Point one five foot-candles. That was the lowest reading on a stairwell landing I was asked to look at, and it passed. The building still failed, because the average along the path came in under one. Nobody on that job had been watching the average.
Emergency lighting gets lumped in with fire alarm because the same people end up testing it and the same inspector signs off on both. It is usually not on the fire alarm panel at all. So here is the whole answer, the way I'd want it handed to me mid job.
What the code is actually asking for
The two documents you will hear about are NFPA 101 (the Life Safety Code) and the International Building Code, with your state or local amendments on top. They agree on the big picture, and the numbers below are the ones I see most. Your AHJ may have adopted a different edition or changed something, so check before you quote any of it on a drawing.
- Emergency lighting covers the path of egress: corridors, stairs, exit discharge, and usually rooms over a certain occupant load.
- It has to come on automatically when normal power fails.
- It has to run for 90 minutes.
- Illumination is measured along the floor of the path, not at the fixture.
That last one trips people up. A bright head on the wall means nothing if the floor under it is dark.
The numbers
Most editions I've worked under use the same set of targets for the path of egress:
| Measure | Typical requirement |
|---|---|
| Duration | 90 minutes |
| Average illumination at start | 1 foot-candle at floor level |
| Minimum at any point at start | 0.1 foot-candle |
| Average at the end of 90 minutes | 0.6 foot-candle |
| Minimum at the end of 90 minutes | 0.06 foot-candle |
| Max to min ratio | 40 to 1 |
Dont quote me on the exact end of duration values for your jurisdiction, because some have tweaked them. The shape stays the same: start decent, fade gracefully, never go black in a spot.
What my walk looked like
Here is that stairwell landing, anonymized. Six readings along the travel path with a light meter held near the floor:
| Point | Reading (foot-candles) |
|---|---|
| Door to landing | 1.8 |
| Mid landing | 1.4 |
| Top of flight | 0.9 |
| Mid flight | 0.6 |
| Bottom of flight | 0.3 |
| Corner behind door swing | 0.15 |
Minimum is 0.15, which clears 0.1. Max to min is 12 to 1, fine. Add them up and divide by six and you get about 0.86. Under the 1.0 average. The fix was one more head on the lower flight, not a rewire. Nobody was thrilled that a passing minimum still failed the job, but that is how the average works.
Transfer time and power
Lighting has to be live fast. The usual language is that the emergency source takes over within 10 seconds of normal power loss, and plenty of systems do much better than that. Battery units do it nearly instantly.
There are two common ways to power it:
- Unit equipment, which is a battery, charger and lamp heads in one housing. Cheapest, most common in small buildings.
- A central inverter or generator feeding dedicated emergency circuits. Bigger buildings, bigger paperwork.
For unit equipment, the circuit feeding the charger needs to be an unswitched leg off the same branch circuit that serves the normal lights in that area. That way, if someone kills the lights at the wall, the unit still thinks everything is fine, and if the breaker trips, the unit sees the loss and lights up. Check the installation sheet for your exact unit for how the feed and any remote head wiring lands, because that varies by model and I'm not going to guess at it here.
Remote heads on a unit get sized against the battery. Watts of the heads times 1.5 hours has to fit inside the battery's capacity, with the manufacturer's margin. Same idea as a fire alarm battery calc, just a different duty cycle. If you want the panel side of that, see fire alarm battery calc by panel brand.
Exit signs are a separate conversation
Illuminated exit signs have their own rules and their own power requirement, and they are often counted in a different line of the code. Many exit signs have built in batteries, some are self luminous. Same 90 minutes applies in most places. Do not assume an exit sign contributes to your path illumination numbers. Some AHJs count it, some don't.
Testing and records
The usual schedule I see is a 30 second functional test every month and a full 90 minute test once a year. Self testing units do the monthly one on their own and flash a status light, but someone still has to look at the light and write it down.
Keep a log. Date, unit, pass or fail, who did it. An inspector who asks for the last twelve months and gets a blank page is going to write that up before they look at a single fixture.
Where the fire alarm connects
Mostly it doesn't, but sometimes it does. A few jobs I've done had lighting control relays that were supposed to drop out or force on with a fire alarm signal, and that was spelled out in the sequence of operations. If your drawings show a relay tied to alarm, treat it as a normal control output and verify it against the sequence. Read the relay's installation sheet for what lands where. If nothing in the drawings ties it in, leave it alone.
Also remember the panel's own battery is a different thing. It runs on its own duty cycle and has nothing to do with the 90 minutes here. More on that in NFPA 72 secondary power requirements.
One more thing
Full disclosure, I built FireDeck, mostly because I was sick of retyping the same room and device lists into every form a job wanted.
Anyway. Walk the path with a meter once before the inspector does, and write the numbers down.