Field Guide • Lighting & Life Safety
How to Wire an Exit Sign and Emergency Light
Updated July 16, 2026 • Written by the field team at Arizona Electrical Solutions. All field guides →
Exit signs and battery-backup emergency lights — bugeyes and exit/emergency combo units — are the simplest life-safety gear you'll ever wire: three conductors and done. But they're commonly miswired, because the hard part isn't the splice — it's picking the right circuit. The NEC calls these self-contained units 'unit equipment,' and Article 700 is specific about where their power comes from.
The logic matters: a battery unit only transfers when it loses AC power. Feed it from a different circuit than the room's lights and the lighting breaker can trip, the room goes black, and the unit sits there happy on its own live circuit — doing nothing. That's a failed inspection at best, a dark stairwell at worst.
This guide covers tapping the area lighting circuit ahead of switching, the three-wire connection, commissioning, testing duties, and when a generator or inverter takes over from unit equipment.
Safety first. This work is for qualified, licensed electricians. Before touching any conductor, de-energize the circuit, apply lockout/tagout, and verify absence of voltage with a tester proven on a known live source. Wear appropriate PPE per NFPA 70E for the task and exposure. Locations come from the approved life-safety plan, and permits and AHJ inspections apply. The locally adopted NEC edition and local amendments govern — verify before rough-in.
What you'll need
- UL 924 listed exit signs, emergency units, or combo units per plans (most accept 120/277 V dual input)
- 4-inch square box or the unit's integral back box
- 12 AWG THHN in raceway or MC cable whip to match the branch-circuit wiring method
- Wire connectors, box connectors, and grounding pigtails
- Voltage tester (proven live-dead-live) and lockout/tagout kit
- Circuit tracer to confirm which circuit serves the area lighting
- Screwdrivers, strippers, level, and a ladder or lift
- Breaker lock-on device if using the separate-circuit exception
- Labels and a written test log for owner turnover
Code references
| NEC 700.12 | Unit equipment fed from the same branch circuit as the area's normal lighting, ahead of local switches, with a narrow separate-circuit exception. |
| NEC 700.3 | Acceptance test of the complete system conducted or witnessed by the AHJ, plus periodic testing and written records. |
| NEC 700.10 | Identification and independence of emergency system wiring — governs generator and central-inverter emergency circuits. |
| NEC 700.16 | Emergency illumination requirements; failure of any single lighting element cannot leave a space requiring emergency light in total darkness. |
| NFPA 101 7.9 | Emergency lighting performance: 90-minute duration, 1 fc average/0.1 fc minimum initial illumination, monthly and annual testing. |
| NFPA 101 7.10 | Exit sign placement, visibility, viewing distance, and illumination for marking the means of egress. |
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
Step by Step
How to Wire an Exit Sign and Emergency Light
1. Confirm locations from the life-safety plan
Locations come off the approved egress drawings, not off your gut. NFPA 101 requires exit signs visible from any direction of egress travel, and emergency illumination must provide an initial 1 footcandle average along the egress path at floor level, no point below 0.1 footcandle. Walk the space before rough-in: nothing should block sightlines to a sign, and heads must aim down the egress path. Field changes go through the design team and AHJ.
2. Identify the area lighting circuit — this is the whole game
NEC 700.12 requires unit equipment to be supplied by the same branch circuit that serves the normal lighting in the area, connected ahead of any local switches. Tap the constant hot feeding the room's lights — before the wall switch, occupancy sensor, relay, or dimmer. If power to that area's lighting fails — breaker trip included — the unit sees it and fires.
Trace and confirm the circuit; don't trust an old panel schedule. One exception: in a separate and uninterrupted area supplied by at least three normal lighting circuits that aren't part of a multiwire branch circuit, a dedicated unit-equipment branch circuit is permitted — from the same panelboard as the normal lighting circuits, with a lock-on breaker. Anywhere else: same circuit, ahead of the switch.
3. Rough in the box and the connection point
Pull your tap from a J-box where the constant hot is available — upstream of all local control. Set a box or use the unit's integral back box, and run raceway or an MC whip matching the lighting circuit's wiring method. Exit signs typically go over doors per the plans; wall-mounted units usually land around 7 ft 6 in. so heads clear door swings.
4. De-energize, lock out, verify
Kill the lighting circuit at the breaker, lock it out, tag it, and verify absence of voltage at the connection point with a tester checked on a known live source first. You're killing the area's lights, so coordinate the outage.
