Bright new LEDs can make a garage feel upgraded—until the breaker disagrees.
A row of 40-watt LED shop lights looks almost too efficient to worry about. But when the garage door opener starts, a battery charger is humming, and a small compressor kicks on, the same 15-amp circuit can suddenly be doing far more work than the fixtures suggest.
The issue is not usually one light; it is the total connected load. Tools with motors draw a brief startup surge, while heaters, chargers, freezers, and old fluorescent ballasts may run for long stretches. Warning signs can be subtle: lights dip when a tool starts, a plug or switch feels warm, or the breaker trips only on busy workdays. Counting every device on that branch circuit—not just the new lighting—gives a much safer picture of its real capacity.
- A 15-amp, 120-volt circuit has a theoretical maximum of 1,800 watts.
- For lighting that may stay on for hours, many electricians plan around roughly 1,440 watts (80% of capacity).
Use the 1,440-watt planning limit
On a typical 120-volt household branch circuit, a 15-amp breaker has a mathematical maximum of 1,800 watts: 120 volts × 15 amps. That figure describes the breaker’s rating, not a comfortable load for lights that remain on for hours.
Shop lighting is usually treated as a continuous load when it operates for three hours or more. A common planning rule is to use only 80% of the breaker rating:
- 15 amps × 80% = 12 amps
- 12 amps × 120 volts = 1,440 watts
That leaves room for normal voltage variation, warm conditions, and the brief startup demand of some equipment. It also reduces nuisance trips and avoids routinely heating the wiring and breaker near their limit.
The 1,440-watt figure applies to the entire circuit, not just the fixtures. A garage receptacle on the same breaker may also feed a battery charger, freezer, radio, or occasional power tool. Their wattage must be counted before deciding how many shop lights belong there.
Local electrical rules may be stricter, particularly in garages, workshops, or older buildings. The breaker label and panel directory are useful starting points, but a qualified electrician can confirm what is actually connected when the circuit’s use is unclear.
Add each fixture’s input watts from its label, then include every other item that normally runs on that breaker. Keep the expected long-running total at or below 1,440 watts on a 120-volt, 15-amp circuit.
Turn remaining watts into a practical light count
- Start with loads already on the circuit
Add the wattage of anything likely to run while the lights are on: a garage-door opener, freezer, battery charger, bench tool, or existing lamps. Subtract that total from the 1,440-watt lighting-planning allowance.
- Use the fixture’s listed input watts
Divide the watts left by the input power printed on each fixture’s label, manual, or specification sheet. This produces a sensible upper planning count before allowing a little room for future additions.
- A 40-watt fixture example
On a dedicated 15-amp circuit with no other expected load, 1,440 ÷ 40 equals 36 fixtures. That is the planning-limit calculation, not a target for filling every outlet; installing fewer leaves useful headroom.
- Do not count “equivalent” watts
A package may call an LED bulb “100-watt equivalent,” meaning it gives light similar to a 100-watt incandescent bulb. Its actual electrical draw might be only 14 watts. Circuit capacity is based on the smaller, real input-watt figure—not the equivalent-wattage claim.
- When labels give amps instead
At 120 volts, multiply listed amps by 120 to estimate watts. For example, a 0.33-amp fixture uses about 40 watts. If the label lists both, use the stated watts and round the fixture count down.
Fixture labels are more reliable than guessing from brightness, bulb shape, or an online listing.
Leave room for the shop’s other loads
A light-count calculation only applies to watts that are genuinely available. On a shared 15-amp circuit, reserve capacity for equipment that can run at the same time as the lights—not every device that happens to be plugged in.
For example, start with the 1,440-watt planning limit, then subtract likely overlapping loads:
| Load running with lights | Typical planning watts |
|---|---|
| Battery charger | 100–300 W |
| Garage-door opener | 400–700 W while operating |
| Small refrigerator or freezer | 100–300 W running |
| Shop vacuum or dust extractor | 800–1,400 W |
A circuit serving 800 watts of lighting and a 1,200-watt extractor is already a poor match, even though the extractor is not continuous. Its motor can draw a higher surge at startup, and a corded saw, compressor, or heater may be added at exactly the wrong moment.
A practical approach is to list the tools used during a normal work session, keep the largest simultaneous motor load off the lighting circuit where possible, and calculate fixture capacity from what remains. A fridge that cycles occasionally still deserves an allowance; a charger unplugged before work does not. Dedicated lighting circuits make the original fixture count far more realistic.
Use the number the fixture actually draws
- Input watts
This is the useful number for circuit planning: the power the complete fixture consumes from the outlet. A lamp advertised as “equivalent to 100 watts” may draw only 15 watts.
- Input amps
Some labels list amperage instead of watts. On a 120-volt circuit, watts are approximately volts × amps, so a 0.35-amp fixture uses about 42 watts.
- Voltage rating
Use the specification for the voltage actually supplied. A fixture marked 120–277 V can draw different current at different voltages; its label or manual should state the input watts.
- Equivalent watts
This marketing comparison describes light output relative to an older incandescent bulb, not electrical demand. It must not be added when totaling a circuit.
