Existing rooftop exhaust fan...

[c_picard]

Good afternoon people,
Let's just say for debates' sake there is a roof exhaust fan in an existing building... and said fan motor is being fed by type AC cable THROUGH the ductwork. Now let's say I was to install a new exhaust fan right alongside the old one, would a better idea be to penetrate roof with RMC and change to Liquidtite? Foreman wants me to duplicate existing. I don't see how it's legal to run cable in duct, is this common practice? Unfortunately I have no one wiser on the ways of the code to turn to, I hope MODERATOR MAN will allow this post.
Thanks for any input.
Chas

 

[tom baker]

In general wiring can not be in ducts unless its necessary for a device that acts on the air. But this is for air plenums and not exhaust ducts.

 

[c_picard]

Well, I suppose the fan/motor is a device that acts on the air. I looked in NEC and evidently MC is OK in enviromental air spaces. I'm sure at least a few of you guys have wired a thousand of these things and maybe just don't think I deserve an answer, oh well. I thought I had found a valuable resource.

 

[growler]

What type of exhaust duct are you talking about. You can get a definite no by reading 300.22 (A) if the if the duct is for Dust, loose stock or flammable vapor removal. I have seen what you are describing used for bath exhaust ( commercial ). I am not sure it's legal ( methane gas, flammable vapor, I may be pushing it a little ) ). But for hood vents for restaurant equipment, or paint shop are even fibers I think you could get a no ruling on the MC inside the duct.

 

[c_picard]

Thanks Growler, I was losing faith for a moment...
It is a commercial job, afew are for bathrooms, one for an electric room via t-stat. I was just wondering what a typical install would look like.

 

[c_picard]

51 lookers, but only one taker. What am i doing wrong? Should I work on my people skills? Or you fellas want me to learn the hard way like you did back in '62...

 

[LarryFine]

We'd be happy with a good description; better yet, a picture or two. It's hard to give a specific answer to a vague question. Don't take this the wrong way.

By that, I mean that if it were simple to answer, you probably wouldn't need to ask it. "We need more detail" is what the above responses are saying.

 

[c_picard]

THanks Larry,
To be more specific, the exhaust fans I'm speaking of range from 1/8 to
1 1/2 hp, they are mounted on a "curb" on the roof. From a distance they sort of look like a mushroom. The existing fans have type AC running through the ductwork, sleeved through a short piece of EMT up to the motor housing. The Ac terminates in a handi-box with a SP switch as a disconnect. Does anyone know the type I mean?

 

[growler]

I doubt if anyone would get bent out of shape if you do run the MC cable in the duct. Around here most inspectors would say go ahead. They are not going to look at it anyway. If I were going to do this to keep from doing a roof penetration I would get as close to the roof as possible before drilling a small hole in the duct then run the cable and seal the hole with calk and metal duct tape. Back in 62 labor was so cheap that we didn't use fan motors we would hire a little guy to sit on the roof and spin the fan manually.

 

[hardworkingstiff]

I've wire a few roof top units. We usually went through the roof (with EMT) between the roof curb and the ductwork. Put a LB up there and flex to the disconnect. Leave the disconnect with flex and go into the "mushroom" rooftop unit to connect to the motor.

These were all new buildings, so I didn't have to worry about leaks, it was the roofers job to keep it from leaking.

 

[izak]

c_picard

I have seen the type of fans you are talking about

the way the emt nipple is located in the fan, there is really no alternative than to put the conduit in the ducting..
what we have done on those is to mount a four square box on the duct, inside the building, close to the roof, drill a hole in the duct and flex out of the back of the box a short distance and couple onto the emt nipple

there are a couple of code rules that prohibit this such as in resturaunt vent hoods or vents for loose stock or flying fibers..

but for a general use exhaust fan, i couldnt find ANYthing in the code to prohibit what i thought initially was a faulty installation.

 

[izak]

in fact.. article 300.22 (A) applies specifically to your application.

check it out

 

[dlhoule]

51 lookers, but only one taker. What am i doing wrong? Should I work on my people skills? Or you fellas want me to learn the hard way like you did back in '62...

Actually in my case it was '60, but in many of the cases it was much later. I didn't respond for 2 reasons. This is the first I've seen it and I have no experience doing other than with a roof penetration. Have never used duct work for the wiring, but that doesn't mean it is not legal. I didn't know.

This is a great place to learn. There are, however, a few of us that are very opinionated. Never take anything too personally.