How this calculator works
The calculator multiplies count by watts, then divides by 80% of the circuit you picked:
connected W = count × watts per luminaire
capacity W = circuit amps × 120 V × 0.8
= 1,440 W on 15 A
= 1,920 W on 20 A
circuits = ceil(connected / capacity)Ten × 10 W on a 15 A circuit is 100 W connected and 1 circuit. That is the figure the engine is tested against. Two hundred × 10 W is 2,000 W, which is two circuits at 80% of 15 A (1,440 W each) and still two at 80% of 20 A (1,920 W). The note switches from "fits on one" to "exceeds one" when connected load passes the cap. Switching, boxes and the adopted code still apply either way.
If count or watts is 0, the engine asks for both and reminds you that 3 W/sf is a load-calc input, not a lighting design. It will not fill in a wattage from floor area.
What each input means
| Input | What it is actually asking |
|---|---|
| Count | How many luminaires (or lamps, if that is your unit) you are planning on this set of circuits. Whole numbers. Not square footage. |
| Watts per luminaire | Actual input watts on the label. For LEDs that is often 6 to 15 W per downlight, not 60. "Equivalent" is marketing. Type the small number. |
| Circuit | 15 A or 20 A. Capacity is 80% of that at 120 V. This page does not pick cable size, breaker brand, or AFCI. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
Ten 10 W downlights on a 15 A circuit
Count 10, watts 10, circuit 15. Connected 100 W. Capacity 1,440 W. 1 circuit, and the note says it fits at 80% on these watts. Switching, boxes and the adopted code still apply. That is the tested example. It is also why LED load is usually not the reason a lighting job is expensive. The ceiling is.
A long run of cans that actually fills a circuit
Count 80, watts 18, circuit 15. Connected 1,440 W, which is exactly the 15 A cap at 80%. 1 circuit on the nose. Change watts to 19 and it becomes 1,520 W and 2 circuits. The note becomes a count conversation, not a dimmer conversation. A dimmer does not add capacity. A second homerun does.
The one where 3 W/sf misleads
A 400 sq ft kitchen, "designed" at the old 3 W/sf general-lighting figure, so 1,200 W typed as if it were a layout, then six actual 10 W LEDs installed. The naive answer is that the kitchen "uses" 1,200 W of lighting and needs to watch the circuit. The six LEDs are 60 W. 3 W/sf still belongs on the load calculator as a service-size input. Using it here as a lighting design overstates LED load, understates that switching and boxes still exist, and does not tell you whether the room is dark. Photometrics are off this page on purpose.
What changes the result
Watts per luminaire move the connected load linearly. Swapping a 10 W LED for a 60 W halogen multiplies that fixture by six. A room that fit on one circuit as LED will not, as halogen, and that is still not a reason to keep the halogen. Type the number on the label you will actually install.
Circuit rating changes capacity from 1,440 W to 1,920 W. That only changes the circuit count when you are near the cap. One hundred watts fits on either. Two thousand watts is two circuits on 15 A and two on 20 A (2,000 / 1,920, rounded up).
Count is the other linear lever. It is also where people double-count: five fixtures × five lamps × watts per lamp. Pick fixtures or lamps and stay there.
Local considerations
AFCI and GFCI requirements for lighting and for the rooms those lights sit in have expanded through successive NEC editions. Which edition your jurisdiction adopted, and its amendments, decide what a permitted lighting job has to include. This page will not list those by state. Switching location, box fill and whether a finished ceiling is opened are local labour, not a wattage question.
When not to use this
Do not use this as a lighting design, a lumen plan, or a permit. It cannot see glare, spacing, daylight, a dimmer that will not like the driver, or a joist in the way of the can.
Do not use 3 W/sf as the watts input. That figure is a load-calculation input on the service-size tool.
Do not use it as a reason to add a circuit yourself. Live work is not DIY. A finished ceiling is how a small lighting job becomes drywall work as well.
If a breaker is tripping, do not keep resetting it and do not add more fixtures to "test." Use the safety check.
HyreElectrical does not install lighting and does not perform electrical work. Matching is still being built. The enquiry form is not a booking.
Related on this site
- Lighting as a service on the homepage Replacing fixtures is usually simple. Adding them is circuits, switching and often drywall.
- Lighting needs work When the request is a layout and when it is a homerun.
- How electrical cost moves Accessibility of the run, not the LED wattage, is usually the price.
- Find electrical help An enquiry, not a dispatch.
Questions this calculator answers
- How many lights can I put on one circuit?
