How To Connect A 4-Wire Smoke Detector

Technical Notes

I’ve received a few calls on how to connect 4-wire smoke detectors so I thought it might be a good idea to share what I know about these smoke sensors.

Why is it referred to a 4-wire detector? As with just about all powered sensors, it requires four wire connections, two of the wires are used for power, typically 12 VDC, and the other two are used for signal. The signal comes from a switch within the smoke sensors that closes its contacts when smoke is detected.  The internal switch is open when the situation is normal, and closes when it detects smoke.

Power going to the smoke detector is supervised, meaning that if power is lost to the detector for whatever reason, the security or automation system that the smoke sensors is connected to will know about that loss of power.  The supervision is accomplished by the use of a relay and a resistor which is installed at the end of the line, or after, the smoke detector. If there are multiple smoke sensors connected to a zone input of the system, the end-of-line (EOL) relay and resistor are installed after the last smoke sensor in the chain. The combination of the EOL relay, resistor, and wiring scheme allows for the supervision status to be in one of three possible conditions:  System Ready, Alarm or Trouble.

You might want to take a look at the provided wiring diagram to fully grasp my next statement. 

Though power applied to the smoke detector(s) and the coil of the EOL  relay are electrically in parallel, the physical wiring is in series.


You cannot use “home-runned” wiring to supply power to a smoke detector, or use a 12 VDC power distribution module with multiple outputs each connected to a smoke detector.  You cannot get supervision with these wiring schemes.

Instead, you need to daisy-chain the wiring such that a single wire from the 12 VDC output of the system goes to the first smoke sensor’s “+” terminal, and then a second wire from the same “+” terminal goes to the next smoke sensor in the chain’s “+” terminal, or to the EOL relay. Use the same daisy-chain wiring from the system’s 12 VDC Return to go to the first smoke sensor’s “-” terminal, and a second wire from the “-” terminal going to the next smoke sensor in the chain’s “-” terminal, or to the EOL relay.  Wiring in this fashion ensures that if any power wire is compromised, the chain is broken which causes the EOL relay to open its contacts.

Enter the signal wiring and EOL resistor.  The signal wiring is connected from the system’s zone “+” and “-”  input to the smoke sensor’s Common (COM) and normally open (NO) terminal, respectively, in the same daisy-chain fashion as the power wiring.  Once the wiring has passed the last smoke detector, one lead of the resistor is connected in series with either one of the  signal wires (in the diagram the resistor is on the NO side) and the other lead from the resistor continues on to one side of the EOL relay’s contacts.

The remaining signal wire is connected directly to the EOL relay’s other contact.  Note that when power is applied and the EOL relay is energized, the EOL resistor is placed in a series loop as seen by the zone input.  The value of the resistor varies with the panel used and in the diagram a resistance of 1000 Ohms is used, but the key point to remember is that when the situation is normal, the zone input “sees” the resistor.  Electrically speaking, a specific amount of current flows through the wiring and EOL resistor which the systems acknowledges as a normal condition. This can be described as “System Ready.” System Ready means just that, the system is OK, there is no smoke detection, all is working normally, the system can be armed if needed, all green and good to go, the system is ready. “We are ready for launch, Houston.”

Now when a smoke sensor detects the presence of smoke, the internal normally open switch closes.  This ties the COM and NO terminal together.  This switch closure effectively shorts out the resistor that the zone input normally sees. Instead of having the normal amount of current flowing through the resistor, the short causes an over-current condition.  The short circuit causes an “Alarm” condition and the system responds accordingly by sounding an alarm or siren and initiating a dial-out call.

In the event of a compromised wire, be it for power or signal, a “Trouble” condition will exist. Trouble occurs when the zone input no longer senses any current flow at all.   Any break in the power wires causes the relay contacts to open, and a break in the signal wires will open the circuit to the zone input.

You can easily test your security or automation system’s zone input to see if it has been properly configured to work with a smoke detector without actually having a smoke sensor installed.  “Spoof” the system by placing the appropriate value of resistor across the zone input.  If configured properly, you’ll get a System Ready. Next, take a short piece of wire and connect it across the zone input shorting out the resistor and you’ll get an Alarm condition.  And lastly, lose the short piece of wire and remove one lead of the resistor from the input to simulate a Trouble condition.

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34 Comments on How To Connect A 4-Wire Smoke Detector

  1. I have a wired interconnected smoke alarm system in my house… It works great, but I was told that I cannot connect it to my security system (DSC PC1555)… I dont know why!

    I was going to go out and buy myself one of these 4 wire smoke detectors (I like the GE449 series) and wire them to my security system. Am I losing the benefits of my interconnected system by doing this? Is there a better way to do this? Or am i being told “No – can’t do” because the sales guy is looking for some commission? Any help would be appreciated. You can post back here or reply to me. Thanks!

