NAC Voltage-Drop Calculations & Wire Sizing

1. Why the far appliance is the whole problem

A notification appliance circuit — a NAC — is the pair of conductors that carries power from a fire alarm control unit, or from a separate notification-appliance power supply, out to the horns, strobes, chimes, and speakers that warn a building’s occupants. On paper it looks like the simplest part of a fire alarm system: a power source, two wires, and a string of appliances. In practice it is where a large share of real installation defects live, because a NAC that measures fine at the panel can still fail to operate the one appliance that matters most — the one farthest from the source.

The reason is voltage drop. Every conductor has resistance, and every ampere the appliances draw pushes a voltage loss across that resistance, following Ohm’s law. The power source may put out its rated voltage at its own terminals, but by the time that power has traveled hundreds of feet of wire and delivered current to appliance after appliance, the voltage remaining at the end of the run is lower — sometimes much lower. A strobe that needs a minimum voltage to fire its lamp at the listed candela, or a horn that needs a minimum voltage to produce its rated sound level, will simply underperform or fail if the voltage reaching it has sagged below what its listing requires.

This is a life-safety failure that hides. The system passes a casual power-on test because the near appliances work and the panel shows no trouble. The defect only reveals itself at the far end, under full alarm load, when the source is at its weakest — precisely the condition a real fire creates. A strobe operating just below its rated voltage may still flash, but not at the candela the occupant-notification design counted on, so the visible signal that a deaf occupant or a person in a noisy space relies on is quietly deficient. A horn below its rated voltage produces fewer decibels than the audibility calculation assumed, so the sound that must rise above ambient noise to wake a sleeping occupant may not. None of this shows up as a trouble signal, because the wiring is intact and supervised; the circuit is electrically continuous but functionally short of what the design promised.

For that reason, notification circuits are not wired by rule of thumb and checked by eye; the delivered voltage at the most-remote appliance is calculated during design and verified during acceptance testing, exactly as the secondary-power battery is calculated and verified. The calculation is not difficult — it is Ohm’s law with two small twists — but it must be done deliberately, from the right starting numbers, and it must be redone every time the circuit changes.

The mental model for this course is a single inequality. Picture the source voltage as the top of a hill and the wire as a long downhill slope: current flowing through the conductors’ resistance bleeds voltage away as you walk out to the last appliance. The design succeeds when the voltage still standing at that last appliance — at the bottom of the slope, under the worst realistic conditions — is at least the minimum voltage the appliance is listed to operate on. Everything in this course is a way to compute the height of that slope and keep the bottom of it above the line.