The two ways to wire residential audibility in Canada: Class A, or Class B with suite fault isolators

Last updated: September 2026

Quick answer

• The rule is a performance requirement, not a wiring method: audible devices inside a dwelling unit or a suite of residential or care occupancy must be connected so a fault at one device does not knock out the devices serving the other suites
• Canadian practice satisfies it two ways: Class A wiring, or Class B wiring with a suite fault isolator. A third option, a separate signal circuit per suite, sits in the same code sentence
• Which path is open to you is jurisdictional. The Ontario sentence addresses a single open circuit. The Vancouver By-law version says open circuit or short circuit
• Because of that phrase, City of Vancouver Bulletin 2022-002 requires Class B plus a suite fault isolator and states that Class A circuits shall not be used in lieu of isolators
• The isolator goes outside the suite it protects, provides wire to wire short circuit protection, and must still let an alarm sound after a trouble has already been registered
• Acceptance is proven by imposing a wire to wire short circuit inside each suite and showing the corridor devices are unaffected
• Hotel and motel sleeping rooms in suites without a kitchen are excepted, per the Vancouver bulletin

The decision a residential job actually forces

At rough in on any residential or care building with sounders inside the units, someone has to answer one question: how do you wire the in suite audible devices so that a fault in one unit does not take out the units next door.

The building code does not answer it for you. It states the outcome it wants and leaves the method open. Three answers are in circulation, and only two of them are wiring classes:

1. Class A wiring on the circuit serving the in suite devices.
2. Class B wiring with a suite fault isolator at each suite.
3. A separate signal circuit for each suite, which avoids the question entirely and is normally only practical on small buildings or where the panel has circuits to spare.

Most of the argument on real projects is between the first two.

Why the choice exists at all

Picture a twelve storey rental building, four suites to a floor, a sounder inside every unit, all hanging off shared notification circuits that run up a shaft and out along the corridors.

Now put one fault in one suite. A tenant drives a screw into a wall to hang a shelf. A renovation nicks a conductor above a dropped ceiling. Water gets into a device. On an unprotected circuit, a short at that one device drags the whole circuit down: suite 402 goes quiet, and so do 403, 404, 501 and everything else on that circuit. The occupants of those suites are asleep, and nothing tells them anything is wrong.

That is the failure mode the requirement exists to prevent, and it is why the topic sits alongside supervision and fault tolerance rather than alongside ordinary power wiring. An isolator is the hardware answer to it.

What the code sentence actually says, and why two cities read it differently

This is the part that explains everything else, and almost nobody reads both documents side by side.

Read the middle row again. Ontario's clause (a) is written around an open circuit. Vancouver's is written around an open circuit or short circuit.

That addition changes the answer, and the bulletin walks you through the logic. Where damage produces an open circuit, it says the "not impair operation of other audible signal devices" criterion may be achieved by using Class A wiring. Where damage produces a short circuit, it says the in suite devices must be provided with Class B wiring and a suite fault isolator. Then Note 4 closes the door. Written for the purpose of Clause 3.2.4.18.(8)(a) of the by-law, it reads:

Class A signal circuits shall not be used in lieu of suite fault isolators for the audible signal devices within dwelling units or suites of residential or care occupancy.

So in the City of Vancouver there is one in suite path, not two. If you have read elsewhere that the Vancouver bulletin offers a choice, read Note 4 yourself.

In Ontario the choice survives. City of Markham's Builder Tip 57 treats clause (a) as satisfied by Class A wiring or by Class B wiring with signal circuit fault isolators installed outside the suite protected, and treats clause (b), a separate circuit per suite, as the third route.

On the sentence numbers. We could not open the Building Code Compendium itself. The Ontario numbering above comes from Markham's builder tip, which cites 3.2.4.18.(9) and (10) under the 2024 code, corroborated by a public code index reproduction of Article 3.2.4.18. The 2015 edition of the same tip cited 3.2.4.20 under the previous code, so on an older adopted edition the number will differ. Check it against your own copy before you put it in a submission.

What each path costs you in practice

The Class A path costs conductor and labour. Every in suite notification circuit needs a return route back to the control unit: more copper up the shaft, more conduit fill, more terminations, and a routing that has to satisfy the separation rules in the installation standard. Over hundreds of drops that adds up. The upside is no per suite hardware to buy, install, locate, label or maintain.

The isolator path costs hardware and discipline. You buy a device per suite, put each one in the right place, which is not the obvious place, record where every one went, and get into every suite on verification day.

How the decision gets made. On a new build the electrical consultant usually settles it at design stage. On an occupied retrofit, suite access tends to decide it, because a test regime that needs entry to every unit is a scheduling problem in a building full of tenants. In some jurisdictions it is decided for you.

The isolator path, in detail

This is the one people get wrong. The full treatment is in the guide to suite fault isolators; what follows is the short version.

• The isolator goes outside the suite it protects. Markham's tip states that signal circuit fault isolators shall be installed outside of the suite protected, attributing the rule to CAN/ULC-S524-06, an edition two revisions back even in the version of the tip updated to the 2024 code. Mount it inside and it sits on the wrong side of the fault it exists to contain, in the same volume where the tenant, the renovation and the water damage all happen.
• It must provide wire to wire short circuit protection for the wiring to the audible device inside the dwelling unit. That is how the Vancouver bulletin describes the device's function, and it is the basis on which every device behind it is deemed to comply.
• It has to work in both trouble and alarm. Nobody thinks about this at ordering time. The bulletin is explicit that some isolators will not let a fire alarm signal sound if the alarm is activated after a short or open has already put the circuit into trouble, and that this is not acceptable. Short first, initiating device afterwards: the signal still has to sound. Short during an alarm: the panel gets a trouble indication and the other devices keep sounding. Fault cleared and trouble reset: the isolator returns to normal.
• Acceptance is a real test. A wire to wire short circuit fault is imposed within each suite, in normal and in alarm conditions, and the corridor devices have to be demonstrated unaffected. Each suite. Not a sample.
• Record where you put them. A technician arriving in five years for an annual has to find every one, and an undocumented isolator above a corridor ceiling is a problem you handed to your future self.

