SettingsSettings for this calculationUS
The total number of light fixtures on this circuit.
On THIS circuit, not in the room. Rooms are routinely split across two circuits precisely so one tripped breaker does not take the lights out, and counting a room's fixtures onto a single circuit both overloads the calculation and misses the reason the split exists. Emergency fittings are usually fed separately and do not belong in this count.
The rated wattage of each fixture (or lamp/driver combination).
The INPUT power the fixture draws from the circuit, driver included — not the lamp's own rating and not the incandescent-equivalent figure printed on retail packaging, which describes brightness rather than consumption. An LED driver takes more than the array it feeds. Where fixtures on the circuit differ, use the largest rather than an average: the breaker sees the sum, and the sum of averages is not the sum.
The nominal voltage supplying the circuit.
120/240 V is North American residential, 277 V US commercial lighting (480 V wye), 347 V Canadian commercial lighting (600 V wye), and 230 V UK/EU. Use the line-to-neutral voltage the fixtures actually see, not the service voltage.
The rating of the protective device on this circuit.
UK domestic lighting is normally a 6 A MCB, sometimes 10 A; a US lighting branch circuit is normally 15 A, occasionally 20 A. Enter the device that is actually in the board, not the one you would like — the check below is only as good as that number.
Total connected lighting load
720 W
The circuit's current draw is 6.0 A, at or below 16.0 A — 80% of the breaker rating, which NEC 210.19 and 210.20 set for a continuously-loaded circuit. Under BS 7671 the design check is instead Ib ≤ In ≤ Iz — load under device, device under cable capacity — with no 80% derate for lighting, so this comparison is on the cautious side of the British rule rather than a statement of it. Being inside the rating on this one check settles nothing about the rest of the circuit — the conductors, the overcurrent device, and the work as installed are all outside it.
- Circuit current draw
- 6 A
- Maximum continuous load (80% of breaker)
- 16 A
They open the calculator with your figures already in it
Lighting Circuit Connected Load Calculator: 720 W — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Connected load = fixture count × watts per fixture; per NEC 210.19/210.20, a continuously-loaded branch circuit's total load must not exceed 80% of the breaker's rating (equivalently, the breaker must be rated at least load ÷ 0.8)
Inputs used
- Number of Light Fixtures
- 12
- Watts per Fixture
- 60
- Circuit Voltage (V)
- 120
- Breaker Rating (A)
- 20
Intermediate steps
- Circuit current draw
- 6 A
- Maximum continuous load (80% of breaker)
- 16 A
Confidence note: The circuit's current draw is 6.0 A, at or below 16.0 A — 80% of the breaker rating, which NEC 210.19 and 210.20 set for a continuously-loaded circuit. Under BS 7671 the design check is instead Ib ≤ In ≤ Iz — load under device, device under cable capacity — with no 80% derate for lighting, so this comparison is on the cautious side of the British rule rather than a statement of it. Being inside the rating on this one check settles nothing about the rest of the circuit — the conductors, the overcurrent device, and the work as installed are all outside it.
What this calculation does not cover
- Watts divided by volts is the current only at unity power factor, and lighting is not at unity. LED drivers and ballasts pull current out of phase with the voltage, so the real figure is watts divided by volts times power factor — at 0.9 that is 11% more current than shown here, and uncorrected budget drivers sit nearer 0.5, which roughly doubles it. The breaker responds to current, not to watts.
- Steady-state load says nothing about the moment of switch-on. Electronic drivers charge their input capacitors in the first milliseconds at many times running current, and enough drivers on one circuit will trip a B-curve MCB or a thermal-magnetic breaker every time the lights are turned on, on a circuit that clears this check with room to spare. That is a driver-count and breaker-curve question, and it is decided nowhere on this page.
Add the equipment this sizes
This result is a specification — 720 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Part of bigger jobs
This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Connected load = fixture count × watts per fixture; per NEC 210.19/210.20, a continuously-loaded branch circuit's total load must not exceed 80% of the breaker's rating (equivalently, the breaker must be rated at least load ÷ 0.8)
Which documents these citations point at
- National Electrical Code (NFPA 70) — 210.19 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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