2-Wire vs 4-Wire Smoke Detectors: Which System Sensor i3 Model?

2-WireThe Fire Alarm Supplier
System Sensor 2W-B i3 2-wire photoelectric smoke detector

A 2-wire smoke detector is powered by, and signals alarm on, the panel's initiating zone, so it must be listed as compatible with that panel. A 4-wire detector takes 12 or 24 V power on one pair and reports through relay contacts on another, and needs a power supervision relay.

In the System Sensor i3 Series the first two characters of the part number tell you which one you have: 2W models are 2-wire, 4W models are 4-wire. The letters that follow add a heat sensor, a sounder or a relay.

What is the practical difference between a 2-wire and a 4-wire smoke detector?

2-wire i3 (2W-B, 2WT-B, 2WTA-B, 2WTR-B) 4-wire i3 (4W-B, 4WT-B, 4WTA-B, 4WTR-B, 4WTAR-B, 4WITAR-B)
Power From the initiating zone itself Separate 12/24 V supply
Operating voltage 8.5 to 35 V 10 to 35 V
Alarm signal Detector draws alarm current on the zone; the panel must limit it (130 mA maximum) Normally open alarm contacts rated 0.5 A at 30 V AC/DC
Panel requirement A UL Listed compatible panel, matched through the compatibility identifier on the detector label A zone for the contacts and a power output for the detectors
Extra parts The panel's end-of-line resistor The panel's end-of-line resistor plus an end-of-line power supervision relay
Reset Alarm latches; reset by a momentary power interruption Alarm latches; reset by a momentary power interruption

Both types run at 50 µA average standby current, have 2.5 %/ft nominal sensitivity and ship with the plug-in mounting base.

What does each letter in the i3 part number mean?

System Sensor's installation sheets describe the models this way:

Model Wiring 135°F fixed thermal sensor 85 dB sounder Form C relay
2W-B 2-wire No No No
2WT-B 2-wire Yes No No
2WTA-B 2-wire Yes Yes No
2WTR-B 2-wire Yes No Yes
4W-B 4-wire No No No
4WT-B 4-wire Yes No No
4WTA-B 4-wire Yes Yes No
4WTR-B 4-wire Yes No Yes
4WTAR-B 4-wire Yes Yes Yes
4WITAR-B 4-wire Yes, isolated Yes Yes
  • 2W / 4W: 2-wire or 4-wire.
  • T: 135°F (57.2°C) fixed-temperature thermal sensor. Thermal models also report a freeze trouble at 41°F and are rated 32 to 100°F; the plain 2W-B and 4W-B are rated to 120°F.
  • A: built-in sounder, 85 dBA, temporal 3 pattern.
  • R: Form C relay rated 2 A at 30 V AC/DC.
  • I: isolated thermal. On the 4WITAR-B, smoke sounds a local, self-resetting alarm that does not report to the panel. Only the heat sensor initiates a panel alarm and changes the relay.

Alarm current on the 4-wire models: 4W-B and 4WT-B draw 20 mA at 12 V or 23 mA at 24 V; 4WTA-B and 4WTR-B draw 35 mA; 4WTAR-B and 4WITAR-B draw 50 mA.

Will a 2-wire i3 detector work with my panel, and how many can go on one zone?

Only if the panel is listed as compatible, and the number per zone comes from that listing, not from the detector. System Sensor's instructions say to connect 2-wire i3 detectors only to compatible control panels identified in its two-wire compatibility chart. The panel manufacturer's device compatibility document gives the same answer from the other side, including the maximum number of detectors per zone.

Two further rules from the installation sheets:

  • The 2WTA-B and 2WTR-B are not to be installed on initiating circuits that contain other makes or models of smoke detectors.
  • Any 2-wire i3 detector on a Style D (Class A) initiating circuit requires the 2W-MOD2 module.

What extra parts does a 4-wire detector need?

An end-of-line power supervision relay and resettable detector power. The relay sits at the end of the detector power run. If power to the detectors is lost, the relay releases and the zone reports trouble. System Sensor's current i3 sheet names the EOLR-1 as the end-of-line relay (older sheets show the A77-716B), and a Fire-Lite panel manual gives the same instruction for 4-wire smoke detectors.

Because the alarm latches until power is interrupted, the detectors must be fed from an output the panel drops on reset. The zone still needs the end-of-line resistor value specified by the panel.

Which i3 models sound all detectors on the loop?

None do so alone. A sounder model (2WTA-B, 4WTA-B, 4WTAR-B, 4WITAR-B) sounds only its own horn when it alarms. To make every i3 sounder on the loop sound, the supply polarity has to be reversed, and System Sensor requires its RRS-MOD reversing relay/synchronization module for that. The RRS-MOD also synchronizes the sounders and lets them be silenced from the panel. Add 25 mA of alarm current for the module.

When do I need the 2W-MOD2 module or a COSMO-2W / COSMO-4W detector?

2W-MOD2. This is the 2-wire loop test/maintenance module. Use it when 2-wire i3 detectors are on a Style D circuit (required), or when the panel is not i3-ready and you want the remote maintenance signal and the EZ Walk loop test. It replaced the earlier 2W-MOD, which System Sensor says not to use with the 2WTA-B and 2WTR-B.

COSMO-2W / COSMO-4W. These i4 Series detectors combine photoelectric smoke and carbon monoxide sensing with a sounder (temporal 3 for smoke, temporal 4 for CO). They cannot be wired straight to a zone: the COSMO-2W requires the COSMOD2W interface module and the COSMO-4W requires the COSMOD4W. One module connects up to 12 detectors, reports smoke and CO to separate zones and interconnects the sounders without an RRS-MOD. The modules also accept 2WTA-B or 4WTA-B detectors, and 4-wire systems still need the EOLR-1.

The store stocks the System Sensor i3 line, and in-stock orders placed by 3 PM ET ship the same business day. Confirm 2-wire compatibility against the control panel's listing documents, and install to NFPA 72 and your local AHJ. For a project list, request a quote.

Sources

Tags: 2-wire, 4-wire, Buying guide, I3 series, Smoke detectors, System sensor