Voltage drop

Voltage drop is the voltage lost along the conductors of a circuit under load, and on a fire alarm notification appliance circuit it is usually what limits the run, not the panel's current rating. The worst case is the last appliance on the circuit, with every appliance on that circuit drawing alarm current, at the panel's end of discharge battery voltage rather than its float voltage. If the voltage at that appliance falls below its listed minimum operating voltage, the circuit fails during a real alarm while passing every quiet test you can do at the panel. The calculation method a Canadian job is held to sits inside CAN/ULC-S524, which is paywalled, so what follows is the physics, which is not jurisdiction specific, plus a plain statement of where the compliance method lives.

Last updated: September 2026

Why the last appliance decides

Current leaves the panel along one conductor, passes through the appliances and returns along the other. Both conductors carry it, so the resistance that matters is the round trip, out and back, not the one way distance.

Each appliance draws its own current, so the segment nearest the panel carries the whole circuit's load and the segment past the last appliance carries none. Voltage falls progressively along the run and the appliance at the far end sees the least of it. Treating the whole alarm load as though it sat at the far end is the conservative simplification.

The three numbers the calculation needs

The voltage you start from. Not 24 V, and not the panel's float voltage. On normal power a nominal 24 V system sits above nominal because the charger is holding the batteries up. During a real alarm on secondary power the batteries are discharging and the supply falls toward the point where the control unit gives up. That end of discharge figure is what you start from. It is panel specific and it comes from the manufacturer's documentation for the unit in front of you. We do not publish a number for it, because borrowing one from a different panel is how a calculation looks right and is wrong.

The appliance's minimum operating voltage. Every listed horn, bell and strobe has one, published in its data sheet. It is the floor, and the voltage at that appliance under full alarm load has to stay above it with margin left over.

Conductor resistance. Resistance rises as the conductor gets smaller and the run gets longer, and copper resistance rises with temperature, so cable in a hot ceiling has more of it than the table value. Resistance per unit length is in the conductor tables in the Canadian Electrical Code. The drop is that resistance, counted out and back, multiplied by the current through it. There is nothing Canadian about the physics.

Where the Canadian compliance method lives

Inside CAN/ULC-S524, the installation standard, which is copyrighted and paywalled. We have not read it and we do not reproduce it.

Section 32 of the Canadian Electrical Code gives conductor minimums, No. 16 AWG for individual conductors pulled into a raceway and No. 19 AWG laid in. Those are minimums, not a sizing method, and on a long notification circuit the drop is what makes you go up a size. We are also not going to hand you an American worked example or a US voltage drop table: the appliance ratings, the conductor tables and the standard behind them are different documents in the two countries.

What a Canadian specification will hold you to

Owner specifications are where you can read a real Canadian number for free. BCIT's published Division 28 technical standard requires fire alarm wiring to be sized for a maximum 3 per cent voltage drop at maximum load at the last device in the run, signalling circuits at No. 16 AWG minimum and in accordance with the manufacturer, and notification appliance circuits loaded to not more than 80 per cent of their listed rating in amperes.

That is one institution's requirement for its own campus, not a code rule, and it does not bind your project. It is worth knowing because a circuit drawn at 100 per cent of rating has already failed that kind of specification before anybody measures anything.

What goes wrong

Sizing at float voltage. The circuit measures fine on normal power and fails on batteries. It is invisible to every quiet test you can do at the panel.

Appliances added without redoing the sum. Strobes added to an existing NAC during a fit-up change the current and the drop for everything downstream of them.

Transponder circuits loaded against the main panel's ratings. A transponder usually carries lower rated circuits than the head end, and the drawing rarely prints the number.

Neighbouring terms

The NAC is the circuit this bites on. The end of discharge condition comes out of the battery calculation and the durations in standby power. The conductor rules are in fire alarm cable in Canada.

Fire Alarm Academy provides educational content only. It does not confer any certification, licence or credential, and it is not affiliated with, endorsed by, or accredited by the Canadian Fire Alarm Association, the Electrical Contractors Association of Ontario, ULC Standards, UL Standards and Engagement, CSA Group, the Electrical Safety Authority, any authority having jurisdiction, or any manufacturer. Always work from the edition of any standard adopted by your authority having jurisdiction.