CSLB C-10 Electrical ContractorPlanning and EstimatingMedium
During a site visit for a commercial tenant improvement, an electrician observes that a portion of the existing lighting circuit wiring is visibly damaged. The circuit is 120V and serves 10 fluorescent light fixtures, each drawing 1.5A. The damaged wiring needs to be replaced and potentially extended. What is the minimum continuous current rating the new overcurrent protective device (OCPD) must have for this lighting circuit, assuming it's a general lighting load?
- A15A
- B25A
- C20A
- D30A
Show answer & explanationAnswer & explanation
Correct answer: C. 20A
Total load = 10 fixtures * 1.5A/fixture = 15A. For continuous loads (which general lighting is considered if operating for 3 hours or more), the OCPD must be sized at 125% of the continuous load (CEC 210.20(A)). So, 15A * 1.25 = 18.75A. The next standard OCPD size is 20A (CEC 240.6(A)).
Why the other options are wrong
- A. Incorrect. 15A is the actual load, but the OCPD must be sized for 125% of continuous loads.
- B. Incorrect. 25A is a standard size but is larger than the minimum required.
- D. Incorrect. 30A is a standard size but is larger than the minimum required and may not provide adequate protection for a 15A continuous load after factoring 125%.
Continuous Load OCPD Sizing
Overcurrent protective devices (OCPDs) for continuous loads (operating for 3 hours or more) must be sized at not less than 125% of the maximum continuous load current.
- Applies to feeders and branch circuits (CEC 210.20(A), 215.2(A)(1)).
- Ensures the OCPD does not trip prematurely due to heating effects.
- The calculated value is then matched to the next standard OCPD size (CEC 240.6(A)).
Memory trick: Long run, add a quarter, then pick the next fuse.