LumaQuote Electrical calculators Lights per circuit
Lighting Circuit Calculator
How many lights can you put on a 15, 20 or 30 amp circuit? Enter the circuit voltage, breaker size and fixture watts to get the maximum per circuit, and how many circuits a job needs.
How many lights on a circuit
Divide the usable capacity of the circuit by the volt-amps of one fixture. Commercial lighting runs for hours, so it is a continuous load and the circuit is limited to 80% of the breaker rating.
Fixtures = volts × breaker amps × 0.8 ÷ (fixture watts ÷ power factor)
Round down. Use the fixture's input watts from the spec sheet, which includes the driver or ballast.
How many lights on a 15 amp circuit
A 15 A circuit at 120 V carries 1,800 VA, and 80% of that is 1,440 VA for continuous lighting. With 10 W LED lamps at power factor 0.9 (11.1 VA each), that is 129 lamps. With a power factor of 1.0 it is 144. With 12 W LED downlights at 0.9, it is 108.
How many lights on a 20 amp circuit
At 120 V, a 20 A circuit gives 1,920 VA continuous, enough for 43 LED troffers at 40 W (power factor 0.9). At 277 V the same breaker carries 4,432 VA, or 99 of the same troffers. That is why commercial buildings in the US light at 277 V, and Canadian buildings at 347 V.
Lights per circuit chart
Maximum fixtures per circuit, power factor 0.9, continuous load (80% of the breaker).
| Watts per fixture | 120 V 15 A | 120 V 20 A | 277 V 20 A | 347 V 15 A |
|---|---|---|---|---|
| 10 W | 129 | 172 | 398 | 374 |
| 15 W | 86 | 115 | 265 | 249 |
| 20 W | 64 | 86 | 199 | 187 |
| 30 W | 43 | 57 | 132 | 124 |
| 40 W | 32 | 43 | 99 | 93 |
| 50 W | 25 | 34 | 79 | 74 |
| 60 W | 21 | 28 | 66 | 62 |
| 100 W | 12 | 17 | 39 | 37 |
| 150 W | 8 | 11 | 26 | 24 |
| 200 W | 6 | 8 | 19 | 18 |
LED retrofits and circuit capacity
A 20 A, 277 V circuit carries 67 fluorescent 2-lamp T8 fixtures at 59 W, or 132 LED fixtures at 30 W (power factor 0.9 for both). A retrofit roughly halves the load on every lighting circuit. In practice the existing circuits stay as they are, but the spare capacity matters when the client wants to add fixtures, extend into a new area or add controls.
Before you quote, an area-by-area lighting audit gives you the fixture counts and input watts this calculator needs.
Limits beyond the math
- Inrush current. LED drivers draw a large current for a fraction of a second when switched on. Many driver makers publish a maximum number of drivers per breaker; with high counts, check it.
- Voltage drop. Long runs, common on parking lots and in warehouses, can need fewer fixtures per circuit or a larger wire.
- Switching and zones. Circuits often follow the switching and control zones, not the maximum count.
- Emergency and night lights. These stay on their own circuits.
Quoting lighting work?
LumaQuote is lighting audit and retrofit proposal software for electrical contractors. Count fixtures and take photos on site with the field app, even with no signal, then turn the audit into a priced proposal on your computer.
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Lights per circuit FAQ
How many lights can be on a 15 amp circuit?
At 120 V, a 15 A circuit carries 1,440 VA of continuous lighting. That is 129 lamps at 10 W with power factor 0.9, or 108 downlights at 12 W.
How many LED lights can be on a 20 amp circuit?
1,920 VA at 120 V, or 4,432 VA at 277 V, for continuous lighting. With 40 W LED troffers at power factor 0.9, that is 43 fixtures at 120 V or 99 at 277 V.
Do I have to use the 80% rule for lighting?
For lighting that runs 3 hours or more at a time, yes. That covers almost all commercial lighting. The breaker has to be rated at 125% of the load, which limits the load to 80% of the breaker.
Why use VA instead of watts?
The breaker and wire carry current, and current depends on volt-amps. A 40 W LED fixture with a power factor of 0.9 draws 44.4 VA, so it uses more of the circuit than 40 W suggests.
Why is commercial lighting on 277 V?
A 277 V circuit carries 2.3 times the power of a 120 V circuit on the same breaker, so the building needs fewer circuits and less copper. In Canada the equivalent is 347 V.
Does voltage drop limit how many lights I can add?
On long runs, yes. Keeping voltage drop to about 3% on a branch circuit is a common target. Check long runs separately or use a larger wire.
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