Which arrangement requires the most electrical power?

• A. A 24-volt anti-collision light circuit consisting of two light assemblies which require 3 amperes each during operation.
• B. A 1/5-horsepower, 24-volt motor which is 75 percent efficient.
• C. Four 30-watt lamps arranged in a 12-volt parallel circuit.
• D. A 1/3-horsepower, 24-volt motor which is 60 percent efficient.

Answer: (D)

Power drawn from the supply comes from both the device’s output needs and how efficiently it converts input to useful work. For motors, input power equals the output power divided by efficiency; for lamps, the total input power is the sum of the lamp wattages in parallel at the supply voltage.

• The two 24 V lights draw 3 A each, totaling 6 A. Input power is 24 V × 6 A = 144 W.

• A 0.2 hp motor at 24 V with 75% efficiency has output power of 0.2 × 746 ≈ 149.2 W. Input power is 149.2 / 0.75 ≈ 199 W.

• Four 12 V lamps in parallel consume 4 × 30 W = 120 W.

• A 1/3 hp motor at 24 V with 60% efficiency has output power of (1/3) × 746 ≈ 248.7 W. Input power is 248.7 / 0.60 ≈ 414 W.

The arrangement with the 1/3 hp motor at 24 V and 60% efficiency requires the most electrical power, about 414 W.