What This Calculator Does
The Maximum Lights per Circuit Calculator estimates how many light fixtures you can safely connect to a single electrical circuit. It multiplies the circuit voltage by the breaker amperage to get total capacity, applies the common 80% continuous-load derating rule, and divides by the wattage of each fixture. The result is rounded down to a whole number of fixtures.
Note: This tool reflects the widely used US/NEC-style 80% continuous-load guideline (common breaker sizes 15A, 20A, 30A and voltages 120V, 240V, 277V). Always confirm with your local electrical code and a licensed electrician — circuit design also depends on wire gauge, other connected devices, and inrush current.
How to Use It
Pick your circuit voltage (120 V is standard for US residential lighting), choose the breaker rating in amps, and enter the wattage of one fixture. Use the actual draw of the bulb or driver — for LED fixtures this is far lower than the equivalent incandescent rating, so you can run many more lights.
The Formula Explained
Capacity in watts = Volts × Amps. The 80% factor reserves headroom for continuous loads (lighting that runs more than 3 hours). Dividing usable watts by watts per fixture and flooring gives a safe whole-number count:
$$\text{Max Fixtures} = \left\lfloor \frac{\text{Voltage (V)} \times \text{Breaker (A)} \times 0.8}{\text{Watts per Fixture}} \right\rfloor$$
Worked Example
A 120 V circuit on a 15 A breaker with 60 W incandescent fixtures: usable watts = \(120 \times 15 \times 0.8 = 1{,}440\ \text{W}\). Dividing by 60 W gives 24 — so up to 24 fixtures. Switch to 10 W LEDs and the same circuit supports 144 fixtures.
FAQ
Why only 80%? Electrical code limits continuous loads to 80% of a breaker's rating to prevent nuisance trips and overheating.
Does this include switches or outlets? No — it assumes a dedicated lighting circuit. Subtract any other loads first.
What wattage should I use? Use the fixture's rated power draw (watts), not the lumen output or the incandescent equivalent.
