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Where Does the End-of-Line Resistor Belong on a Fire Alarm Circuit?

July 13, 2026

A recent Toronto fire alarm question asked:

If a signal circuit has only one bell, can the end-of-line resistor be terminated at that bell?

The short answer is yes—when the circuit truly serves only that one field device and the installation meets the conditions in CAN/ULC-S524.

That answer is more useful when we understand why the resistor is there, where it normally belongs, and what the single-device exception does not permit.

What the end-of-line device actually does

On a conventional Class B fire alarm circuit, the end-of-line device is part of the circuit’s electrical supervision.

It allows the fire alarm equipment to monitor the wiring and recognize a fault that could interfere with operation. Depending on the circuit and equipment, an open, short, or ground fault should produce the required trouble response rather than remaining hidden.

That is why an end-of-line resistor is not simply a component placed wherever it is easiest to reach. Its location helps establish that the wiring leading to the field devices is actually being supervised.

If the resistor is installed at the panel instead of at the end of the field wiring, the panel may continue to “see” the resistor even when the outgoing circuit is disconnected. The trouble light can remain off while the devices in the field are no longer connected.

A neat panel is not the same thing as a supervised circuit.

The normal S524 arrangement

CAN/ULC-S524:2019 Clause 47.1 provides the normal rule.

Except where a specific exception applies, an end-of-line device used for electrical supervision is installed:

The end-of-line device also requires identification showing the zone served.

For a circuit with several bells, horns, strobes, detectors, or other conventional field devices, the EOL normally belongs at the end of the supervised run—not at the first convenient device and not back at the panel.

The single-device exception

CAN/ULC-S524:2019 Clause 47.2 creates an important exception:

Where an end-of-line device serves a single field device on a circuit, it may be located within that field device, provided the EOL remains easily accessible for inspection and voltage testing.

That means a fire alarm signal circuit serving only one bell can use this exception.

The EOL may be located at or within that bell when:

The exception is real, but it is narrow.

It does not mean an EOL resistor can always be hidden inside a bell. It does not apply to a multi-device circuit simply because one device happens to be easiest to access.

Accessibility still matters

The words “easily accessible for inspection and voltage testing” matter.

A single bell may be mounted high on a wall, above an obstruction, or in an area that requires special equipment or access arrangements. Putting the EOL inside the device is not automatically acceptable just because there is only one device on the circuit.

The installation still has to support practical inspection and testing.

If a technician cannot reasonably access the resistor, identify it, or take the required circuit measurements, the installation has missed an important condition of the exception.

The EOL must be labelled

CAN/ULC-S524 also addresses identification.

Where the end-of-line device is located within a field device under the single-device exception, an identifying label must remain visible after installation.

That small label prevents the next technician from having to guess where the circuit terminates.

Without it, someone troubleshooting the system may search for a separate EOL plate that does not exist—or assume the resistor was improperly omitted.

Good supervision should also be understandable supervision.

Do not guess the resistor value

The correct resistor is determined by the listed fire alarm equipment and the manufacturer’s published installation instructions.

It is not selected because:

Different control units, boosters, modules, and circuits can require different EOL values or wiring arrangements.

An incorrect resistor might make the panel appear normal without providing the supervision the equipment was designed to perform.

Before changing or relocating an EOL, confirm:

A normal panel indication does not prove the resistor is correct.

Not every supervised circuit uses the same arrangement

The familiar EOL resistor arrangement usually applies to conventional terminated circuits.

Fire alarm systems can also include:

Do not assume every supervised circuit ends with a loose resistor or that the same placement rule applies in exactly the same way.

Identify the circuit first. Then follow the applicable standard and the manufacturer’s instructions.

What inspection and verification should prove

CAN/ULC-S536 requires applicable input- and output-circuit end-of-line devices to be tested for open-circuit, short-circuit, and ground-fault conditions, with the results recorded.

The verification standard also addresses circuit fault testing and voltage measurements, although the applicable edition must be confirmed for the project.

The practical objective is straightforward:

The resistor is only one component. The real test is whether the complete circuit is supervised and performs correctly.

A field checklist for a single-bell circuit

Before accepting an EOL at the only bell, verify:

  1. The circuit truly serves only one field device.
  2. The device is on a conventional supervised output or signal circuit.
  3. The EOL location is easily accessible for inspection and voltage testing.
  4. A visible label identifies the EOL arrangement and zone served.
  5. The resistor value matches the manufacturer’s instructions.
  6. The wiring and circuit voltage are within the equipment specifications.
  7. Open, short, and ground-fault conditions produce the required responses.
  8. The applicable project specification, adopted standard edition, and AHJ requirements have been considered.

If those conditions are satisfied, the single-device arrangement is not a shortcut. It is an installation expressly contemplated by CAN/ULC-S524:2019.

The real takeaway

The normal rule is simple: put the end-of-line device at the end of the supervised circuit, beyond the last field device.

The exception is just as important: if the circuit serves only one field device, the EOL may be located within that device when it is accessible, identified, and installed correctly.

So, can the EOL resistor be terminated at the only bell on the circuit?

Yes—provided it is truly a single-device circuit and all of the exception’s conditions are met.

References

The applicable adopted standard edition, manufacturer’s instructions, project specifications, and AHJ requirements should always be confirmed for the specific installation.

Working with Canadian fire and building codes? Ask Codebook Carl — answers are sourced directly from the code books with exact clause citations.