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Connecting 2 Wire Smoke Detectors

Using 2- and 4-Conductor Fire Wire

2 wire smoke detectors are simpler and cheaper to use than 4 wire units. Combining the power and detection loops in a single pair of wires reduces equipment costs, and makes installation faster.

Connecting 2-Wire Smoke Detectors

Both 2- and 4-wire residential smoke detectors are typically supervised using an “end-of-line” resistor, or EOLR. Most alarm panels will include any EOLR’s necessary for proper smoke detector wiring.

In addition, a 4-wire smoke detector circuit also needs an end-of-line relay, for power supervision. This unit is not included with most panels. While the relay isn’t expensive, it is an extra cost, and another item to keep track of.

Not all home security systems can support 2-wire units. For those panels, 4 wire smoke detectors are the only option. Read more about them in Connecting 4 Wire Smoke Detectors.

For information on finding and fixing problems with smoke detector wiring, see these pages:

Smoke Alarm Circuit Troubleshooting

Troubleshooting Smoke Alarm Wiring

Regular cleaning can help avoid smoke detector problems in the first place. See how to do it at System Sensor Smoke Detectors, Cleaning and Testing.

4-conductor fire wire

When it comes to connecting 2 wire smoke alarms, the terminology can get confusing.

These units are referred to as “2-wire” smoke detectors, and will work just fine when connected using 2-conductor wire. However, they are very often installed using 4-conductor wire.

Using 4-conductor wiring even when it’s not required gives you several benefits, which I’ll cover in a minute. First, let’s see how to use each type of wire.

Fire Wiring Color Code

The wire colors mentioned here match the fire wire shown above. “Out” refers to “out” from the alarm panel:

  • Red = Positive (+) loop out
  • Black = Negative (-) loop out
  • Blue = Positive (+) loop return
  • Brown = Negative (-) loop return

If you’re using cable with other colors, just make up your own color code. It makes no difference to the wires, or to me, which colors do what, as long as you stay consistent.

Smoke detector circuit to locate EOL resistor at panel

Regardless of which kind of cable is used, pulling it is the same. A single 2- or 4-conductor fire wire is pulled from the main panel location to the last smoke detector in the system (the “end-of-line” smoke). Along the way, the wire passes the other smoke alarms in the system (the “looped” smokes).

For tips on where smoke detectors should be installed, see Smoke Alarm Placement for Home Security Systems.

Once the wire reaches the last smoke, it is tied off. Then, working back toward the main panel, extra wire is pulled at each looped smoke location. This “service loop” should be long enough (2 feet or so) to easily allow making connections to each unit.

Extra slack is also left at the main panel. This allows for fudging the panel location around a bit, and makes final panel wiring easier.

Smoke alarm wiring - single run

Connecting 2 Wire Smoke Detectors

Follow the specific fire alarm schematic supplied by the smoke detector manufacturer to make connections. A smoke detector wiring diagram is normally included with every detector, and will show you how to correctly hook up the device.

Be sure to leave at least one copy of this paperwork in the main alarm panel, for future reference. In case of any smoke detector problems, troubleshooting and testing will be much easier.

To make connections at the end-of-line 2 wire smoke

  • Remove 4-6 inches of the outer jacket.
  • Strip off about ¼-inch of insulation from each conductor.
  • Connect each wire to the 2 wire smoke terminals, as shown in the wiring diagram.
  • If you’re using 2-conductor wire, connect the end-of-line resistor (EOLR) to the terminals as shown.
  • If you have 4-conductor wire, the blue and brown wires are connected to the terminals in place of the EOL resistor.

System Sensor 2-Wire smoke detector wiring

To connect the looped smoke detectors -

For 4-conductor wire:

  • Remove several inches of the outer jacketing. Be careful not to cut into the conductors inside.
  • Separate the red and black wires, and cut them at about the midpoint of the stripped out section.
  • Remove ¼-inch of insulation from the ends, and connect to the 2 wire smoke alarm as shown.

For 2-conductor wire:

  • Cut the wire, and remove a few inches of the outer jacket.
  • Strip insulation from each of the ends, and connect as pictured.

For details on how to connect wiring to screw terminals, check out Smoke Detector Circuit Basics.

Back at the main alarm panel

  • 2-conductor wire is stripped, and the red and black wires are connected to the + and - smoke loop terminals on the circuit board, or…
  • 4-conductor wire is stripped as usual. The red and black wires go to the smoke loop, just as with the 2-conductor wire. The blue and brown wires are attached with crimp connectors to each terminal of the end-of-line resistor.

End-of-line resistor located in panel

Benefits of Using 4-Conductor Wire with 2 Wire Smoke Detectors

There are several great reasons to choose 4-conductor cable when wiring home smoke detectors. Here are the most important:

Multiple runs possible

Many home alarm systems only provide a single set of terminals for hooking up smoke detectors. Unfortunately, some floor plans make it necessary to pull more than one wire for smokes.

See how to solve this problem at Smoke Detector Wiring - Connecting Multiple Runs.

More options

When a new house gets pre-wired for a security system, we usually don’t know which specific alarm panel will be installed. Many “spec” homes are prewired by the builder, even with no alarm system in the immediate plans.

To “cover our bases”, we use 4-conductor wire, which will handle any possible panel/smoke alarm combination.


Even if we know for sure that a given house will be getting an alarm panel that can use 2 wire smoke detectors, most houses change owners within a few years. Maybe the next homeowner will replace the original alarm panel with one that needs 4-wire smokes.


Fire wire is also commonly used (and/or required by code) for the transformer and siren wiring. For either of these devices, 4-conductor wiring is not required, but it is preferable.

Because of this, many home alarm companies find it easier to use 4-conductor fire wire for everything, rather than also stock 2-conductor wire that won’t be used as often. After all, 4-conductor cable pulls just as easily as 2-conductor, and the difference in price is minimal.

For more information on wiring a typical alarm panel, see Ademco Vista 20P Wiring Diagram.

Confused by home alarm pricing? Get the inside scoop on home security prices from an experienced alarm professional.

If you haven't done so already, get a few Free Home Security Systems Quotes from companies in your area. This will give you a good idea of what a system would cost to have installed, as well as how much you could save by doing it yourself.

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