  2. Hi, Amba,

    Maybe your wired interconnected smoke alarm system doesn’t have the signaling pair of wires required by the security system. Your system might only be using only high voltage (120 VAC) wiring to communicate with other smoke detectors which makes it incompatible for use with the low voltage (less than 30 VAC/VDC) DSC security system.

    It would be beneficial if you can learn more about your existing some alarm system. Until we know about the current sytstem, comparisons cannot be made with the GE449 series of detectors. Also, ask your sales guy why the smoke system won’t work with the DSC. Hopefully the answer will have nothing to do with making a buck.

  3. I am not sure of the voltage in the interconnect wires (did not measure them), but it looks like it is 9V +/- 2V… according to
    this website… I dont know if i can trust them.

    But the detector itself is a kiddie 1275 ionization smoke alarm with a battery back up[Data Sheet User Manual]… I know it is the cheap one from stone age!

    My alarm system is a DSC PC 1555 [ Manual]. Page 6 has the wiring diagrams.

    Thanks for your help!

  4. I am not sure of the voltage in the interconnect wires (did not measure them), but it looks like it is 9V +/- 2V… according to a website… I dont know if i can trust them.

    But the detector itself is a kiddie 1275 ionization smoke alarm with a battery back up[Data Sheet User Manual]… I know it is the cheap one from stone age!

    My alarm system is a DSC PC 1555 [ Manual]. Page 6 has the wiring diagrams.

    Thanks for your help!

    PS: I will wait for moderator approval on the message with links, but if you can make some sense out of this, please let me know.

  5. Hi, Amba,

    I researched the Kidde 1275 and it’s a 120 VAC unit with a 9 volt back-up battery. It does not have a pair of low-voltage signaling wires. So what your sales guy told you is correct in that you cannot use your smoke detectors with the DSC.

    Also, I checked out the DSC and its 4-wire smoke detector connections. It is compatible with the GE449 series of detectors.

  6. Wonderful… Thank You.

  7. Joe Teneriello // November 8, 2008 at 10:32 am // Reply

    Hello Sir,

    I am setting up 12 – 4wire smoke detectors in my home within 4 zones, (basement, first floor, second floor, attic).

    Can i dasy chain the power to all 12 smokes and still isolate the zones without a problem? If yes, what about the EOL resistor and relay….would i need 1 for each zone split in the chain or can i use just one single EOL resistor and relay at the very end of the chain?

    Thank you very much.


  8. Hi, Joe,

    If you daisy-chain all 12 detectors, you’ll use only one EOL relay, one resistor, and one zone input on the security panel. In the event of smoke detection, the panel will not be able to isolate which one of the detectors went active. Was smoke detected in the basement, the first floor? The panel can’t know because only one zone input was used.

    Here’s an example regarding your four zones. Let’s say that there are three smoke detectors in the basement, three on the first floor, and three each per remaining zone. Four zone inputs on your securtity panel will be used to monitor those four spaces. You’ll use one EOL relay and resistor for each zone, for a total of four each EOL relays and resistors, installed after the last smoke detector in the chain. Reference Figure 2 in the wiring diagram, and you’ll see a setup using two detectors. Add one more detector after the last detector and before the EOL and you ‘ll be good to go.

    Hope this helps you out, Joe.

  9. Joe Teneriello // November 11, 2008 at 4:02 pm // Reply

    Thank you for the reply! I’ve got it down now

  10. Steve Langford // November 18, 2008 at 3:14 pm // Reply

    I have a similar situation. I am trying to add zones with DSC 5108 zone expander. I am being told that this device will not support smoke detectors even though I have the ability to program any zone for 24 hour fire supervision???? I have been told that I would not have reset ability??? I am only planning to have one other current drawing device (glass break) and the two combined do not exceed the current rating of the expander. Looking at the GE 449 CTE. Thanks for any help you can give!

  11. Hi, Steve,

    Sorry, but I have zero experience on DSC products. Maybe you can add your smoke detectors to the main DSC board, and relocate non-resetting sensors, such as a magnetic contact switch, to the zone expander?

  12. Steve Langford // November 19, 2008 at 12:53 pm // Reply

    Thanks, but I am adding the expander to a new building with a new partition and it is not very practical to run all of the wiring the 300 feet between the expander and the main panel. I was hoping for a simple solution…may just have to forgo the fire protection….. 🙁

    I really appreciate the response!

  13. Timothy Burris // December 6, 2008 at 2:54 pm // Reply

    ISSUE: Wiring problem replacing 15-year-old smoke detectors.

    The smoke detectors are wired into a security system installed in our townhouse. However, the system is old and out of date, and NOT CONNECTED. A security company informed us that they could not reactivate this system, but would need to install a new one, at a cost that we can’t justify.

    We thought it was time to replace the smoke detectors, in view of some recent remodeling and some false positives, and I bought BRK9120B units.

    PROBLEM: I only discovered when I started to install the first new one that the old one was a four-wire unit, and the new ones are three-wire units.