- Driver startup
LED drivers can take a brief inrush current when switched on, especially when many fixtures share one switch. The steady input-watt total remains the main figure, but leaving capacity beyond the 1,440-watt planning limit helps prevent nuisance breaker trips.
Confirm what the breaker actually serves
- Start with the shop in a normal state
Leave usual lights, chargers, and equipment connected. A circuit that looks dedicated when the shop is empty may also feed a freezer, exterior light, or neighboring receptacle.
- Switch off the suspected breaker
Use the panel label only as a starting point. Turn off one breaker at a time; do not remove the panel cover or touch anything inside it.
- Check every likely outlet and fixture
Walk the shop and adjacent spaces with a plug-in lamp or receptacle tester. Check ceiling lights, wall outlets, garage-door outlets, exterior receptacles, and anything fed from the same area.
- Record what goes dead
List each dead light and receptacle on the panel schedule. If another room, appliance, or tool outlet loses power, subtract its expected load before counting shop lights.
- Restore power and test uncertain points
Turn the breaker back on before leaving the panel. A fixture that stays lit may be on another circuit, switched separately, or supplied by an emergency or battery-backed system.
A plug-in meter is useful for loads connected through a receptacle: chargers, fans, tools, and a plug-in shop light. It does not measure a ceiling fixture wired directly to a junction box.
For hardwired fixtures, add the input watts on each fixture label or installation sheet. Measuring current on branch-circuit wiring requires access to the conductors and suitable test equipment; that is a sensible job for a qualified electrician, not a reason to open the panel.
What the numbers look like in a real shop
A fixture count can sound large until the wattage is written down. On a dedicated 15-amp lighting circuit, 12 LED shop lights drawing 40 watts each use 480 watts. That is only one-third of the 1,440-watt planning limit, leaving 960 watts of long-running capacity.
Higher-output fixtures change the picture, but not always dramatically. For example:
| Lighting setup | Total fixture draw | Share of 1,440 W planning limit |
|---|---|---|
| 8 LED strips at 25 W | 200 W | 14% |
| 12 linkable shop lights at 40 W | 480 W | 33% |
| 8 high-bay LEDs at 150 W | 1,200 W | 83% |
| 10 older fluorescent fixtures at 100 W | 1,000 W | 69% |
A shared circuit deserves a more conservative calculation. Suppose six 40-watt LED fixtures need 240 watts, while a small refrigerator averages 180 watts when its compressor runs. Reserving 500 watts for a shop vacuum, benchtop tool, or other motor load brings the expected total to 920 watts. That remains below 1,440 watts, but the refrigerator compressor and tool can both start at once; adding more permanent lighting would offer little benefit and less margin.
The practical lesson is that modern LEDs rarely overload a circuit by count alone. Eight 150-watt high-bays can be less forgiving than two dozen 40-watt shop lights, and an otherwise light lighting load may still share a breaker with the shop’s most demanding equipment.
Improve the light before adding load
A dim bench does not automatically mean the circuit needs more fixtures. First, aim existing light at the work: mount fixtures perpendicular to benches, add a focused adjustable task light, and keep tall shelving from blocking the beam. A garage lighting layout guide can help space general fixtures so the floor and work surfaces receive more even light.
Reflectance matters more than it first appears. Light-colored walls, a white ceiling, and clean fixture lenses return useful light; dark open rafters, dusty diffusers, and a bright source behind the worker create shadows instead. Task lighting at a drill press, assembly bench, or tool wall can improve visibility without materially changing the general-lighting load.
A dedicated lighting circuit is worth considering when a shared breaker trips, lights visibly dim as a motor starts, or measured/planned load leaves little margin for normal shop use. An electrician should also investigate warm switches or receptacles, buzzing, scorch marks, repeated unexplained trips, or uncertain wiring. Replacing a 15-amp breaker with a larger one is not a capacity upgrade: the wire, devices, and circuit design must match the breaker rating.
A larger breaker can let undersized wiring overheat before it trips. Add a correctly designed circuit instead, with a qualified electrician determining wire size and protection.
Make a final load check before energizing the lights
- Record the real input load
List each fixture’s labeled input watts, plus every device expected to run at the same time. Use measured wattage from a plug-in meter where possible.
- Compare the total with the planning limit
Keep continuous lighting and shared loads below 1,440 watts on a 15-amp, 120-volt circuit. Treat uncertain loads as higher, not lower.
- Protect some spare capacity
Do not design to the breaker’s 1,800-watt rating. Leave extra room for motor startup, battery chargers, and additions that may be plugged in later.
- Test during ordinary shop use
Run the lights with the refrigerator, extractor, or tools normally used nearby. Check for breaker trips, flicker, hot plugs, or unusual driver noise.
- Revise the count if conditions change
A new tool or fixture changes the calculation. Update the load list before relying on the circuit again.
Heat, repeated trips, discoloration, or loose connections call for an electrician rather than a larger breaker.
- A breaker rating is not a target load.
- The usable fixture count changes whenever shared equipment changes.
The safe number of shop lights is the number that fits within measured remaining capacity while the shop is operating normally. Conservative headroom is more useful than squeezing in the last possible fixture.
