Smoke Detector Circuit Basics

Fire Protection for Home Security Systems



System Sensor Smoke Detector



A smoke detector circuit is built into many home security systems. Also known as a "smoke loop", this circuit uses low-voltage smoke detectors, powered by the alarm system panel.

Unlike standard 110-volt home smoke detectors, which only make noise at the house, the low-voltage units allow fire alarms to be reported to a central monitoring station. Help can be on the way almost as soon as smoke is detected, often before the occupants are aware of it.

Some panels that support fire alarm connections only provide a single set of terminals. Often though, it's necessary to pull more than one smoke wire to reach all the detectors in the system. See how to handle this situation here: Smoke Detector Wiring - Connecting Multiple Runs.





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In many areas, electrical and fire codes require the use of special fire-rated cable for connecting residential smoke detectors. This “fire wire” has jacketing material that is rated to withstand high temperatures longer than standard wire, so it should be the last to burn in case of a fire. This gives the system a better chance to alert occupants and transmit a report to the central monitoring station.

Fire wire is normally 22-gauge for residential use, and can contain either 2 or 4 conductors.



Smoke detector wiring





The low-voltage smoke detector wiring is completely separate from the standard 110-volt smoke alarms, and is wired in a specific way.



Trouble Conditions

The security panel is designed to sense any problems on the smoke detector circuit. These trouble conditions can include:

  • Open Circuit – A wire is cut or disconnected at some point in the loop.
  • Short Circuit – A portion of the wiring is effectively bypassed. Usually due to loose or damaged wires touching each other.
  • Ground Fault – A wire is in contact with a grounded conductor. This often means wiring has been damaged, and is touching ductwork, framing, or other metal.

The ability to sense problems like this is called “supervision”. Supervised loops often include an “end-of-line resistor”, EOL resistor, or just EOLR. This resistor is located at the end of the wiring run (oddly enough), and is used to detect a shorted condition.


The System Sensor smoke detector circuits, shown below, are typical of most other manufacturers.


2 wire & 4 wire fire alarm wiring diagram





The diagram shows the two basic types of smoke detectors, 2-wire and 4-wire. Most home alarm panels are designed to handle one type or the other, but some can handle both. Here’s how they work:


Two-wire Smoke Detector Circuit

Power to operate the smoke detector is supplied on the same 2 wires as the detection circuit. A 2-conductor cable goes from the main panel, connects to each smoke in sequence, and terminates with an EOLR at the last detector.

In normal operation, the alarm panel “sees” the EOLR, so it “knows” that all the connections are good. If the detector senses smoke, it creates a short across the pair of wires, and the panel goes into alarm.

If a wire is cut, or pulls loose from a screw terminal, the panel no longer sees the EOLR, and it signals an open trouble condition.

In case the wires are shorted, the panel will go into alarm, since this is the same condition caused by the detector when it senses smoke.

Learn more about Connecting 2 Wire Smoke Detectors.



Four-wire Smoke Detector Circuit

Power for the device is supplied on one pair of wires, or loop, while the detection circuit is connected to a second pair. The cable runs from the main panel, and connects to each smoke detector in the system. At the last unit, the detection circuit terminates with an EOLR, just like the 2-wire smoke.

In operation, the detection loop acts the same as with a 2-wire smoke detector circuit. The panel senses the EOLR as long as the loop is closed, and signals a trouble condition if it opens. If smoke is detected, the pair of wires is shorted, causing an alarm.

The power loop is supervised differently with the 4-wire detector. A relay is connected with its coil powered by the power loop, and its contacts in series with the detection loop.

As long as the relay gets power, the detection loop stays good. If a power wire is cut or comes loose, the relay drops out. This opens the detection loop, and the panel signals an open trouble condition.

For more information, see Connecting 4 Wire Smoke Detectors.




Making the Connections

Most smoke detector manufacturers include a wiring diagram with every unit, so you can quickly see how to make connections. It’s a good idea to leave a copy in the main alarm panel, for future reference. (This goes for any device installed in the system, for that matter). Besides the wiring diagram, the installation sheet will also contain instructions on how to test and maintain the unit.

When making connections at the last detector in the run, you’ll obviously be stripping the wire ends to attach to the screw terminals.


When making connections to a looped smoke detector location, you’ll have a loop of wire to deal with. In this case, always cut the wire. While it may be easier to simply strip off some insulation and fold the wire over, don’t do it. Why?

If, someday down the road, the smoke detector wire is pulled on, the wire may come loose from the screw terminal. If you’ve simply folded over the wire, the smoke detector circuit will still appear to be intact from the alarm panel’s point of view. No trouble condition will be generated, even though the wire is now no longer connected to the smoke detector!

This leaves you unprotected, but still thinking the system is working properly. The panel is actually unable to trigger an alarm from that detector.

Had the wire been cut at the time of connection, the ends would have separated when pulled loose, causing an open trouble condition.



For tips on finding problems with smoke detector wiring, check out these 2 articles:

For wiring at the main panel, see Smoke Alarm Circuit Troubleshooting.


For wiring at the smoke detectors themselves, see Troubleshooting Smoke Alarm Wiring.


For details on connecting smoke detectors and other devices to a typical alarm panel, see Ademco Vista 20P Wiring Diagram.



Hardwired smoke detector wiring





Residential smoke detectors





Types of Smoke Detector Circuit Terminals

Screw terminals vary from unit to unit. Older style smoke detectors use standard screws with teeter plates. Teeter plates hold wire more securely than a screw head alone, and also make connecting more than one wire easier.

When attaching two wires to the same terminal, put one conductor on each side of the teeter plate. This causes the screw to apply even clamping force to each piece of wire. If both wires are placed on the same side of the plate, one may be held securely while the other is not.


Hardwired smoke detector wiring





Residential smoke detectors





Many newer designs of smoke detector use terminal strips or detachable terminal blocks. These speed up wiring a bit, but you’ll need a fine-tipped screwdriver to tighten them.

With any kind of wiring connector, don’t over-tighten the screws. They are small enough that they can be easily stripped, making the wire forever prone to coming loose. Just snug them up good, without using your “Kung Fu” grip.

For more information on residential smoke detectors for home alarm systems, see this page on Hardwired Smoke Detectors.

For tips on where smoke detectors should be located in a house, see Smoke Alarm Placement for Home Security Systems.

For information on cleaning smoke alarms, see System Sensor Smoke Detectors, Cleaning and Testing.

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