Lighting circuits: fixture count times watts, not a design
This is load on a branch circuit, not lumens on a desk. Ten 10 W luminaires are 100 W, which fits on one 15 A circuit at 80%. The old 3 W per square foot figure is a load-calculation input on the service-size tool. It is not how you design a room.
80%of 15 A or 20 A at 120 V1,440 W on a 15 A circuit, 1,920 W on a 20 A circuit. Connected load is count × watts. Not photometrics. Live work is not DIY. HyreElectrical does not install lighting.
Type how many luminaires, watts each (the LED figure on the label, not an "equivalent"), and 15 A or 20 A. Ten 10 W lamps are 100 W connected, one circuit at 80% of 15 A.
Eighty 15 W cans are 1,200 W, still one 15 A circuit; add enough and it becomes a count conversation, not a dimmer conversation. Switching, boxes and the adopted code still apply. Live work is not DIY.
Count, watts, 15 A or 20 A
LED watts on the label, not an equivalent. Nothing is emailed. Not a lighting layout.
Count × watts. 80% of a 15 A or 20 A, 120 V circuit. Planning, not a stamp and not photometrics. Live work is not DIY. HyreElectrical does not install lighting.
LED broke the old rule of thumb, and most lighting planning is still using it
The instinct that a room full of downlights needs its own circuit comes from a world where each of those downlights drew fifty or sixty watts. That world ended, and the arithmetic ended with it.
Our arithmetic on two representative wattages. The exact figures depend on your fittings; the ratio is the point.Our own illustration. No product is named and no measurement is claimed.
The old constraint was real. Forty incandescent downlights was thousands of watts, which is more than a 15 A circuit can carry continuously. Lighting genuinely needed several circuits, and dividing a house into lighting zones by load was sensible engineering.
The same forty fittings today are a few hundred watts. That is comfortably inside one circuit with room left over. On load alone, an entire modern house of lighting could sit on very few circuits — and that is exactly the conclusion people find uncomfortable, because it contradicts an intuition formed on hot lamps.
So why does anyone still run multiple lighting circuits? Because load stopped being the binding constraint and other things took over. Not putting an entire floor in the dark when one breaker trips.
Keeping a bathroom’s protection separate from a bedroom’s. Switching arrangements. Future flexibility. Those are all good reasons, and none of them is a wattage calculation.
The honest summary. A modern lighting plan is divided for resilience and control, not for capacity. If someone justifies a circuit count purely by adding up LED wattage, the arithmetic will not support them, and it is worth asking what the real reason is — because there usually is a good one.
And a caution against the opposite error. Very few circuits is not automatically better either. One tripped breaker taking out every light in the house is a genuinely bad night, and the marginal cost of an extra lighting circuit during construction is small compared with adding one afterwards.
The Code’s lighting allowance is a floor for calculation, not a description of your lights
The lighting allowance is the second place the old world persists, and it confuses people who try to reconcile the Code’s number with their own.
The allowance itself.NEC 220.82(B)(1) requires “3 volt-amperes/ft2 for general lighting and general-use receptacles”. For a 2,000 square foot dwelling that is 6,000 volt-amperes of general lighting and receptacle load, before a single fitting is chosen.
Your actual lighting is nothing like that. A whole house lit with LED may draw a few hundred watts with everything on. The Code’s figure is roughly an order of magnitude larger than the real connected lighting load in a modern dwelling.
That is not an error in the Code. Read the wording again: it is “general lighting and general-use receptacles”.
The allowance is standing in for everything that gets plugged into an ordinary outlet as well — lamps, televisions, vacuum cleaners, tools, chargers, a space heater somebody brings home in November. Lighting is the smallest part of what it covers.
Which is why it has not fallen as lamps got efficient. The plug loads did not fall. If anything the number of things plugged into a house has risen steadily, and the allowance is doing that work now rather than the lighting work it is named after.
And why the demand factor exists on top of it. NEC 220.82(B) then applies “The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads” — because nobody has every lamp and every appliance running at once.
