20 HVAC ContractorControls, Electrical and SafetyHard
A single compressor-motor unit has a nameplate rated-load current (RLA) of 18 amps. Per NEC 440.12, what is the minimum ampacity rating required for the disconnecting means serving this compressor (based on the 115% rule), rounded up to the nearest standard disconnect size?
- A30 amps
- B20 amps
- C60 amps
- D18 amps
Show answer & explanationAnswer & explanation
Correct answer: A. 30 amps
NEC 440.12 requires the disconnecting means to have an ampere rating of at least 115% of the motor's RLA: 18A × 1.15 = 20.7A. Since disconnect switches are manufactured in standard sizes (30, 60, 100A, etc.), the technician must select the next standard size at or above 20.7A, which is 30 amps.
Why the other options are wrong
- B. 20A is below the calculated 20.7A requirement and not a standard disconnect size typically stocked for this application.
- C. 60A is oversized well beyond the minimum requirement, though not incorrect for safety, it is not the minimum required size.
- D. 18A equals RLA exactly and does not meet the required 115% minimum.
Compressor Disconnect Sizing (115% Rule)
NEC 440.12 requires a motor-compressor disconnecting means to be rated at least 115% of the motor's nameplate rated-load current (RLA), rounded up to the nearest standard size.
- Formula: Minimum disconnect rating = RLA × 1.15
- Standard disconnect sizes: 30, 60, 100A, etc.
- Applies to horsepower-rated or ampere-rated disconnect switches
Memory trick: 115% keeps the switch safely ahead of the load.