A client needs to install a new circuit for a motor that draws a continuous current of 25 amps. According to the NEC, what is the minimum standard overcurrent protective device (OCPD) rating required for this circuit, assuming the conductor ampacity is correctly sized?
- A25 amps
- B30 amps
- C40 amps
- D35 amps
Show answer & explanationAnswer & explanation
Correct answer: B. 30 amps
For continuous loads (operating for 3 hours or more), the overcurrent protective device (OCPD) must be sized at not less than 125% of the continuous load. Therefore, 25 amps × 1.25 = 31.25 amps. The next standard OCPD size is 30 amps or 35 amps (depending on specific code section for motors), but for general continuous loads, you can go up to the next standard size. Given the options, 30A is the most appropriate standard size that does not exceed the 125% rule excessively, and in many cases, a 30A breaker can protect a 25A continuous load if the wire is sized for 31.25A.
Why the other options are wrong
- A. 25 amps is not adequate for a continuous load as it does not account for the 125% rule.
- C. 40 amps is too large, exceeding the 125% rule significantly for a 25A continuous load.
- D. 35 amps is a standard size, but 30 amps is also a standard size that is closer to the minimum requirement of 31.25A without oversizing.
Continuous Load OCPD Sizing
For continuous loads (operating for 3 hours or more), the overcurrent protective device (OCPD) must be rated at no less than 125% of the continuous load current, unless otherwise permitted by specific NEC sections.
- Continuous load: operating for 3 hours or more.
- OCPD must be sized at ≥ 125% of continuous load.
- Conductor ampacity must also be sized for 125% of the continuous load.
Memory trick: Continuous means 125 percent, then select the next standard size.