What is the minimum conductor size for a branch circuit supplying thirteen lighting fixtures rated at 1.25A at 120V each, retrofit LEDs draw 90W total per fixture, and run 10 hours daily?

What is the minimum conductor size for a branch circuit supplying thirteen lighting fixtures rated at 1.25A at 120V each, retrofit LEDs draw 90W total per fixture, and run 10 hours daily?

• A. 12 AWG
• B. 16 AWG
• C. 14 AWG
• D. 10 AWG

Answer: (A)

The key idea is sizing the branch-circuit conductor for a continuous lighting load. For thirteen fixtures rated at 1.25 A each, the total current is 13 × 1.25 = 16.25 A. Since the lamps run for 10 hours a day, this is a continuous load, so apply the 125% multiplier: 16.25 × 1.25 = 20.31 A. The conductor must have an ampacity at least 20.31 A. Looking at common copper conductor ampacities, 14 AWG is about 15 A (not enough), while 12 AWG is about 25 A (sufficient). Therefore, the smallest conductor that meets the requirement is 12 AWG. (If you used the LED wattage 90 W per fixture to base the load, you’d get a smaller current and could size down to 14 AWG, but with the given rating data, 12 AWG is the minimum.)

The key idea is sizing the branch-circuit conductor for a continuous lighting load. For thirteen fixtures rated at 1.25 A each, the total current is 13 × 1.25 = 16.25 A. Since the lamps run for 10 hours a day, this is a continuous load, so apply the 125% multiplier: 16.25 × 1.25 = 20.31 A. The conductor must have an ampacity at least 20.31 A. Looking at common copper conductor ampacities, 14 AWG is about 15 A (not enough), while 12 AWG is about 25 A (sufficient). Therefore, the smallest conductor that meets the requirement is 12 AWG. (If you used the LED wattage 90 W per fixture to base the load, you’d get a smaller current and could size down to 14 AWG, but with the given rating data, 12 AWG is the minimum.)