    QUESTION: Is it possible for us to wire in these new smoke detectors. If ‘yes’, which wires go where?

    Timothy Burris

  14. Timothy Burris // December 6, 2008 at 3:36 pm // Reply

    Follow-up: The four wires I described are as follows:
    Power input: blue wire to + terminal; green wire to – terminal
    Red wire to Signal terminal
    White wire to Ground

    Timothy Burris

  15. Hi, Timothy,

    It sounds like the four wires you currently have are for low voltage only, two for power and two for signal, whereas your new smoke detector is a 120 VAC unit. They are not compatible.

  16. Dear Sir,
    I will like to order some somke detector in your store,Are will pay you by paypal are you accet it and get me the shipping cost to my address.


    • nutznboltz // June 12, 2009 at 9:06 am // Reply

      Hi, John,

      Sorry, no, we don’t accept PayPal. Please contact Customer Service at 800-266-8765, inform them of what you need, and they’ll be able to calculate the shipping costs.

  17. I will have 14 smokes, 5 heat detectors, and 5 carbon monoxide detectors on my system. The home has 4 leveks, The contractor has proposed a NAPCO Gemini Zone Control and 3 Zone Expanders—I dont understand how the Zone Expanders fit in the interconnect wiring of the detectors, Can you explain how these equiptment will be wired

  18. Hi Chuck,

    Do you know which Napco model your contractor is proposing? Most Napco panels are burglar alarm systems that can also be configured for fire and gas detection zones. The main board typically handle a max of 8 zones. Hence, the 3 zone expanders might be used for handling the fire, heat, gas plus security zones.

  19. there is missing point i cant understand
    this Zone input (security panel ) how it will check the current through the resistance if this loop dont have any power source?
    when the internal contact is open between NO and COM i can check the resistance value but when somke come and contact closed it will be closed circuit for resistance where is the current source?

  20. how the current will come through the resistance loop?

  21. The current source is provided by the security panel. The internal electronics of the panel is smart enough to know how much current normally flows through the resistor during a no-smoke condition. When there is smoke, the resistor is shorted-out by the smoke detector and the change in current sets indicates a smoke alarm condition.

  22. what ithe resister value to be used at the end of smoke detector

  23. Hello, viswanathan,

    The EOL resistor will vary depending on the security panel used. For example, an HAI OMNI panel requires 1Kohm, a GE Networx 8e requires 3.3 Kohm, and a Napco Gemini P9600 requires 2.2 Kohm. Each panel requires a specific value of resistor that will produce the proper amount of trickle current through the resistor indicating to the panel that the circuit is closed, or in the normal, non-alarm state.

  24. What should you be looking for in your home security system of choice? Frankly there are many different characteristics that combine to provide you complete protection, and you\’ll find that some companies and some systems excel where others fall off. It really depends on what exactly you need from your system.

  25. Here are three points to consider. First, is the system for security only, or will it be for security and home automation, such as lighting control and thermostat control. Of course, security-only systems are less expensive.

    Second, how many sensors, or zones, are required?

    Third, hard-wired vs wireless capability, or a combination of both. New construction will typically use hard-wired sensors because wiring can be installed before the drywall goes up. Wireless may be the only way to go if you can’t pull wires.

  26. My husband unplugged the smoke detector from the wall, but I think the wires are still active because something is still beeping. We are perplexed as to why there is still beeping after the darned thing was detached. Is something wrong with the wiring? We have had a problem with roaches. Can they activate the wires and cause them to sound? Please help! We are beyond mad that the beeping hasn’t stopped Despite detaching the detector!

    • The smoke detector is probably connected to another device possibly an alarm panel which has a keypad that’s producing the beeping sound. Roaches cannot activate wires and cause them to sound.

  27. Hi. Thank you for this article, it has been very helpful. I have installed several hardwired smoke detectors in my home and wired them per your instruction to my panel (all to one zone). My questions is re the programming for the smokes. Specifically, are all the smokes are programmed to one zone? (I understand they’re all wired to one zone, but are they, likewise, all programmed to one zone?) Thanks!

  28. renatobacat // June 26, 2015 at 7:26 pm // Reply

    Hi,i have a problem of my 4 wires connection of my smoke detector, if i will connect more than 2 ,10 smoke detector the system always alarm,this is conventional type..can u give me an idea to this prblem,pls.rply asap.thanks

  29. Hi sir , I connected FZM 1 module with 20 nonaddressable detector ,and I want check when I open the top cover of smoke detector the panel detect or not ,but it doesn’t show the address of module .normally it s supposed to show the module address if we remove the top cover of detectors followed by FZM . And another thing is if I fired the detector panel shows the address of module . I checked module it is good . can u help me ?

  30. How do I go from a four wire smoke alarm (black, red, white, and brown) to a new thre wire detector? New one has black, white, and red wires.

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