How to use the Lighting Circuit Load Calculator
The Formula
Lighting circuits run for hours—often treated as continuous loads. Loading a breaker past 80% of its rating risks heat, nuisance trips, and dimming when other loads share the neutral. Count every luminaire on the branch before you close the drywall.
Practical Example
Total W = fixtures × W each. Amps = W ÷ V. Utilization % = 100 × amps ÷ breaker amps. Compare to 80% of breaker amps (12 A on a 15 A breaker, 16 A on 20 A) for continuous-load headroom.
About Lighting Circuit Load Calculator
Why lighting load planning matters
Lighting circuits run for hours—often treated as continuous loads. Loading a breaker past 80% of its rating risks heat, nuisance trips, and dimming when other loads share the neutral. Count every luminaire on the branch before you close the drywall.
Load math
Total W = fixtures × W each. Amps = W ÷ V. Utilization % = 100 × amps ÷ breaker amps. Compare to 80% of breaker amps (12 A on a 15 A breaker, 16 A on 20 A) for continuous-load headroom.
LED vs. legacy watts
LED retrofits cut amps dramatically—do not size new breakers using old incandescent nameplates on the same fixture count. Use actual driver watts from cut sheets.
Frequently asked questions
Q: 240 V lighting? A: Enter line voltage the circuit uses; amps = W ÷ V. Q: Dimming? A: Plan for rated wattage, not dimmed level. Q: Code compliance? A: Planning tool—follow local NEC or IEC and licensed design for permits.