How to Install a Motion Sensor Floodlight on an Exterior Box
Why this matters
A motion floodlight is a twenty-minute fixture swap that generates more callbacks than it should. The light that stays on all night, the one that fires every time a car passes, the box full of water by spring, the heads aimed into the neighbor's bedroom: each one comes from a step that got skipped because the swap looked like any other. The general fixture swap is 'Light Fixture Replacement Standard'. This card is what is different outdoors: the box and its rating, sealing so water gets out as well as staying out, aiming, and setting the sensor so it does what the customer asked for.
The worked job runs through the steps: a twin-head LED motion floodlight replacing an old twin-lamp holder on the corner of a detached garage, 11 ft above the driveway, controlled by a switch inside the garage door.
Ladder and power first
The ladder. An extension ladder at about a 1 to 4 angle (1 ft out for every 4 ft of height to the contact point), feet on firm ground, not on the slope of a driveway apron, and footed by a second person or tied at the top. Keep three points of contact and the work within reach of your centerline (29 CFR 1910.23, or 1926.1053 on construction work); a fixture you have to lean to reach gets a ladder move, not a stretch. On the job, the ladder went on the lawn beside the driveway, not on the gravel strip.
The power. The switch inside the door is not an isolation point: it may break only the hot, may be the wrong switch, and someone may flip it. Turn off the breaker, lock and tag it, and prove dead at the fixture with live-dead-live on a CAT III or better meter, proved on a known live source before and after (NFPA 70E Article 120, 120.5 in the 2021 edition and renumbered in 2024; work practices at 29 CFR 1910.333(b)(2)). Test every conductor in the box, including the white: in an outdoor box fed from a garage subpanel, a white carrying another circuit's return is a shock you will not expect. Old fixtures and eaves collect wasp nests; look before you put a hand behind the canopy.
Step 1. Confirm the box is a wet-location box
An exterior wall exposed to rain is a wet location. NEC 314.15 (2023 edition, as adopted) requires boxes in wet locations to be listed for wet locations, and NEC 410.10(A) requires luminaires there to be marked suitable for wet locations. The new fixture is marked; the box is the open question.
What to look for: a weatherproof cast box with threaded hubs and a listing mark inside, or a plastic box marked for wet locations, mounted so its face is flush and sealed to the siding. What fails: an indoor device box set in the siding, a plastic old-work box, a box whose face sits behind lap siding with a gap the fixture's gasket cannot seal against. A siding mounting block solves the last one and gives a flat surface.
On the job: a 4 in round cast box, three 1/2 in threaded hubs, the bottom hub open with no plug. The listing mark was readable inside. The open hub was the reason the old fixture's lamp sockets had rusted: every rain ran in. Unused openings in a box are closed (NEC 110.12(A)), and outdoors that means a threaded plug sealed in the hub. The bottom hub got a listed threaded plug with sealant on the threads.
Step 2. Confirm the box can hold the fixture
A box at a luminaire outlet has to be designed for the purpose. Under NEC 314.27(A)(1) as adopted, a wall box at a luminaire outlet is good for 50 lb unless the marking inside it states another maximum (a luminaire of 6 lb or less may also hang from other boxes on at least two No. 6 or larger screws), and a heavier luminaire needs separate support. Twin-head floodlights weigh a few pounds; the risk is not weight but the box being loose in the wall. Grab it and pull. A box that moves will let the fixture's gasket open on one side.
On the job: the cast box was screwed to the garage stud through its back and did not move. The new fixture weighs about 3 lb.
When the box fails step 1 or 2. Replace it rather than relying on the fixture's gasket to make up for it. On vinyl or lap siding, the clean fix is a siding mounting block with an integral wet-location box: cut the siding to the block's template, slip the block's flange under the course above so water sheds over it, and fasten the box to framing. The fixture then seals against a flat face, and the job takes the better part of an hour instead of twenty minutes, which is worth saying to the customer before you start rather than after.
Step 3. Wire it, and plan the switch
Land the fixture's black on the switched hot, white on the neutral, and the fixture's green or bare to the box's EGC (to the box with a green screw on a metal box, and to the circuit's EGC). Use connectors sized for the conductor combination, and keep the splices clear of the box's gasket surface.
The switch now becomes a power supply, not an on-off control. The sensor needs to be powered all the time to work, so the switch has to stay on. On many units, flicking the switch off and back on within a few seconds puts the light into a manual-on override until the next dawn or the next cycle; the method varies by model and is in the fixture's instructions. Tell the customer, and on the job a label went on the switch plate: "Floodlight: leave ON".
