Master Electrician Exam (NEC)Overcurrent ProtectionHard
A new commercial refrigeration unit with a total connected load of 45 amperes (continuous) is being installed. The circuit conductors are rated for 75°C. What is the minimum continuous current rating required for the overcurrent protective device (OCPD), considering NEC 210.20(A) for continuous loads?
- A60 amperes
- B45 amperes
- C50 amperes
- D56.25 amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 60 amperes
According to NEC 210.20(A), the overcurrent device for continuous loads must be sized at 125% of the continuous load. For a 45-ampere continuous load, 45 A * 1.25 = 56.25 A. Since 56.25 A is not a standard overcurrent device size (NEC 240.6), the next higher standard size is permitted, which is 60 amperes.
Why the other options are wrong
- B. This rating is equal to the continuous load but does not account for the 125% requirement for continuous loads.
- C. This is a standard size, but it is less than the calculated 56.25A, so it would likely trip prematurely.
- D. This is the calculated minimum (45A x 1.25). However, it is not a standard overcurrent device size.
Continuous Load OCPD Sizing
For continuous loads (expected to operate for 3 hours or more), the overcurrent protective device (OCPD) must have a rating not less than 125% of the continuous load current.
- Continuous load is 3 hours or more (NEC 100).
- OCPD must be sized at 125% of the continuous load (NEC 210.20(A)).
- Next higher standard OCPD size is permitted if 125% value is not standard.
Memory trick: Continuous loads are a '125%' commitment, so size the 'OCPD' accordingly, then go 'Next'!