A hardwired emergency light with battery backup requires a continuous AC source to charge the internal battery and a specific relay logic to illuminate the LEDs during a power failure. The core wiring requirement for standard 120V AC units (like the Lithonia ELM2L or Dual-Lite LZR series) is an unswitched hot for charging, a neutral return, and an equipment ground. If the unit controls egress lighting that must switch off during normal hours, a secondary switched hot (switch leg) is also required.
The Emergency Light with Battery Backup Wiring Diagram: Symbol Key & Terminal Map
Before pulling wire, you need to translate the schematic on the inside of the unit's housing into physical connections. Here is what the standard symbols mean in this context:
- AC Source (Circle with Sine Wave): Represents your 120V AC mains supply from the breaker panel.
- Battery Stack (Parallel Lines): The internal DC power source (typically 6V/9.6V NiCd, or 3.2V LiFePO4 in newer 2025/2026 models).
- DPDT Relay (Rectangle with Coil): The internal transfer switch. When AC power is present, the coil is energized, keeping the battery disconnected from the LEDs. When AC drops, the coil de-energizes, and the normally-closed (NC) contacts route battery power to the LEDs.
- LED Array (Diode with Outward Arrows): The light-emitting diode load.
Physical terminal blocks on commercial emergency units are standardized. Below is the exact terminal-to-wire mapping based on US NEC color codes for 120V AC circuits.
| Wire Color (NEC) | Terminal Label | Function | Voltage / State |
|---|---|---|---|
| Black | L or AC HOT | Unswitched Hot (Charges battery & powers normal operation) | 120V AC Continuous |
| White | N or AC NEU | Neutral Return | 0V (Nominal) |
| Bare / Green | G or GND | Equipment Grounding Conductor | 0V (Fault path only) |
| Red (or Black w/ tape) | SW or TRIG | Switched Hot (Optional: turns off normal lighting) | 120V AC (Only when wall switch is ON) |
Node-by-Node Wiring Trace: Source to Load
This textual trace assumes a standard 120V AC branch circuit using 14 AWG or 12 AWG copper NM-B (Romex) or THHN in conduit. We are tracing a 'switched' configuration, which is the most complex and common for egress corridors where lights must turn off at night but still act as emergency backups.
- Panel to Junction Box: The unswitched hot (Black) and neutral (White) originate from a dedicated or shared 15A/20A breaker. They enter the ceiling junction box.
- Switch Loop Integration: A 3-wire cable (Black, Red, White, Bare) runs from the ceiling box to the wall switch. The Black wire acts as the 'always hot' feed down to the switch. The Red wire returns as the 'switched hot' (switch leg) back up to the ceiling box.
- Splicing at the Ceiling Box:
- Connect the panel Neutral (White) to the unit's Neutral (White) and the switch loop Neutral (if present) using a wire nut.
- Connect the panel Ground (Bare) to the box ground screw and the unit's Ground (Green/Bare).
- Connect the panel 'Always Hot' (Black) to the unit's L (AC HOT) terminal. This ensures the internal battery charges 24/7, regardless of the wall switch position.
- Connect the 'Switched Hot' (Red) returning from the wall switch to the unit's SW (Trigger) terminal.
- Internal Device Logic (Normal Power): AC voltage at the 'L' terminal powers the internal battery charge circuit. Simultaneously, if the wall switch is ON, voltage at the 'SW' terminal energizes the internal relay coil. The relay pulls the contacts, connecting the internal AC LED driver to the mains, illuminating the light.
- Internal Device Logic (Power Failure): Mains voltage drops to zero. The battery charge circuit stops. The internal relay coil de-energizes. The spring-loaded relay contacts snap back to their Normally Closed (NC) position. This physically connects the internal DC battery directly to the LED array, illuminating the light for the mandated 90 minutes per NFPA 101 Life Safety Code.
Verifying Your Connections with a Multimeter
Never assume a wiring diagram is correct without bench-testing the physical installation. Set your multimeter to the following modes to verify the circuit before applying main power and closing the housing.
1. Verify AC Mains (L to N):
Set the meter to AC Voltage (V~). Place the red probe on the Black (L) wire and the black probe on the White (N) wire. You should read between 114V and 126V. If you read 0V, the breaker is off or a backstabbed connection has failed.
2. Verify Ground Path Continuity:
De-energize the circuit. Set the meter to Continuity (or Ohms). Place one probe on the unit's bare ground wire and the other on a known grounded metal part of the building (like a metal junction box or conduit). The reading must be less than 1.0 ohm. A high reading indicates a broken ground path, which is a severe shock hazard and a code violation.
3. Verify Internal Battery Polarity and Health:
Open the unit and locate the battery connector. Set the meter to DC Voltage (V⎓). Place the red probe on the red battery wire and the black probe on the black battery wire. A healthy 6V NiCd pack will read around 6.2V to 6.5V at rest. A 3.2V LiFePO4 pack (common in modern UL 924 listed LED units) will read around 3.3V. If the reading is below 80% of nominal, the battery has sulfated or degraded and must be replaced before the unit will pass a fire marshal inspection.
Frequently Asked Questions
How do I wire an emergency light to an unswitched hot circuit?
If you are wiring to an unswitched hot (meaning the light stays on 24/7), you only need a 2-wire cable (Black, White, Bare) from the breaker panel. Connect the Black wire to the unit's 'L' terminal, the White to 'N', and the Bare to 'G'. You will cap off or ignore the 'SW' terminal on the unit. This is common in stairwells and commercial corridors where egress lighting must never be turned off by a wall switch.
Why is my hardwired battery backup light staying on when power is restored?
If the LEDs remain illuminated after AC power is restored, you likely have a 'loss of neutral' or a shared neutral ghost voltage issue. If the neutral wire is loose or shared with a different phase on a multi-wire branch circuit, the internal relay coil may not receive the full 120V required to pull the contacts open. Check the neutral splice in the junction box with a torque screwdriver (ensure it's tight). Alternatively, the internal relay contacts may have welded together due to an arc fault; if tightening the neutral doesn't fix it, the internal PCB must be replaced.
Can I wire multiple emergency lights to the same 20-amp breaker?
Yes, you can daisy-chain multiple units on a single 20A breaker, provided you calculate the total load. Modern LED emergency combo units draw very little power—typically between 2W and 5W during normal operation. A 20A breaker at 120V provides 2400W of capacity (derated to 1920W for continuous loads per NEC 210.20). You could theoretically wire hundreds of LED units on one circuit. However, local fire codes often require emergency lighting circuits to be segregated from general receptacle circuits to prevent a tripped breaker (from a vacuum cleaner or power tool) from disabling the egress lighting. Always consult your local Authority Having Jurisdiction (AHJ) regarding dedicated circuit requirements.
