Conduit in/under slabs


  #1  
Old 10-10-09, 12:22 PM
K
Member
Thread Starter
Join Date: Oct 2009
Location: Georgia USA
Posts: 3
Received 0 Votes on 0 Posts
Conduit in/under slabs

I frequently have to bury PVC conduit in or under driveways. In cases where the conduit is running TOTALLY under a slab all the way to a retaining wall or post light location at the slab edge, I have always cut a ~ 3" deep trench under the driveway put in the conduit covered it over and lived happily ever after. The reason I do not like to bury conduit IN the slab is that doing so in thin slabs always causes a crack in the concrete right above the conduit.

However, this past week I did this again and along comes the inspector and he tells me that I have to bury the conduit at least 12 inches under the slab OR embed it into the slab per NEC 2008. Is this the new norm or ???

I do, in fact, not find anything in the NEC 2008 code that speaks to burial depth for conduit placed in earth UNDER a concrete driveway or slab. I do see that it is permissible to place the conduit IN the slab.

Maybe I missed something?

Thanks.
 
  #2  
Old 10-10-09, 02:12 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,964
Received 6 Votes on 6 Posts
Welcome to the forums! My book is in the shop and not accessible, or I would quote you the reference, but conduit must be buried at least 12" under walkways, etc. None the less, it could be a local thing, too and he sets the rules.
At least you didn't do like one guy did here....buried UF under a pour of steps to the lake. I never found all the faults in it. Concrete shifts and doesn't care what's in the way. Finally had to re-trench beside the steps (about 100') and lay new. Pedestal lighting every 20' on both sides of the walkway, so finding all the problems was impossible. Had to run under the walkway with conduit to the other side to pick up each light. Total mess.
 
  #3  
Old 10-10-09, 02:34 PM
K
Member
Thread Starter
Join Date: Oct 2009
Location: Georgia USA
Posts: 3
Received 0 Votes on 0 Posts
Yep.. Running UF within concrete has always been problematical. Long ago, it was permissible to put NM cable inside conduit and run it underground. I never have had a section of that fail, but not that is not to code either. However, it IS OK to run UF inside conduit INSIDE a slab and I have done that many times and it works fine. BUT.. after a time, a crack usually appears right over the pipe, even when it is underneath any rebar. That is why I like to put my conduit just under the dirt under a slab. But.. Likely "somebody" somewhere had a problem and so the code changes.
 
  #4  
Old 10-10-09, 04:26 PM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
If the slab is 4" or thicker poured concrete then the conduit can sit right under the slab. If it is thinner or something other than poured concrete (bricks, pavers, asphalt, etc) then 12" burial is required.
 
  #5  
Old 10-10-09, 08:37 PM
K
Member
Thread Starter
Join Date: Oct 2009
Location: Georgia USA
Posts: 3
Received 0 Votes on 0 Posts
Thanks ibpooks! That is what I thought. Can you tell me where to look in the NEC 2008 code book for that bit of advice? I have looked for it quite a bit and did not find any information on anything but putting conduit IN a slab or under 12 or 18" of dirt.

thanks.
 
  #6  
Old 10-12-09, 08:50 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
Article 300.5 "Underground Installations" and its associated table "Minimum Cover Requirements".

Under a minimum of 4 in. thick concrete exterior slab with no vehicular traffic and the slab extending not less than 6 in. beyond the underground installation....Non-metallic Raceways...4 in

Cover is measured from the surface of the slab to the top of the conduit at its shallowest point, so 4 inches of concrete = 4 inches of cover.

I have never heard of embedding conduit in the slab horizontally. It always lays in a trench under the slab. In fact, embedding it in the slab would give you less than 4" of cover which would violate code.

Since you mentioned driveways I'll include this one too: One- and two-family dwelling driveways and outdoor parking areas, and used only for dwelling-related purposes....Non-metallic raceway (no GFCI) = 18 in.....non-metallic raceway (with GFCI) = 12 in.

All other locations....Non-metallic raceway (no GFCI) = 18 in.....non-metallic raceway (with GFCI) = 12 in.
 
  #7  
Old 01-12-12, 12:11 PM
H
Member
Join Date: Jan 2012
Location: usa
Posts: 1
Received 0 Votes on 0 Posts
Thumbs up conduit under slab

After you determine the code. If you have dirt under the concrete and not rock, you can jet a conduit under the concrete to the point you wish to install the island.


First measure the distance from the edge of the slab to the point you want the conduit to surface. (This should be the shortest distance to the edge of the concrete from the island point you desire.

Dig a hole a little deeper than the bottom of the slab or to the depth code requires, near the edge or the concrete.

Get a 1/2" non-matellic conduit a couple feet longer than the total length to reach the desired point from the edge of the slab.

Attach a 1/2" PVC male hosepipe fitting to one end of the conduit and another Female hosepipe coupling to the other end of the conduit.
Attach a short straight hose nozzle ( a "JET" nozzle, this is the kind that is not adjustable and is the sleekest to a small tip where the water jets out) to the pvc coupling you attached to the conduit.

Connect a hosepipe to the female coupling and turn on the water full blast.

Insert the JET nozzle to the dirt beneath the slab at the desired depth and pointed directely in the direction of the desired point of the island. Keep the Jet and conduit Horizontal to the slab at the desired depth and Let the JET stream of water Jet the hole all the way until you have inserted the desired measured length of conduit under the slab.

Only when finished JETTING, do you disconnect the hosepipe while leaving the conduit in place (then the water will drain back out from under the slab through the conduit). Now CUT a HOLE in the slab at the nozzle end of the conduit. After you removed the nozzle from the hole you cut in the slab at the point of the island, then fish your wire through the conduit from the outside edge to the island point. Now add your elbow and whatever and concrete or foam the hole you cut back sealed with the wire sticking up and you are there.

I know it sounds easy, but it is messy and it is difficult to make the conduit nozzle end, end up at the exact disired location, but you can get it close enough to work if you don't hit any rocks or obstructions when JETTING --Good luck my friend.
 
  #8  
Old 01-12-12, 05:22 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,252
Received 113 Votes on 98 Posts
This thread is over two years old.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: