# Piping for Class A/B Fire Alarm Sytems



## sweetp

Greetings all! I have been scouring some of the previous posts past and present and haven't been able to come up a definitive answer to a couple of questions I have concerning F/A systems. I understand the differences between the two, how they function, why, where and when they're are installed, and have been involved in several installs but mostly w/ cables installed on bridal rings, ect.. Have been catching up on the current NFPA 72 requirements along with current NEC codes. What I am trying to determine in the piping scheme are you allowed to run Class A SLC circuits in the same conduits as the Class B, mostly notification circuits? The NEC seems to point to it be allowed, but not very clearly, that I can see anyway..and NFPA 72 code, doesn't seem to clarify, (other than requirements for the incoming and return paths regarding min and max lengths to other devices), though I just may not have come upon the right section yet...:001_huh: Also any information on Pre-Action systems and special requirements for the sprinkler system would be of great help...Im sure I could ask a hundred questions...and I may further down the road as we get into this..but don't want to get greedy! 
This is a great site filled with a myriad of knowledgeable, friendly and helpful people
of which I have I learned much in my short time here. Thanks in advance for any and all help! Much appreciated! Be safe! :thumbsup:


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## HARRY304E

sweetp said:


> Greetings all! I have been scouring some of the previous posts past and present and haven't been able to come up a definitive answer to a couple of questions I have concerning F/A systems. I understand the differences between the two, how they function, why, where and when they're are installed, and have been involved in several installs but mostly w/ cables installed on bridal rings, ect.. Have been catching up on the current NFPA 72 requirements along with current NEC codes. What I am trying to determine in the piping scheme are you allowed to run Class A SLC circuits in the same conduits as the Class B, mostly notification circuits? The NEC seems to point to it be allowed, but not very clearly, that I can see anyway..and NFPA 72 code, doesn't seem to clarify, (other than requirements for the incoming and return paths regarding min and max lengths to other devices), though I just may not have come upon the right section yet...:001_huh: Also any information on Pre-Action systems and special requirements for the sprinkler system would be of great help...Im sure I could ask a hundred questions...and I may further down the road as we get into this..but don't want to get greedy!
> This is a great site filled with a myriad of knowledgeable, friendly and helpful people
> of which I have I learned much in my short time here. Thanks in advance for any and all help! Much appreciated! Be safe! :thumbsup:


Look in Article 760.


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## halfamp

you can use the same raceway for class A and B circuits. it's about survivability, so if it's class A, you're supposed to have two pathways, one in and one out, of each device


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## sweetp

Thanks for the input guys! Much appreciated! I've read thru 760, thanks Harry...that's what lead me to believe I could, so Half, If I wanted to let's say run a main trunk line to somewhere, let's say in the middle, hit a trough..j box...then split my zones, (Initiating(CL-A) and indicating (CL-B)) at that point...and maintain a separate conduit for the redundant loop, that would be acceptable?


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## halfamp

yes, as long as (theoretically) each of your initiating Class A devices has a separate path in and out of the device. Ideally, from your FACP you'd have your main trunk exit one end of the room, and your Class A return leg coming back in from another side of the room, if that makes sense.

Imagine it like a circle if you will. Class A will still work if you put a cut in that circle somewhere


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## sweetp

Yes, thank you, Half I do understand that requirement for the class A system. We just need to hit several aux F/A panels on different floors, trying to consolidate as much as possible while still maintaining the code...it's a fairly complex system..with many duct detectors, damp. motors, Sec. interfaces...BMI interfaces..Window operators...fan shut downs..ect...Im sure you get the picture..and not a lot of room to go anywhere!


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## NacBooster29

If you're installing more than one slc circuit in conduit, check the manufacture specs. Some will require twisted shielded cables. 
At the very least I would try to run twisted cable on your slc. I just finished a fairly large job where we ran thhn through conduit, and we were picking up induction on the slc loop. 
It turns out that twisted cable helps with this issue.


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## sweetp

Thanks Nac!, yes all of the cables are twisted pair, and depending on their function shielded also. Mostly on the Class B circuits, audio loops, and power circuits.


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## xaH

The only separation requirements that I'm aware of for Class A is the incoming and outgoing loops must be in separate raceways or cables. I am not aware of any code rules that forbid Class A and B circuits in the same raceway.


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## Nab

Only issue I see is, is it an addressable system with voice evac? This is the only issueI would invision you having. Not sure how town your working in is, but for me in the NH, MA area every fire department has different rules. The issue with an addressable system with speakers strobes is the slc (data loop) can cause interference with speaker strobes. Not with horn strobes however.


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## Nab

NacBooster29 said:


> If you're installing more than one slc circuit in conduit, check the manufacture specs. Some will require twisted shielded cables.
> At the very least I would try to run twisted cable on your slc. I just finished a fairly large job where we ran thhn through conduit, and we were picking up induction on the slc loop.
> It turns out that twisted cable helps with this issue.


Out of curiosity, what brand of panel. I ran into inductance off of a monitor module in a fire light panel. the box was attached to the same conduit as the conventional loop of the monitor module and my speaker wires even though they were twisted pair. Had to run a shielded FPLP straight to the amp to get rid of the interference.


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## NacBooster29

It was also a fire light panel.
They don't put it in manuals. But if you run into that issue again.
Place a 4.7 k resistor across +, - on the slc.
The resistor allows capacitnce to drain from the + , to negative.

Having an slc loop with capacatance on it screws up its polling.
Often causing invalid reply. Or missing device troubles.


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