- Count times watts, compared with 80% of the circuit: 1,440 W on 15 A at 120 V, 1,920 W on 20 A. Ten 10 W LEDs are 100 W and fit easily. Ten 150 W incandescent cans would be 1,500 W and would not fit on a 15 A circuit at 80%. Use the number on the luminaire, not the "60 W equivalent" marketing line.
- Why 80%?
- A planning cap so the branch circuit is not drawn at its breaker rating as a continuous load. 15 × 120 × 0.8 = 1,440 W. 20 × 120 × 0.8 = 1,920 W. The adopted NEC edition and whether the lighting is continuous are the electrician calculation. This page is the 80% shape, not a stamp.
- Is this the 3 watts per square foot rule?
- No. 3 VA per square foot is a general-lighting input to a service load calculation, and that is how the load calculator uses it. It is not a lighting design, and it is not how many LED downlights belong in a kitchen. Putting 3 W/sf into this tool as if it were a layout would overstate modern LED load and still tell you nothing about glare, switching or boxes.
- Can I mix receptacles on a lighting circuit?
- Sometimes, depending on the room, the adopted code, and whether the circuit is already carrying a bathroom GFCI, a garage freezer, or a dining-room receptacle load. This tool counts luminaires only. It will not bless a mixed circuit. Ask the electrician who will land it.
- Do LED lamps mean I can ignore the circuit?
- No. Watts dropped. Switching, box fill, derating in insulated ceilings, dimmer compatibility, and AFCI or GFCI requirements did not. A kitchen full of cans can still want more than one circuit because of how it is switched and where the runs go, even when the watts would fit on one.
- What watts do I type for a fixture with several lamps?
- The total the luminaire draws, from the label. A bar with five 4 W lamps is 20 W if that is what the driver consumes. If you only have per-lamp figures, multiply, then type that as watts per luminaire and keep the count as fixtures, or count lamps and type watts each. Do not double-count.
- Is this a photometric plan?
- No. There are no lumens, no candela plots, no spacing criterion, no daylight, no task lighting. If you need to know whether a kitchen is dark, that is a designer or a lighting layout. This page will only tell you whether the connected watts fit on one or more 15 A or 20 A circuits at 80%.
- Can I add the circuit myself?
- New lighting circuits, moving switches, and anything in a finished ceiling are typically permitted work. Live work is not DIY. This page will not walk you through landing a homerun or opening a panel. HyreElectrical does not install lighting.
- How does this show up on a bid?
- As labour, cable, boxes, dimmers and making-good, not as a wattage. Type those dollars on the project cost tool if you have them. A one-circuit job in an open basement and a one-circuit job through a finished ceiling are not the same invoice.
- Do dimmers change how many lights fit?
- No. A dimmer does not add capacity. Connected watts still count at the fixture input, whether the slider is at 10% or 100%, because the circuit has to be able to carry the full connected load. Dimmer compatibility with the driver, minimum load, and buzz are separate questions, and they are off this page. If the watts already exceed 80% of the circuit, the answer is another homerun, not a dimmer.
- Does a tripping breaker mean too many lights?
- Sometimes. Sometimes it means a fault. If a breaker will not stay reset, that is a stop on the safety check, not a reason to keep swapping lamps. Do not keep resetting it.
Sources and methodology
Figures dated 26 August 2026. Last reviewed .
- NFPA 70, National Electrical Code (National Fire Protection Association, retrieved 2026-08-26. Cited as the model code. 80% of 15 A or 20 A at 120 V is the planning shape on this page, not a reprinted table. General lighting at 3 VA/sf belongs to load calculations, not to this layout-less count.)
- Home lighting and electricity efficiency (U.S. Department of Energy, Energy Saver, retrieved 2026-08-26. Cited for LED wattage being the number that matters, not incandescent "equivalents." Not a photometric method.)
- Home electrical safety guidance (Electrical Safety Foundation International, retrieved 2026-08-26. Cited for the standing rule that live work and panel work are not homeowner walkthroughs.)
Related
- Electrical load calculator 3 VA/sf general lighting lives there, as a service-size input, not as a room design.
- Panel space checker A new lighting circuit still needs a space, even when the watts are tiny.
- Electrical project cost Type the lighting line from a bid. Watts here are not dollars.
- Electrical safety check A breaker that will not hold is a stop, lighting load or not.
- Electrification readiness Lighting watts are small next to heat pump, dryer and EVSE. Those three are the 240 V conversation.