CSLB C-10 Electrical ContractorPlanning and EstimatingEasy
A residential client is planning a new laundry room. The plans specify a 120V, 20A branch circuit to serve a 4.5 kW electric clothes dryer. What is the minimum continuous load percentage that must be applied to the calculated dryer load when sizing the overcurrent protective device (OCPD) for this circuit?
- A80%
- B150%
- C125%
- D100%
Show answer & explanationAnswer & explanation
Correct answer: C. 125%
According to NEC 210.20(A), the OCPD for continuous loads must be sized at not less than 125% of the continuous load. A continuous load is one where the maximum current is expected to continue for 3 hours or more.
Why the other options are wrong
- A. 80% is incorrect; this percentage is sometimes used for non-continuous loads when applying demand factors, but not for OCPD sizing of continuous loads.
- B. 150% is incorrect; while some motor loads or specific applications might require higher percentages, the general rule for continuous loads is 125%.
- D. 100% is incorrect; continuous loads require an OCPD sized at 125% of the load.
Continuous Load OCPD Sizing
Overcurrent protective devices (OCPDs) for continuous loads (expected to operate for 3 hours or more) must be sized at a minimum of 125% of the continuous load's ampere rating.
- Continuous load: max current for 3+ hours
- OCPD must be >= 125% of continuous load
- NEC 210.20(A) and 215.2(A)(1)
Memory trick: Continuous current needs a 'boost' for the breaker.