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Thread: Smoke Alarms - Hardwired & Interconnecting

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    DoD Army bjferri's Avatar
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    Default Smoke Alarms - Hardwired & Interconnecting

    I want to retrofit my old house with hardwired interconnecting smoke alarms. I'd like to start of course in the basement where I have a dedicated old 12/2 metal conduit circuit that was used in the past but now has not home.

    The smoke alarms have a red, white, and black wire. The red is used for interconnections. I was told the circuit for smoke alarms can't be protected by a ground fault interruptor. Now I understand the metal conduit from my 12/2 is the ground.

    Does this mean I apply electric tape to the conduit before fastening it to the metal electric box to lose the ground fault interruption? This is all abstract to me what happens by cutting off the ground fault, or not...??? If someone can please explain I'll thank you in advance.

    Also, to interconnect one smoke alarm to the next, can I use the ground (since it is not being used) of the 12/2 for the red to interconnect? Or, do I have to use a 12/3? Or, something else?

    I don't know what else to ask...

    Thank you...

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    Retired Defense Industry Engineer jadnashua's Avatar
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    To do this right, you need the smoke detectors on their own circuit (their own circuit breaker with nothing else attached), and 14/3 (12/3 is overkill but okay if you had it). A gfi circuit is an electronic circuit that monitors the hot and neutral and if there is an unbalance, it disconnects power. The safety ground is not a direct part of the operation of a gfi. GFCI circuits are required for kitchen counter outlets, bathroom outlets, outdoor outlets, and garage outlets.

    Basically, each of the detectors needs hot and neutral, and the third wire is used to allow them to be interconnected (so if one goes off, the others do, too). You should not use the ground wire for this, since if it made contact with a real ground anywhere, it would either damage the dectors or inhibit the interconnections to work...you need a dedicated, insulated wire for this.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DoD Army bjferri's Avatar
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    Right now I do have a dedicated 12/2 coming from the circuit box. This has the metal conduit since it is old and I was planning on using this to connect the first smoke alarm. Can I use this? If I can, how do I break the ground from the metal conduit when connecting into the metal 4" box?

    Are you suggesting 12/3 to interconnect?

    If you think I should start from scratch from the main circuit box I can. If I did would I use a 14/2 or 14/3?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You misunderstand what gfi is altogether. Ground is a safety thing, and you need to leave it intact - it can't be used as a current carrying or signal connector at all, so you can't 'break' that connection. The wire you have to the first detector is fine, since it only needs power. But, from that one to the rest, you need three dedicated wires: hot, neutral, and interconnect. You can't use the ground for that third connection. So, instead of running new wire everwhere, you only need it between the dectors. As noted previously, that circuit breaker or fuse should not supply power to any other device other than the detectors - you don't want something to trip that circuit and render your detectors inoperative.

    If the circuit breaker feeding this branch is a gfi breaker, then you need to change it out to a 'normal' one. The reason for this is that when a detector gets tripped, instead of all of the power going from the hot to the neutral lead, some of it goes through the signal wire. This causes an imbalance which trips the gfi circuit.

    When purchasing hardwired detectors, consider those with a battery backup. If you use a lithium battery, they can last the lifetime of the detector (which is recommended at a max of 10-years). If you have frequent or extended power outages, the battery may not last the full 10-years, though.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DoD Army bjferri's Avatar
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    Now I follow. Thanks.

    You say "The wire you have to the first detector is fine, since it only needs power. But, from that one to the rest, you need three dedicated wires: hot, neutral, and interconnect."

    Last question. If it's a 12/3 I use for the interconnections, what do I do with the ground wire?

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by jadnashua
    If the circuit breaker feeding this branch is a gfi breaker, then you need to change it out to a 'normal' one.
    The breaker that supplies the smokes is required by 210.12 to be an Arc-fault device.

    Quote Originally Posted by bjferri
    Now I follow. Thanks.

    You say "The wire you have to the first detector is fine, since it only needs power. But, from that one to the rest, you need three dedicated wires: hot, neutral, and interconnect."

