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.
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:
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.
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:
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.
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.
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
For 4-conductor wire:
For 2-conductor wire:
For details on how to connect wiring to screw terminals, check out Smoke Detector Circuit Basics.
There are several great reasons to choose 4-conductor cable when wiring home smoke detectors. Here are the most important:
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.
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.
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