Why only 80% of the circuit?

Lighting that operates 3 hours or more is a continuous load. NEC 210.20(A) requires the overcurrent device to be rated at least 125 percent of a continuous load, which is mathematically the same as limiting the circuit to 80 percent of the breaker rating. The tool builds in that 80 percent factor.

How many fixtures fit on a 20 A 120 V circuit?

A 20 A 120 V circuit is 2400 VA, and 80 percent of that is 1920 VA of usable continuous capacity. With 60 W fixtures that is 32 fixtures per circuit; with 100 W fixtures it is 19. The tool computes this for any wattage you enter.

Should I use lamp wattage or fixture input wattage?

Use the fixture's total input wattage from its label, which for LED and fluorescent fixtures includes driver or ballast losses and is higher than the bare lamp wattage. Sizing on lamp wattage alone underestimates the load and can overload the circuit.

Does power factor matter?

For accurate VA loading it can. The circuit limit is in volt-amperes, so a fixture with a 0.9 power factor draws more VA than its watt rating. When the label lists input VA or amps, use that directly rather than converting from watts.

Why use 277 V for commercial lighting?

Commercial buildings commonly have 480Y/277 V systems, and 277 V single-phase lighting circuits carry far more fixtures per circuit than 120 V because the VA capacity scales with voltage. A 20 A 277 V circuit gives about 4432 VA usable, more than double a 120 V circuit.

Lighting Circuit Load Calculator

Laying out a lighting panel comes down to one question: how many fixtures can each branch circuit carry. Because lighting is a continuous load, the answer is not the full circuit rating but 80 percent of it. This calculator applies that rule and tells you fixtures per circuit and total circuits for the space.

How it works

The circuit’s volt-ampere capacity is its voltage times its breaker amps. The continuous-load rule caps usable capacity at 80 percent of that:

circuit VA        = voltage × breaker amps
usable VA         = circuit VA × 0.80
fixtures/circuit  = floor( usable VA / fixture VA )
circuits needed   = ceil( total fixtures / fixtures per circuit )

The 80 percent factor is the inverse of the NEC 210.20(A) requirement that the breaker be rated 125 percent of a continuous load. Using the floor and ceiling functions keeps fixture counts whole and never overloads a circuit.

Worked example

A 20 A 120 V circuit provides 2400 VA, of which 1920 VA is usable. With 60 W fixtures that is 32 fixtures per circuit. A space with 40 such fixtures therefore needs 2 circuits, the second lightly loaded. At 277 V the same 20 A breaker gives 4432 VA usable, more than doubling the fixtures per circuit.

Notes

Always size from the fixture’s labeled input wattage or VA, not the bare lamp wattage, since LED drivers and ballasts add real load. Where the label gives input amps, use those directly. Leave headroom for future additions rather than packing each circuit to the exact maximum.