    Last question. If it's a 12/3 I use for the interconnections, what do I do with the ground wire?
    Twist all the grounds together and push up in the box out of the way.

  7. #7

    Exclamation

    The great AFCI debate...

    Article 210.12(B) states, "Dwelling Unit Bedroom Circuits. All 15 or 20A, 120V branch circuits that supply outlets in dwelling unit bedrooms must be AFCI-protected by a listed arc-fault circuit interrupter."

    Now smoke detectors aren't connected to outlets in the bedroom they're hardwired. Also circuits serving fire alarm alarm circuits are exempt from AFCI requirements per 760.21 and 760.41. Problem is NFPA 72 of the National Fire Alarm Code doesn't define interconnected smokes as fire alarms, but "regular" alarms. Yeah, that makes lots of sense.

    My personal opinion, which I realize might catch heat for, is put the fire alarm circuit on a standard breaker. This your life we're talking about.

    And yes, you have to use 3 conductor cable, plus ground. Never use a ground for anything but a ground. If you're local code requires 12/3 on 15A circuits like ours does in NYC, then run 12/3.

    Joe

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    Retired Defense Industry Engineer jadnashua's Avatar
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    To provide ground to the rest of the daisy chain, there is usually a threaded hole in the box that you can attach a pigtail to so you have something to tie the ground lead for the new x/3 cable going out from the first box. If you use metal boxes for the others, the inspector will probably want to see the ground connected to the box as well as the in/out wires even though the detector doesn't use the ground wire...this keeps the metal box safe if a wire gets pinched.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DoD Army bjferri's Avatar
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    So I do want to daisy chain the ground to the other smoke alarms and not tuck them out of the way? And an arc fault is not necessary?

    I thought a 12/3 goes on a 20 amp circiut. A 14/3 is 15 amps, right? I'm in NJ.

  10. #10

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    Yes that is correct.

    I would run that circuit in 14 guage using a 15 amp breaker. It is a low use circuit and doesn't require the heavier wire.

    Tom

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    DoD Army bjferri's Avatar
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    Thanks for all the help. You guys are the best source of information!

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    So now that you have done that with you own panel are you concerned about burning down your house?
    Absolutely not; I've "staggered" my breakers to reduce potential heat buildup. In other words, each row of two AFCI or GFCI breakers is separated by a row of standard breakers. And in cases where that couldn't be done, and I must stack AFCI and GFCI breakers, I do with so with the lowest potential expected load on them, like halway circuits, guest bedroom outlets, and such.

    I can remember when GFCIs were first introduced to the code and you know I listened to the same stories then about them as I am hearing today about arc-fault.
    I don't think this is quite the same, because GFCI had always been fairly effective at the get-go, and there were GFCI receptacles when it was a code requirement. I have yet to see an AFCI receptacle. The only real concern on GFCI was nuisance tripping, and they weren't required on critical circuits like a refrigerator (single receptacle).

    Now as we see from the debate here, AFCI is required on critical circuits. I view any security circuit - especially smokes - as critical. I agree with HJ here (who happens to also be an outstanding plumber, by the way) 100% on this. Also, in many tests, AFCI has been proven ineffecitve on variety of arcs - but I suppose some protection is better than none (hence, why I have them). Let's put it this way, if you were on life support, would you rather the breaker feeding the equipment to be standard or AFCI?

    It should also be noted that quite a few AHJ's have pretty much ignored AFCI requirements, and don't even want to see them.

    You might realize we are a little skitzy about electricians named Joe !!
    Hahaha... and apparently we were practically neighbors in Queens!

    Joe

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    General Contractor dx's Avatar
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    Quote Originally Posted by joe in queens
    It should also be noted that quite a few AHJ's have pretty much ignored AFCI requirements, and don't even want to see them.
    Joe
    Correct. In my area none of the AHJs require AFCI anywhere and I bet most inspectors would take issue with an AFCI on a smoke alarm. Nobody I know has actually tried it, but in casual conversation a couple inspectors said they would turn down an AFCI on a smoke.

    dx

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