5. Make the three-wire connection
The standard hookup is three conductors: unswitched hot to the unit's hot lead (on 277 V systems use the 277 V lead and cap the unused one per the manufacturer), neutral to neutral, and equipment ground to the unit's ground lead or screw. No switched conductor is needed — the charger and power-loss sensing both live on the unswitched hot.
Some units ship an extra lead so lamp heads can follow the local switch while the charger stays on constant power. If the design doesn't use it, terminate it per the manufacturer — usually tied to the unswitched hot or capped. Never land the charger lead on the switched leg.
6. Connect the battery and button up
Batteries ship with a lead disconnected. Land the battery connector per the manufacturer. Mount the unit square and level, aim the heads down the egress path, and set exit-sign chevrons to match the direction of travel on the plans.
7. Energize and commission
Energize and check the charge indicator LED on every unit. Press and hold the test switch: AC drops, the unit transfers to battery, and the lamps or sign face come on. Release and confirm it returns to charge.
For acceptance, let batteries charge per the manufacturer — commonly 24 hours — then run a full 90-minute discharge. NFPA 101 requires at least 1.5 hours of illumination, permitted to decline to a 0.6 footcandle average and 0.06 footcandle minimum at the end. NEC 700.3 requires the AHJ to conduct or witness the acceptance test of the complete system — schedule it.
8. Hand off the testing duties
Under NFPA 101, traditionally tested units get a 30-second functional test monthly and a 90-minute test annually, with written records kept for the AHJ. Self-diagnostic units automate the tests but still need periodic checks of their status indicators. Put the schedule and a log sheet in the O&M package and walk the facility person through a test switch — five minutes now saves a citation later.
9. Know when unit equipment isn't the answer
On larger buildings — high-rises, hospitals, big assembly spaces — emergency lighting often runs on a generator or central battery inverter instead of a wall of bugeyes. Those are true emergency systems: dedicated transfer equipment, with wiring kept independent of normal wiring per NEC 700.10. The tap-the-area-lighting rule applies to unit equipment, not generator-fed emergency circuits. Hybrids exist too — fixtures with listed emergency battery drivers, inverter-fed exit signs — so read the fixture schedule first.
Watch Out
Common mistakes
- Tapping the switched leg instead of the constant hot — every switch-off reads as a power failure and drains the battery overnight.
- Feeding the unit from a spare circuit that doesn't serve the area lighting — if the lighting breaker trips, the room goes dark and the unit never transfers.
- Using the separate-circuit exception without a breaker lock-on device or from a different panelboard — both conditions are mandatory.
- Leaving the battery shipping disconnect in place — press the test switch and nothing happens.
- Running the 90-minute acceptance test before the battery has charged per the manufacturer, then condemning good units that die early.
- Chevrons pointing the wrong way or heads aimed at a wall instead of down the egress path — inspectors catch both constantly.
- Handing over the job with no test log — NFPA 101 expects monthly and annual testing with written records, and the owner inherits that duty.
FAQ
Frequently asked questions
Why does an exit sign have to be on an unswitched hot?
A battery unit only transfers when it loses AC power, so it must sense power where occupants would lose light. Wired there, a flipped switch doesn't drain the battery, but a real power loss on that circuit fires the unit.
Does the exit sign have to be on the same circuit as the room lights?
Yes, NEC 700.12 requires unit equipment on the same branch circuit serving the area's normal lighting, ahead of local switches. The exception is a dedicated circuit in an area served by at least three normal lighting circuits, from the same panelboard, with a lock-on breaker.
How long do emergency lights have to stay on when power fails?
NFPA 101 requires not less than 1.5 hours, which is why the annual test is a 90-minute discharge. Illumination may decline over that period but not below a 0.6 footcandle average and 0.06 footcandle minimum along the egress path.
How often do exit signs and emergency lights get tested?
Traditional units get a 30-second test monthly and a 90-minute test annually, with written records kept for the fire marshal. Self-testing units run those tests automatically but still need periodic indicator checks.
If the building has a generator, do exit signs still need batteries?
Not necessarily. Fixtures fed from a generator or central inverter through proper transfer equipment and independent Article 700 wiring don't require batteries in each unit. Many designs use battery units anyway, so follow the drawings.
What do the small LED and button on the unit do?
The LED is the charge indicator, showing AC power is present and the charger is working. The button is the test switch: it drops AC so the unit transfers to battery.
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