A material takeoff for a new commercial office space includes several lighting branch circuits. Each circuit is 120V, 20A, and will supply 12 fluorescent lighting fixtures, each drawing 1.5A. The circuit will be protected by a circuit breaker. What is the minimum continuous load rating (in VA) that must be used for sizing the overcurrent protective device for each circuit?
- A1800 VA
- B2400 VA
- C2880 VA
- D2160 VA
Show answer & explanationAnswer & explanation
Correct answer: D. 2160 VA
Lighting loads are considered continuous, requiring the overcurrent protective device to be sized at 125% of the continuous load. For 12 fixtures at 1.5A each, the total current is 18A. Multiplying this by 125% gives 22.5A, which, when converted to VA (22.5A * 120V), is 2700VA. However, the question asks for the minimum continuous load rating *for sizing the OCPD*, which means the actual continuous load multiplied by 125%. The total VA load is 12 fixtures * 1.5A/fixture * 120V = 2160 VA. The OCPD must be sized for 125% of this, but the question asks for the load rating *used for sizing*, which is 125% of the actual load.
Why the other options are wrong
- A. 1800 VA is the non-adjusted total VA for 10 fixtures, not 12, or the current * voltage for 15A.
- B. 2400 VA is the maximum capacity of a 20A, 120V circuit (20A * 120V).
- C. 2880 VA (24A * 120V) is beyond the 20A circuit rating.
Continuous Load Sizing
For continuous loads (expected to operate for 3 hours or more), the overcurrent protective device (OCPD) must be sized at no less than 125% of the load's ampere rating.
- NEC 210.20(A) and 215.2(A)(1) mandate the 125% rule for continuous loads.
- This rule applies to both branch circuits and feeders.
- The OCPD must be rated for 125% of the continuous load, or a combination of noncontinuous and 125% of continuous loads.
Memory trick: Continuous loads: 'Time' for a 'Boost' to your 'Protection'.