A motion floodlight on a dimmer or a smart switch is a conflict: the sensor's electronics and a dimmer's waveform interfere, and the light flickers or never turns off. If the customer has one on that leg, it comes off, unless both devices' instructions explicitly allow the combination.
Step 4. Seal the top and sides, leave the bottom open
Most wet-location fixtures ship with a foam gasket, and the instructions usually call for exterior sealant around the canopy as well. Where the bead goes, seen from the front with the fixture on the wall:
sealant bead across the top
******************************
* *
* canopy, gasket behind, * bead down
* 4 in cast box behind * each side
* that, its bottom hub *
* plugged and sealed *
* *
* - - - - - - - - - - - - *
bottom left open: water that gets
behind the canopy drains out here
The box stays closed and the canopy does not. Water that gets behind the canopy anyway, by wind-driven rain or condensation as the fixture cools at night, has to be able to drain. A canopy sealed all the way around holds that water against the splices; by spring the box is a cup.
On the job: gasket on, sealant along the top and sides, a 1 in gap left at the bottom center. The instructions for this model showed the same.
Step 5. Aim the heads below horizontal
Aim each head down, so the light lands on the area the customer wants lit and not into the eyes of someone approaching, a neighbor's window or the street. Many fixtures limit how far up a head can point because the lamp or its heat sink is rated for downward use; follow that limit. Some towns have lighting ordinances that limit glare or light crossing the property line; check yours on commercial jobs especially.
On the job: one head aimed down the driveway, about 30 degrees below horizontal; the other at the side door path, about 45 degrees below.
Step 6. Aim and set the sensor
The sensor is usually a passive infrared detector: it sees warm objects moving across its zones, which is why a person walking across its field triggers it more readily than one walking straight at it. Aim it slightly down, at the approach you want to catch, and away from things that move and are warm: the street, a neighbor's driveway, an HVAC condenser, a dryer vent, a tree branch in the sun.
Three controls, set from the instructions:
- Sensitivity (range). Start at the middle. Full range on a driveway that faces a street catches every passing car.
- Time. How long the light stays on after the last motion. On the job: 5 minutes, the customer's request.
- Dusk (photocell) level. Sets how dark it must be before the sensor is allowed to turn the light on. Set it toward full dark for an all-night security light, toward dusk for a light someone uses coming home in the evening.
Most units have a test position that allows triggering in daylight with a short on-time. Use it for the first aim, then set the real values.
Step 7. Restore power and test
Fixture mounted, canopy screws tight against the gasket, ladder clear of the fixture. Close the breaker standing to the side of the panel, then turn the wall switch on. Many units light for a warm-up period at power-up; let it finish.
The finish test is a walk-test at dusk, because the photocell will keep a properly set light off in daylight and a daylight test only proves the test mode.
On the job, at dusk:
- Walk up the driveway from the street. The light came on with the walker about 25 ft from the garage, inside the drive and not at the street. A passing car on the street did not trigger it.
- Walk the side path. It triggered at the corner of the garage.
- Stand still out of the sensor's view and time it. It shut off 5 minutes 10 seconds after the last motion, matching the 5 minute setting (the extra seconds are normal sensor reset delay on many units).
- Check the heads: both lit the intended areas, and from the neighbor's side of the fence neither lamp face was visible.
If it never turns off, look for something warm moving in the field (the condenser fan, a vent) before blaming the fixture. If it triggers late or not at all, lower the sensor's aim before turning sensitivity up.
What changes the answer
- Under a deep, enclosed porch roof. That may be a damp location rather than wet, which changes the listing the fixture and box need. Judge the actual exposure; a fixture rated for wet is never wrong in a damp location.
- Brick or stucco wall. The box is set in the masonry or on a surface-mount wet-location box; the sealing rule at the canopy is the same, and the mortar joint around the box gets sealed as well.
References
- NFPA 70, National Electrical Code, 2023 edition (2020 where numbering moved) as adopted, local amendments checked: 110.12(A), 314.15, 314.27(A)(1), 410.10(A)
- NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2); ladder use per 29 CFR 1910.23 (general industry) or 29 CFR 1926.1053 (construction)
- Fixture manufacturer's installation instructions (sealing, aiming limits, sensor settings)
- See related: Light Fixture Replacement Standard