Class A Circuit

Last updated: September 2026

Quick answer

• A Class A circuit runs out through every device and back to the panel, so it can be fed from both ends.
• A single open conductor costs you no devices. The panel splits the loop, feeds each half, and shows a trouble.
• Outgoing and return conductors must take physically separate routes, or both legs go in the same accident.
• Polarity must stay consistent around the loop, because the far section is fed from the return end once the circuit splits.
• Installation follows CAN/ULC-S524, not NFPA 72. American Class X and Class N carry no weight here.
• Ontario's Building Code does not require Class A generally. It appears as one permitted way to wire audible devices in residential suites.
• Budget double the cable back to the panel, and check the manual: some Canadian panels halve the permitted run on Class A.

What a fault actually does

On Class B an open takes out everything past the break. On Class A it does not. The panel loses continuity, isolates the break, powers one section from the outgoing terminals and the other from the return, and every detector, pull station or horn keeps working. Mircom's FA-300 manual lists "open circuit or Class A trouble" as a condition distinct from a plain open.

A short is a different animal. Class A protects against opens, not shorts. On a conventional initiating device circuit or signal circuit a short still shuts the circuit down, and on an addressable data communication link it is isolators that limit the damage.

Why the return has to go a different way

The physical difference is one extra pair from the last device back to the panel. The rule that goes with it is that the return must not follow the outgoing conductors. Run both legs through one conduit or core hole and one fire, drill bit or anchor opens both, leaving a plain open on each side. You have then paid Class A money for a Class B result.

CAN/ULC-S524 is paywalled and we have not read it, so no clause number or separation distance appears here. The figures on American pages come from NFPA 72 and are not Canadian requirements. See NFPA 72 versus CAN/ULC-S524 for why that guidance does not transfer.

Polarity and the end of line device

Notification appliances are polarised, and the panel reverses polarity to alarm the circuit. When the loop splits, the far section is fed from the return terminals, so in and out polarity has to be right at every device, module and base isolator. Get that wrong and the appliances past the break stay silent in the one situation Class A exists for.

The end of line device is the other surprise. The textbook answer is that the return to the panel does the supervising, so none is needed. On real Canadian panels that is not universal: the FA-300 manual still calls for an end of line resistor on Class A, landed where the return terminates, and halves the maximum run. Read the manual for the panel going in.

Where Canadian code actually asks for it

The Ontario Building Code requires installation to CAN/ULC-S524 and verification to CAN/ULC-S537. Where it touches circuit class is audibility inside residential suites: Sentence 3.2.4.18.(9) requires that a single open at one device not impair the audible devices serving other suites. The City of Markham reads that as permitting Class A wiring, or Class B with signal circuit isolators outside the suites, a choice worked through in residential audibility. Everywhere else, Class A is a specification decision, not a code minimum.

What we could not confirm

The standard is paywalled, so no clause number appears here for routing Class A return conductors. We also could not open the 2024 Building Code Compendium's referenced documents table, so we cannot say which S524 edition binds your permit.

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