The plan review comment was one line. "Provide documentation of speech intelligibility for the emergency voice system." No section number, no hint what the reviewer wanted to see, and the project was already framed in.
That comment is why I now treat a voice job as four separate questions instead of "horns, but with speakers." Here they are in the order a tech actually runs into them.
Do you even need voice?
Usually the building code decides this, not NFPA 72. The model building codes tend to require an emergency voice/alarm communication system in high-rises, some large assembly spaces, and a few other occupancies once you cross certain size or occupant thresholds. The exact triggers shift by code edition and by what your state or city adopted.
So don't guess from memory. Find the adopted code, find the occupancy, and then ask the AHJ if the answer isn't obvious. Plenty of buildings that look like they should need voice get by with horns and strobes, and a few small ones get it required by a local amendment.
What NFPA 72 asks of the system
Chapter 24 is where emergency communications live. In plain terms it wants:
- Pre-recorded or digitized messages that are clear and tell people what to do.
- Live voice from a fire command center or other approved location, which overrides everything else.
- Messages that are intelligible in the spaces people occupy. Not loud. Intelligible.
- Circuits that keep working long enough to finish the job, which is the survivability piece. Some buildings need pathway survivability levels, which can mean rated cable, rated enclosures, or both.
- Supervision on the amplifiers, the speaker circuits, and the audio paths.
Read the section for your edition. I'm describing the shape, not quoting it.
Intelligibility is the part people skip
Intelligible means a person standing there can understand the words. A speaker array can be dead quiet on paper and perfectly clear, or loud and mushy because the stairwell is concrete and echoes like a gym.
Designers usually handle this with speaker placement and tap settings, and then somebody verifies it on site. Some jurisdictions want measured results. Some are happy with a listening test by the inspector. Check your AHJ, and ask early, because you don't want to learn their preference after the ceiling is closed.
Common trouble spots: stairwells, atriums, parking structures, mechanical rooms, anywhere with hard surfaces and a long reverb tail.
Sizing the amplifiers
Most voice systems run 25 V or 70 V audio. Each speaker is tapped at some wattage, and you add up the taps on each circuit to get the load. Then add headroom. A lot of designers leave 20 to 25 percent, but follow your manufacturer and your spec.
Here's a mid-size example I put together from a three story building:
| Area | Speakers | Tap | Watts |
|---|---|---|---|
| Lobby | 8 | 2 W | 16 W |
| Level 2 corridor | 22 | 1 W | 22 W |
| Level 3 corridor | 22 | 1 W | 22 W |
| Stairwells | 4 | 1 W | 4 W |
| Total connected | 64 W |
64 W plus 25 percent headroom is 80 W, so a 100 W amplifier is the next common size up. One wrinkle: if the panel can't drive every zone at once, the amplifier or the message priority logic might limit what plays where. Read the sheet for your exact equipment.
Standby power
Voice systems eat more current in alarm than a pile of horns, and the battery calc needs to reflect that. The general idea from NFPA 72 is a long standby period followed by a longer alarm period for voice than for a plain notification system. I believe it's 15 minutes at full alarm load for emergency voice, but confirm against your edition and your AHJ before you size the batteries.
Rough numbers for the same building, using assumed values:
| Item | Current | Time | Amp hours |
|---|---|---|---|
| Standby (panel, amp idle, devices) | 0.45 A | 24 h | 10.8 Ah |
| Alarm (amp at full 64 W load, about 70% efficient) | 3.8 A | 0.25 h | 0.95 Ah |
| Subtotal | 11.75 Ah | ||
| With 20% derating | 14.1 Ah |
That rounds up to a pair of 18 Ah batteries. Real amplifier draw depends on the model and what you play through it, so use the manufacturer's alarm current, not my estimate. The full method is in the NFPA 72 battery calculation breakdown.
Don't forget voltage drop on the speaker runs either. On a 70 V line it matters less, but on a 25 V line with long corridors it still adds up. This voltage drop walkthrough covers the method.
Wiring notes, without the terminal map
The audio riser, the speaker circuits, and the firefighter phone circuits are usually three separate wiring families, and the installation sheet for your amp or panel shows exactly what lands where. I'm not going to print terminal numbers here because they change by model and revision. The thing worth knowing is that speaker circuits are supervised differently from NACs, so the end of line device is whatever the manufacturer says, not whatever you had in the truck.
If you're using the building's speaker circuits for tones as well, check that your circuit layout still works for the temporal three pattern.
What I'd bring to the final
Bring the intelligibility documentation, the amp schedule with connected watts, a battery calc that includes the voice load, and the survivability details if the building requires them. Test every zone with the live mic, not just the recorded message. And ask the inspector how they want to verify the audio before they show up, not while they're standing in the stairwell.
Typing the same speaker counts into the amp schedule, the battery sheet and the submittal, then finding the one that never got updated, is the grind on these jobs, and full disclosure, I built FireDeck because of it, so I'm biased.
Voice jobs aren't hard. They're just fussy.