Suite isolators are not a one off oddity. A UL Solutions presentation delivered to BC building officials in May 2023 lists six categories of fault isolator recognised in the installation standard: field device data, network data, audio buss, field device power buss, equipment power buss, and suite. The same presentation describes the 2019 edition's fault tolerance rule as covering all circuits leaving the control unit, with the boundary set by the fire alarm zones the National Building Code requires rather than by an area figure.

The hotel and motel exception

The Vancouver bulletin carves out one case. It says the requirement is not intended to put a suite fault isolator on an audible device located within the sleeping room of a hotel or motel suite that does not have a kitchen, and that ordinary Class B wiring without fault isolators is sufficient there.

The kitchen is the hinge: a unit with one is somewhere people live, cook, renovate and hang things on walls, and a hotel room without one is not. Treat this as one jurisdiction's bulletin rather than a national exemption, and confirm it locally before you price a job on it.

What you are actually asking when you search "Class A versus Class B"

Here is the honest answer, and then here is where it stops.

The general principle is that one of the two arrangements gives the circuit a return route back to the control unit, so that a single break in the run does not lose everything past the break, and the other does not. That is the mental model, and it is enough to follow every bulletin quoted on this page.

We are not going to go further, and the reason is the point. The formal definitions of Class A and Class B, the exact fault tolerance each class has to deliver, and the class designations themselves all live inside CAN/ULC-S524, which is copyrighted and sold. We do not reproduce paywalled standard text here, and we will not paper over the gap by importing the NFPA 72 definitions, because NFPA 72 is a different standard inside a different code system and its class definitions are not interchangeable with the Canadian ones. Publishing an American definition under a Canadian heading is exactly how trade sites end up quietly wrong. If you need the definitions, buy the standard. For the longer version of why the two are not swappable, see NFPA 72 versus CAN/ULC-S524.

Which edition you are working to matters too. CAN/ULC-S524:2019 was published on 26 June 2019 and now shows in the Standards Council of Canada database as superseded by CAN/ULC 524:2024, published 22 December 2024. Your legal obligation is the edition your building code references, not the newest one on the catalogue page. That gap is the subject of which ULC edition your province uses.

Audibility itself, briefly

The wiring decision only matters because the sound has to reach a sleeping person. In Ontario's Article 3.2.4.18 the numbers that drive the design are not less than 75 dBA in a sleeping room, and within a floor area not less than 10 dBA above ambient and not less than 65 dBA. Devices inside a dwelling unit also need a means of manual silencing limited to 10 minutes, unless the building qualifies for the automatic silencing arrangement, which is itself tied to putting each suite on its own signal circuit.

Devices in sleeping rooms of residential and care occupancies also have to emit a low frequency signal. Canadian manufacturer guidance describes this as a 520 Hz square wave, mandatory in Ontario for projects submitted after 31 March 2025. Both figures come from a manufacturer's page rather than code text we could open, so treat them as reported. The full picture is in fire alarm audibility in Ontario, and the two terms have their own entries: the sleeping room requirement and the low frequency signal.

This is a contested area, not just a confusing one. Writing in IAEI's Canadian column in 2016, consulting engineer Ark Tsisserev observed that audibility requirements are not consistently applied by designers, electrical contractors and regulators, and that inconsistent interpretation between authorities creates real cost. The gap between the Ontario and Vancouver versions of the same sentence illustrates it.

What the verifier tests

Whichever path you took, it gets proven under CAN/ULC-S537. Expect the verifier to impose a wire to wire short circuit inside each suite, in normal and in alarm conditions, and to confirm the corridor devices keep sounding. Markham's tip points at both S524 and S537 for the test method.

Two practical consequences. You need access to every suite, which on an occupied building is a scheduling exercise that has to start weeks earlier than people expect. And your terminations get tested for real, because somebody is deliberately shorting the circuit at the device and the system has to behave. Marginal workmanship surfaces here rather than at 3 a.m. two winters later. Who may sign the verification is a separate question with a local answer: Vancouver, for one, publishes the qualification routes it accepts and keeps a list of acceptable organisations and qualified persons.

Where provinces differ

Vancouver is not a stand in for British Columbia. The city has its own building by-law under the Vancouver Charter, and Bulletin 2022-002 interprets that by-law, so a project in Surrey or Kelowna works from the provincial code instead. The rest of the province is covered in the British Columbia guide.

Expect this pattern nationally: the performance sentence looks nearly identical everywhere because it descends from the National Building Code, and the variation sits in a single phrase or a municipal bulletin telling you which method your authority accepts. Find the bulletin before you buy the cable.

What remains unclear, and what to confirm with the AHJ

• We did not open the Building Code Compendium or CAN/ULC-S524. Every sentence number here comes from a municipal AHJ document or a public code index, not the code text itself. Verify numbering against your adopted edition.
• The Note 4 prohibition applies to the City of Vancouver. We found no equivalent statement in any other Canadian jurisdiction, and Markham's tip plainly contemplates Class A. Do not assume either position travels.
• We could not locate a comparable free bulletin from a third Canadian municipality. If your authority has published one, it outranks this page for your project.
• Ask your authority two questions before rough in: which method they will accept for in suite audible devices, and whether they want isolator locations on the drawings or only in the as builts.

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