Emergency light voltages—made simple. This technical note explains line (AC) input vs. battery/remote-head voltages, how dual-voltage chargers work, and what to match on labels so your units charge, transfer, and run for the full 90 minutes. For a broader life-safety overview, see the Emergency Lighting Guide.
Last updated: October 2025
Overview: Two Voltage Worlds
Every UL 924 emergency light lives in two voltage worlds: the line (AC) side that powers the charger during normal operation, and the battery (DC) side that drives the LEDs during a power outage. On the label you’ll see the AC input (e.g., 120/277 V) and, separately, the battery or remote-head voltage (e.g., 6 V or 12 V). Get both right and you’ll meet the full 90-minute runtime without surprises.
Voltage Diagram (AC vs DC)
Line (AC) Input: 120/277 V & Universal Chargers
- Dual-voltage chargers (120/277 V): most unit equipment auto-senses either voltage. Tie to an unswitched branch so the charger stays active.
- Universal 100–240 V: some models accept global mains; still verify frequency tolerance (50/60 Hz) on the nameplate.
- Transfer: on loss of AC, solid-state transfer switches to battery in < 1 s (typ. < 0.5 s).
Tip: If a local circuit is switched, use a listed emergency transfer device so the charger remains energized while normal lights can still be switched.
Battery Circuits: 3.6/6/12 V & Lamp Loads
Emergency lamps run on low-voltage DC from the battery pack—commonly 3.6 V (small internal packs), 6 V, or 12 V. Battery capacity is sized in watts (or volt-amps) for 90 minutes.
- Load math: add the wattage of all heads (on-board + remote). Load × 1.5 h must be ≤ battery watt-hours.
- Buffer: leave 10–20% margin for temperature, aging, and LED tolerances.
- Chemistry: SLA/Ni-Cd are common; LiFePO4 recharges faster and lasts longer in many specs.
Remote-Capable & Remote Heads (6 V vs 12 V)
Remote-capable base units power extra lamp heads installed away from the enclosure. This is where battery voltage matters:
- Match voltages: 6 V heads to 6 V bases; 12 V heads to 12 V bases—no mixing.
- Wire runs: longer runs favor 12 V to reduce voltage drop; upsize copper on long home runs.
- Budget wattage: remote heads consume from the base unit’s 90-minute watt rating. Re-check runtime after adds.
Deep dive: Remote-Capable & Remote Heads Guide
Common Mismatches & How to Avoid Them
- Heads/base voltage mismatch: 12 V heads on a 6 V base (or vice versa) won’t meet runtime—match labels.
- Overloading the battery: added remote heads push beyond 90-minute capacity; either downsize spacing or choose a higher-watt base.
- Line mis-tap: dual-voltage units wired to the wrong input lead; always confirm lead selection and breaker voltage.
Voltage Checklist (Spec & Install)
- Confirm AC input (120/277 V or universal) on charger label.
- Verify battery/remote voltage (3.6/6/12 V) and match all remote heads.
- Calculate 90-minute load (on-board + remote) and leave 10–20% margin.
- Plan conductor gauge & distance to minimize voltage drop (12 V preferred on long runs).
- Connect to an unswitched branch; if switched, use a listed transfer device.
- Document initial 90-minute test with lamp aiming photos for the AHJ packet.
FAQ
Can I power 12 V remote heads from a 6 V base?
No—match the head voltage to the base unit. Mixing voltages will fail runtime and may damage components.
How do I know if my charger accepts 240 V?
Check the nameplate: it must state 100–240 V or include a 220–240 V input range. If not present, assume 120/277 V only.
What’s the fastest way to verify load?
Add the wattage of each lamp head and compare to the base unit’s 90-minute watt rating—then add 10–20% headroom.