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#175768 - 03/10/08 10:21 PM Screw in Conduit  
Admin  Offline

Joined: Oct 2000
Posts: 3,459
I don't know if this should be in the "photos for discussion" or the "Violation" section. These photos seem to demonstrate the reason for the new requirement 300.4(E) in the '08 NEC.


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#175769 - 03/10/08 10:38 PM Re: Screw in Conduit [Re: Admin]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
An excellent example to post!

300.4(E) is a new section, that specifically states that pipe run under such decking be spaced at least 1 1/2" away from the decking. No one-hole straps, no mineralacs, no 'thin' strut .... standard strut is 1 5/8" deep.

Yet another change that caught me by surprise.

#175773 - 03/11/08 04:34 AM Re: Screw in Conduit [Re: renosteinke]  
JohnJ0906  Offline
Joined: Oct 2006
Posts: 65
Baltimore MD USA
Just to add one thing: the screw didn't penetrate the conduit. However, if the conduit had been secured properly, it probably would have. As it was, it pushed the conduit away from the roof.

#178724 - 06/10/08 08:43 AM Re: Screw in Conduit [Re: JohnJ0906]  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
Not only that, but, as this is the roof deck, the temperature just under this metal pan can exceed 160 degrees F. in the summer. Look at the bottom part of Table 310.16 for the required ampacity adjustment for any conductors inside these conduits.

It is best to NEVER install conduit directly under the roof decking. Run your conduit and cable on the bottom part of the truss, and secure it in place with tye-wire if a ceiling is to be installed, or strut if it will be exposed.


#178734 - 06/10/08 11:53 AM Re: Screw in Conduit [Re: earlydean]  
junkcollector  Offline
Joined: Apr 2007
Posts: 36
Central MN
My dad once told me about a story similar to this that happened at the factory he works at: When the building was getting a new roof, the roofers where screwing down some sort of foam with long screws. One of the screws went right into a 2 1/2 inch conduit encasing a 480 volt feeder. Ka-boom!

Luckily no one was injured.

The same building also has several conduits running on the ceiling (similar to the photo) that contain 4160 volt conductors.

Last edited by junkcollector; 06/10/08 11:54 AM.

#178763 - 06/11/08 05:48 AM Re: Screw in Conduit [Re: junkcollector]  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
I don't think there's any question that 300.4(E) is justified!

#178766 - 06/11/08 09:20 AM Re: Screw in Conduit [Re: SteveFehr]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,135
I can't find the picture but I caught a great "screw in conduit" violation some years ago.
It was commercial metal stud construction and they were using short pieces of EMT to support EMT, tek screwed across the steel studs and then a one hole strap was tek screwed in the support piece to secure the EMT. They got dyslexic on one and screwed the one hole strap into the pipe with the wire, "securing" the support.

Greg Fretwell

#178924 - 06/16/08 06:14 AM Re: Screw in Conduit [Re: earlydean]  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
Originally Posted by earlydean

It is best to NEVER install conduit directly under the roof decking. Run your conduit and cable on the bottom part of the truss, and secure it in place with tye-wire if a ceiling is to be installed, or strut if it will be exposed.

Be careful here. To be in complete compliance with some building codes you must have a written statement from the engineer of record to support anything off the bottom chord of a roof truss. The engineer of record must be the one who stamped the roof structure design.


#179102 - 06/25/08 11:29 PM Re: Screw in Conduit [Re: resqcapt19]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,135
I just went through the engineering maze and one of the things I had was truss engineering. On one of the notes it will specify what the allowable load is on the bottom chord but you have to look for it. In my case for regular 2x4 residential trusses 27' span the bottom chord load was max 10 psf. That has to include all the drywall, insulation, wiring, lights, fans and anything you might put in the attic so 10 pounds goes away pretty fast.

Greg Fretwell

#179251 - 07/03/08 11:15 AM Re: Screw in Conduit [Re: gfretwell]  
WNYJim  Offline
Joined: Jun 2002
Posts: 39
Rochester, NY, USA
I was in a new building in Washington state recently and noticed the lack of conduit runs but several drops down to machines.
Looking closer I saw slots cut in the roof pan decking.
Conduit was laid into the valley of the roof decking before the board, tar and stone went on!
And I didn't have my camera with me.
Can you envision the problems they will have?

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