Ducts, Plenums, and Other Air-Handling Spaces

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elbac

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In reference to 725.3(C)

The following text is taken from Mike Holt's book, "Understanding Low-Voltage and Power Limited Systems," page 263.

(C) Ducts, Plenums, and Other Air-Handling Spaces. Where necessary for the direct action upon, or sensing of the contained air, Class 2 and Class 3 cables can be installed in ducts or plenums if they are installed in EMT, IMT, or RMC as required by 300.22(B).

Question:
Who, what, where, when, why, or how would an air-handling system ever need cable(s) or conduit to make it "necessary" or need to "sense" anything?

As far as I know, and I also heard Mike Holt say the same thing, these HVAC guys don't want anything in their duct work. I don't know, maybe I just don't get it.
 
Re: Ducts, Plenums, and Other Air-Handling Spaces

Dunno, think BIG air ducts. Think smoke detectors.

The code only allows this wiring, it doesn't say you are going need it.

-Hal
 
Re: Ducts, Plenums, and Other Air-Handling Spaces

I do alot of work in large air handlers. Some are so big, that you need a step ladder inside the unit to work. There are alot of devices that are installed in these from temperature sensors to damper actuators, with a host of others in between.
 
Re: Ducts, Plenums, and Other Air-Handling Spaces

Thanks hbiss & jbwhite for the reply posts.
Yep, I've been thinking BIG air ducts lately.
One of my sons was telling me about the huge
air handlers they were pulling cable through.
I also asked a HVAC friend of mine that was just
telling me about various detectors they install inside these huge air handlers.
 
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