Two layers of conservatism, and the result is a service that has coped with fifty years of appliances nobody anticipated.
The practical takeaway for planning. Do not try to reconcile your lighting schedule with the Code’s allowance. They are answering different questions: yours is about circuits and switching, the Code’s is about sizing a service. Adoption and amendment of the National Electrical Code is local.
A state or city may be on an older edition, may amend it, and may add rules of its own. Confirm the adopted edition and the amendments with the authority having jurisdiction before relying on any code figure here.
What actually limits a lighting circuit now
Six constraints, roughly in the order they bind on a modern job. Load is fifth. The last column is what to do about each one.
Constraint
What it is
When it bites
What to do
Inrush from LED drivers
Electronic drivers draw a very brief, very high current spike at switch-on. Many fittings switching together multiply it.
On large arrays of downlights on one switch — the classic symptom is a breaker that trips only at the moment of switching on, never in use.
This is a real design consideration and a specialist one. Splitting the array across switching or circuits is the usual answer, and the driver manufacturer’s data is where the numbers live.
Dimmer and driver compatibility
A dimmer designed for a resistive filament and a fitting with an electronic driver do not automatically get along.
Flicker, buzzing, a limited dimming range, or lights that will not go off at the bottom of the travel.
Match the dimmer to the fittings using the manufacturer’s own compatibility list. This is the single most common complaint after a lighting job and it is entirely avoidable at specification time.
Switching and control layout
How many places a light is controlled from, and whether the control is a switch, a dimmer or a system.
At design time. Adding a control position later means opening a wall.
Decide this on a plan before anything is wired. It costs nothing on paper and a great deal afterwards.
Resilience — what goes dark together
Whether one tripped breaker leaves a staircase, a bathroom or an entire floor without light.
The first time something trips at night.
Deliberately split lighting so that no single circuit removes all light from a route out of the building. This is a better reason for multiple circuits than load has been for twenty years.
Connected load against circuit rating
The old constraint. Continuous loads are held to 80 percent of the circuit rating.
Now, mostly on very large arrays or where non-LED sources remain.
Still worth calculating, and still the thing this tool does — but expect it to pass comfortably on a modern job.
Protection requirements
NEC 210.12(B) covers 14 named categories of dwelling area, and NEC 210.8(A) reaches receptacles in a long list of locations.
At specification, and again at inspection.
Ask which circuits will be AFCI or GFCI protected and confirm the fittings and drivers are known to behave with them. A device that trips is usually a device that worked; investigating the cause is the work.
Our own ordering of the constraints, written for planning rather than design. It is not a design method and it names no product. HyreElectrical does not perform, supervise or warrant electrical work, does not open panels, and does not pull permits.
What lighting costs to run, now that the answer is boring
Worth stating once, because a fair amount of lighting is still sold on an energy argument that the previous decade already won.
The arithmetic. Thirty LED downlights at 9 W is 270 W. Run for four hours a day, that is about 394 kWh a year, which at the US average residential price of 18.34¢/kWh is roughly $72 a year. This is our arithmetic on a published price and two assumed inputs, not a published figure.
The same thirty fittings at 65 W each would have been about $522 a year on the same assumptions. That difference is the entire historic energy argument for LED, and it has already been made in almost every house.
Which is why the argument has moved on. Between two LED options today, the running-cost difference is negligible.
The differences that matter are color quality, dimming behavior, glare, the lifetime of the driver, and whether you can still buy a replacement in five years — and only the last of those has anything to do with the electrical installation.
A note on integrated fittings. Where the LED is not replaceable, the failure of a driver is the failure of a fitting. That is a real consideration on a job with forty of them in a ceiling, and it is worth asking about at specification rather than discovering in year seven.
And the rate is yours, not ours. EIA’s June 2026 residential prices run from 13.37¢/kWh to 52.72¢/kWh across states. Use your own bill.
What a lighting plan should settle before anyone runs a cable
Every item here is free to decide on paper and expensive to change afterwards. That asymmetry is the whole reason to have a plan.
