On a 120-volt, 20-amp circuit, how many 500-watt lights can be used safely?

On a 120-volt, 20-amp circuit, how many 500-watt lights can be used safely?

• A. Two
• B. Three
• C. Four
• D. Five

Answer: (B)

The main idea is to limit continuous loads on a branch circuit to 80% of its rating and to calculate the current draw of each fixture. A 500-watt light at 120 volts draws about 500/120 ≈ 4.17 amps. For a 20-amp circuit, the safe continuous current is 0.8 × 20 = 16 amps. Three lights would use about 3 × 4.17 ≈ 12.5 amps (about 1500 watts), which stays under the 16-amp limit. Four lights would use about 4 × 4.17 ≈ 16.7 amps (about 2000 watts), which exceeds the safe continuous limit. So, three lights is the maximum safe number on this circuit.

The main idea is to limit continuous loads on a branch circuit to 80% of its rating and to calculate the current draw of each fixture. A 500-watt light at 120 volts draws about 500/120 ≈ 4.17 amps. For a 20-amp circuit, the safe continuous current is 0.8 × 20 = 16 amps. Three lights would use about 3 × 4.17 ≈ 12.5 amps (about 1500 watts), which stays under the 16-amp limit. Four lights would use about 4 × 4.17 ≈ 16.7 amps (about 2000 watts), which exceeds the safe continuous limit. So, three lights is the maximum safe number on this circuit.