Conduits in roof cavity

[augie47]

How would you address this situation.
New construction, an enclosed overhead walkway between buildings.
Plans call for metal roof decking, with Styrofoam on top insulation and a rubber roof.
The plans also call for power conduits between buildings to be installed on top of the decking, surrounded by the insulation and then covered by the rubber roof.
The install is under '05 NECK so 300.4(E) is not applicable even if it applies to this type install.
It appears to me it is somewhat akin to installing conduit "in an oven".

 

[raider1]

First you would have to adjust the ampacity of the conductors in accordance with the temperature adjustments of 310.16.

Second I don't see why you could not call the conduits on top of the metal decking and under the styrofoam as being subject to physical damage and require some type of protection from the possibility of being hit with a long screw used with the styrofoam.

Chris

 

[G._S._Ohm]

It appears to me it is somewhat akin to installing conduit "in an oven".

What is your estimated max. roof temp.? This depends in large part on the roof emissivity.
http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html
From this table rubber is coming in at pretty absorbent. Not so good.

Can you circulate air in the conduit or a cooling liquid through a small pipe in the conduit? Another way to cool the conductor insulation would be to have the cable conduit inside of another larger conduit in which cooling air is circulated, either by convection or by forced air.

 

[Jim W in Tampa]

Being it is under insulation and assume open air under it might be hard to determine the temp.Also puts it as subject to damage and is far from a great install. Your the inspector,could you demand a letter from electrical engineer as to size of wire to adjust ?

 

[chris kennedy]

I just did one of four yesterday. This will be under 6" of foam insulation so I didn't even think about derating as this conduit will be closer to the dwelling unit ceiling temp than the roof temp. It did get derated by default as this is a 15A lighting circuit and I don't carry 14 on my truck.

My initial response to this proposed installation was similar to Chris J's, subject to damage in 20 years when they strip the roof again. Screws are not an issue here as they glue the insulation down with tar. But this is what the owner of my company, the AHJ and the condo board agreed upon. I don't pick-em, I just stick-em.:roll:

 

[Cow]

Second I don't see why you could not call the conduits on top of the metal decking and under the styrofoam as being subject to physical damage and require some type of protection from the possibility of being hit with a long screw used with the styrofoam.

Chris

I agree. If they're using long screws to secure the foam/insulation to the metal roof decking, then running a conduit under the foam is a HORRIBLE idea. Whoever runs the conduit that way get's what they deserve if a screw goes through it....:roll:

If they're sticking it down with adhesive, then I wouldn't sweat it. I would be slightly concerned about screws in the future should they replace the foam, but that's a problem for then, not for now.

 

[jusme123]

While I do not agree with the installation, I would install rigid conduit . Rigid conduit is to code when run on interior decking.

 

[jusme123]

[/QUOTE]

What was the conduit feeding that is sitting behind the mushroom exaust

 

[chris kennedy]

What was the conduit feeding that is sitting behind the mushroom exaust

That was the rotted EMT we replaced.