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Old 10-16-2010, 03:35 PM   #1
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Smoke Detectors on Different Circuits


We’ve hooked up smoke detectors throughout the house. Two groups of smoke detectors are on two separate circuits; however, we have a communication wire that connects all smoke detectors together. The instructions indicate the communication wire should not cross different circuits. Since ours do, what is the drawback to having a single communication wire connect smoke detectors across two separate circuits?

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Old 10-16-2010, 04:25 PM   #2
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Smoke Detectors on Different Circuits


I'm not too sure, but if the instructions tell you not to, I surely wouldn't.....

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Old 10-16-2010, 05:36 PM   #3
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Smoke Detectors on Different Circuits


Why would you run two circuits for your smokes?
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Old 10-16-2010, 07:36 PM   #4
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Smoke Detectors on Different Circuits


Thanks for responding.... It does seem kinda nuts to do something like this, but when we built the addition, the electrician put in another subpanel to accommodate the rest of the power and some of the smoke detectors are located in the new addition.
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Old 10-16-2010, 07:46 PM   #5
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Smoke Detectors on Different Circuits


If you use a relay to isolate the signal wire, you would be OK!
The danger in using two circuits with a common signal tie, is that 220 volt potential is possible and is likely to fry the detectors.
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Old 10-17-2010, 01:15 AM   #6
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Smoke Detectors on Different Circuits


OK, but what's that, and how do I do it?
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Old 10-17-2010, 12:25 PM   #7
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Smoke Detectors on Different Circuits


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OK, but what's that, and how do I do it?
A relay gets you nothing.stop trying to burn things up.just connect smoke det's on same ckt and be safe.if you can't do it right call an electrician please.don't want to see or hear you have gotten hurt.
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Old 10-17-2010, 04:23 PM   #8
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Smoke Detectors on Different Circuits


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A relay gets you nothing.stop trying to burn things up.just connect smoke det's on same ckt and be safe.if you can't do it right call an electrician please.don't want to see or hear you have gotten hurt.
What you say is true, if its possible! However, the OP said that his new addition is wired from a sub-panel.
Ideally, the new addition smoke detectors would have been wired from the main panel and on the same circuit as the old.
A relay does give isolation and is the only possible solution beyond wiring the new detectors back to the main panel.
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Old 10-17-2010, 04:35 PM   #9
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Smoke Detectors on Different Circuits


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OK, but what's that, and how do I do it?
When a smoke detector is triggered, it sends 120 volts out to all the other detectors on the signal lead.
Rather than connect the signal lead to the next detector, the lead is connected to the coil of a relay. The relay contacts are connected to the secondary detector in such a manner that the detectors a/c feed voltage is fed through the contacts and back to the secondary detectors signal lead.
Your electrician will know how to do this and can supply you with a code compliant relay, that would be suitable.

Keep in mind that 2 relays would be required! One to signal from the original end and another to signal back to the original.

I have to say, that this is rather convoluted and if at all possible, it would be best to run a new 14/3 cable back to the main panel and be done with it!

Last edited by Wildie; 10-17-2010 at 04:41 PM.
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Old 10-17-2010, 04:35 PM   #10
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Smoke Detectors on Different Circuits


balony...first not code compliant to use relays.if on same ckt why is there a problem.you will still have to connect the inter connect together.prob still exists.even if you use a relay.why in hell do you guy's want to jury rig every thing?
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Old 10-17-2010, 04:45 PM   #11
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Smoke Detectors on Different Circuits


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balony...first not code compliant to use relays.if on same ckt why is there a problem.you will still have to connect the inter connect together.prob still exists.even if you use a relay.why in hell do you guy's want to jury rig every thing?
hey! get your glasses and read the posts
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Old 10-17-2010, 04:59 PM   #12
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Smoke Detectors on Different Circuits


OK, let's play nice now....

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Old 10-17-2010, 05:07 PM   #13
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Smoke Detectors on Different Circuits


did read it they are on two diff ckt's.and you dont run a 14-3 or 12-3 which ever wire size you use from panel to smoke det.he will have to get det's on same ckt to work...
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Old 10-17-2010, 06:35 PM   #14
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Smoke Detectors on Different Circuits


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When a smoke detector is triggered, it sends 120 volts out to all the other detectors on the signal lead.
Rather than connect the signal lead to the next detector, the lead is connected to the coil of a relay. The relay contacts are connected to the secondary detector in such a manner that the detectors a/c feed voltage is fed through the contacts and back to the secondary detectors signal lead.
Your electrician will know how to do this and can supply you with a code compliant relay, that would be suitable.

Keep in mind that 2 relays would be required! One to signal from the original end and another to signal back to the original.

I have to say, that this is rather convoluted and if at all possible, it would be best to run a new 14/3 cable back to the main panel and be done with it!
by the way,the red sense wire only see's 12 volts.

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