ok so NAC sizing. the part everybody rushes through and then stands in a corridor watching a strobe flicker at the end of a 400-foot run wondering why.
a lot of techs treat the NAC like a simple headcount. count your devices, add up the current, stay under the panel limit, done. and honestly that'll get you partway there. but the real trap isn't the total current, it's what happens to voltage by the time the wire reaches the last device. that's where jobs get kicked back and where retrofits go sideways.
let's walk through it with real numbers.
Where the current limit comes from
NFPA 72 doesn't hand you a NAC current limit. That number lives in the panel's installation manual, and it varies a lot. A Fire-Lite MS-9200UDLS runs 3A per NAC. Some Silent Knight panels give you 3.5A or 4A. Check your specific panel, not a remembered ballpark.
The limit is a hardware protection on the output circuit. Hit it and you can trip an internal fuse or send the panel into trouble. So rule one: know your panel's rated NAC output before you pull a single foot of wire.
Adding up the load
Every notification appliance has a listed alarm current on its cut sheet. Pull the numbers for the exact model and candela rating you're using. Don't use the spec-sheet max as a "safe" estimate either, use the actual rating for the specific device you're installing.
Here's what a corridor with 8 horn/strobes might look like (these are representative numbers, not a copy-paste spec):
| Device | Qty | Alarm current each | Subtotal |
|---|---|---|---|
| Horn/strobe, 15cd | 4 | 155 mA | 620 mA |
| Horn/strobe, 75cd | 4 | 185 mA | 740 mA |
| Total load | 8 | 1,360 mA |
So the circuit load is about 1.36A. On a 3A NAC, that looks fine. You've got headroom. But before you close the binder, run the de-rate check.
Why you de-rate
The 3A rating is the maximum the circuit can source. But you're not sizing for a sunny day, you're sizing for worst-case battery standby. Your panel's battery calc assumes a certain amount of standby current draw, and then the system switches to full alarm at the end of that 24-hour (or 60-hour, check your AHJ) standby period. The battery voltage has sagged. The panel output has dropped.
Most designers apply a rough 80% de-rate to available NAC current as a margin buffer, so 3A becomes 2.4A effective. Our 1.36A load still fits. Good.
Some AHJs want you to show this explicitly in the submittal. Some don't care. Know your jurisdiction.
Now the voltage drop problem
Here's where the headroom disappears.
Even if the current totals look fine, a long wire run eats voltage. The appliances at the far end of the circuit need a minimum voltage to operate, usually somewhere in the neighborhood of 16V, but check the cut sheet for your specific device. If they drop below that threshold, you get unreliable activation or no activation at all.
The voltage drop calculation is straightforward: multiply the circuit current by the round-trip wire resistance. The round-trip part catches people. A 200-foot run means 400 feet of wire total (out and back), and that's what you plug into your resistance calc.
Quick example with the same 8 devices above, wired on 16 AWG:
| Parameter | Value |
|---|---|
| Wire gauge | 16 AWG |
| Resistance (16 AWG) | approx. 4.1 ohms per 1,000 ft |
| One-way run length | 200 ft |
| Round-trip length | 400 ft |
| Total resistance | 1.64 ohms |
| Circuit current | 1.36A |
| Voltage drop | 2.23V |
If your panel's worst-case output under battery is around 20V (check the manual, this varies), that leaves roughly 17.8V at the last device. Most horn/strobes will operate. But shave another 50 feet off the wire gauge budget, or add a couple more devices mid-run, and you're threading a needle.
Bump the run to 350 feet and you're at 2.8V drop. That gets uncomfortable fast.
Splitting the circuit or adding a booster
Two real options when you run out of voltage budget.
Split the circuit. If your panel has multiple NAC outputs (most do), divide the corridor into two circuits. Cut the run in half, cut the voltage drop roughly in half. This is usually the cleaner fix and costs you nothing but a little wiring.
Add a NAC booster. For long runs or buildings where you can't loop back to the panel, a standalone booster like a System Sensor NACLP or Gentex equivalent lets you drive a remote branch off a local power supply. The booster takes the panel's supervision signal and re-amplifies it. More hardware and more to battery-calc, but sometimes it's the only practical answer on a large floor plate.
A third thing to look at: wire gauge. Going from 16 AWG to 14 AWG cuts resistance by about a third. On a borderline long run that might be all you need. Run the calc both ways before you commit to conduit fill.
The submittal part
Your AHJ needs to see the math, not just the result. That means a device schedule with currents, a circuit summary showing total load vs. rated capacity, and a voltage drop table for each NAC. On a straightforward corridor job that's maybe 20 minutes of work once you've done it a few times. On a complex multi-floor job it's a couple of hours of form filling and double-checking formulas.
Full disclosure: that's the part I got tired of doing by hand, so I built a tool called FireDeck that runs the calcs and assembles the submittal package. Obviously biased. The math above is the math either way. But if you're staring down a 40-device job and the same spreadsheet routine for the fifth time this month, there's a free trial at firedeck.app.
anyway. hope this saved somebody from a re-inspection.