Where each light is controlled from, and by what
Switch positions, two-way and multi-way arrangements, and whether any of it is a control system. Adding a control position later means opening a finished surface.
Which fittings are dimmed, and with which dimmer
Checked against the fitting manufacturer’s compatibility list, not assumed. This is the most common source of post-installation complaint and it is a specification problem, not a wiring one.
What goes dark together when a breaker trips
Draw it. No circuit should be able to remove all light from a staircase or a route out of the building.
Which circuits will carry AFCI or GFCI protection
NEC 210.12(B) covers a long list of dwelling areas and NEC 210.8(A) reaches receptacles in many locations. Confirm at specification, not at inspection.
Whether any array is large enough for switch-on inrush to matter
Large numbers of drivers on one switch is a genuine design question with a specialist answer. Raise it before the ceiling closes.
How exterior and wet-location fittings are rated and protected
A fitting suitable for a covered porch is not necessarily suitable for an exposed one, and the protection requirements differ outdoors.
Whether a lamp can be replaced, or whether the fitting is the lamp
On integrated fittings the driver’s life is the fitting’s life. Worth knowing at purchase for anything installed in a ceiling in quantity.
A circuit count justified purely by adding up LED wattage
The arithmetic will not support it, and it usually means the real reasoning — resilience, switching, protection — has not been articulated. Ask for that reasoning instead; it is generally sound.
What this tool cannot do
The planner is not a lighting design. It has no opinion about how many fittings a room needs, where they go, what color temperature suits it, or whether the result will be pleasant. Those are the questions a lighting designer answers and this is not one.
It does not model inrush. Switch-on current for large arrays of electronic drivers is a real constraint, it is manufacturer-specific, and we did not verify any figure for it. It is described here and not calculated.
It does not check dimmer compatibility. That is a product-by-product question answered by the manufacturers’ own lists.
It does not size a conductor or check voltage drop. Conductor ampacity (NEC Article 310), including temperature correction and the terminal-temperature limits that decide the real answer, is not quoted numerically on this site. It is a design question with more inputs than any web form has, and a wrong number there is a fire.
It does not know your protection requirements. Adoption and amendment of the National Electrical Code is local. A state or city may be on an older edition, may amend it, and may add rules of its own.
Confirm the adopted edition and the amendments with the authority having jurisdiction before relying on any code figure here. Which circuits require AFCI or GFCI protection depends on the edition your jurisdiction has adopted and any local amendments.
And it prices nothing. There is no national average price for a panel upgrade, a service change, an EVSE circuit or a rewire on this site, because we hold no dataset of bids and the figures that circulate in roundups come from lead-generation sites with unpublished methods.
HyreElectrical does not perform, supervise or warrant electrical work, does not open panels, and does not pull permits.
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
Inputs on this tool, in the order they appear on the form.
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 labor, 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 inquiry form is not a booking.
Count times watts, against 80% of the circuit: 1,440 W on a 15 A, 120 V circuit, 1,920 W on 20 A. Ten 10 W LEDs are 100 W and fit easily; ten 150 W incandescent cans are 1,500 W and do not fit on 15 A. Use the fixture's rated watts, not its equivalent.
Why 80%?
It is a planning cap so a branch circuit is not loaded to its breaker rating as a continuous load: 15 × 120 × 0.8 = 1,440 W, and 20 × 120 × 0.8 = 1,920 W. Whether the lighting counts as continuous under your adopted code is the electrician's calculation.
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, which is how the load calculator uses it. It is not a lighting layout, and entering it here would overstate modern LED load while saying 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, spacing or task lighting here. Whether a kitchen will be bright enough is a job for a lighting layout. This page only tells 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 labor, 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 adds no capacity: connected watts count at full, because the circuit must carry the full load whatever the slider says. Dimmer and driver compatibility, minimum load and buzz are separate questions. If the watts exceed 80% of the circuit, the answer is another circuit, 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 sizing basis 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 calculator3 VA/sf general lighting lives there, as a service-size input, not as a room design.
Panel space checkerA new lighting circuit still needs a space, even when the watts are tiny.