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Old 08-13-2017, 04:53 PM   #1
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Default Fire Alarm wiring

As a part of a commercial upgrade job we are doing, the prints specify that we have to install a couple of smoke detectors and "connect to the closest smoke detector". I am aware of the circuit wiring of one nac following another with a termination resistor at the end. I notice that all of the existing class B conduit wiring is in series (the conduit not the wiring) - this visually represents easily the chain of one nac following the next to the final termination resistor - while this conduit line makes it easy to following which smoke follows the next, is this a requirement? Would it not be possible to install my new smoke detector using one conduit run branched off existing line, and then, if necessary install two pairs of wiring, allowing the sensing signal to be routed off the main line to my device and back again through the same conduit - from there continuing along the chain?

The question here is simply whether a smoke detector needs two conduit runs or not to clarify the direction of the run. A drawing would show this more clearly as one line of sensing devices in a row with the new one branching off one proposed in the middle of the line.
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Old 08-13-2017, 06:03 PM   #2
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Originally Posted by stdave View Post
As a part of a commercial upgrade job we are doing, the prints specify that we have to install a couple of smoke detectors and "connect to the closest smoke detector". I am aware of the circuit wiring of one nac following another with a termination resistor at the end. I notice that all of the existing class B conduit wiring is in series (the conduit not the wiring) - this visually represents easily the chain of one nac following the next to the final termination resistor - while this conduit line makes it easy to following which smoke follows the next, is this a requirement? Would it not be possible to install my new smoke detector using one conduit run branched off existing line, and then, if necessary install two pairs of wiring, allowing the sensing signal to be routed off the main line to my device and back again through the same conduit - from there continuing along the chain?

The question here is simply whether a smoke detector needs two conduit runs or not to clarify the direction of the run. A drawing would show this more clearly as one line of sensing devices in a row with the new one branching off one proposed in the middle of the line.
There is no general rule for F/A systems as the technology has changed over the years.

There was a time when loops were required to attain supervision.

Now it's gotten to the point that branched circuits can work -- and some systems even have addressable notification appliances.

Consequently no-one can provide decent advice over the Internet.

You'll have to dig into the technicals for what has been installed -- by contacting its maker.

The FACP largely dictates what, where and how you may expand the system... and there are a LOT of variations.
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Old 08-13-2017, 08:59 PM   #3
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if you're sure it's class B, I don't think there would be a code problem. In a class A system you shouldn't and can't run both in same conduit and supply and return wires must be horizontally 4' apart .
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Old 08-13-2017, 09:10 PM   #4
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Is this an addressable system?
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Old 08-13-2017, 09:23 PM   #5
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Is this an addressable system?
doesn't seem to be. he mentioned class B with resistor
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Old 08-13-2017, 09:44 PM   #6
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If it is really class B IDC wiring to the nearest smoke detector, just run 4 conductors of the same gauge to your 2 new detectors . Running class B in and out in the same conduit is a non issue. A on the other hand is all about redundancy. You loose your redundancy by using the same path for both pairs.
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Old 08-13-2017, 10:21 PM   #7
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OK - that's helpful. Thanks all for the input.
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Old 10-03-2017, 08:45 PM   #8
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I know im a bit late.... but if its class b wiring, mostly done in same raceway. Class A definitely needs to be seperate raceways or it kind of defeats the purpose.

Ahj sometimes will have imput in how things get wired.

Here in boston everything needs to be class a. When the FD runs a test they will make us pull down a horn/strobe and make sure all others still work

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