The NEC considers pipe to be adequately protected when it is either under, or even with, the bottom of the slab. I'd like to open this topic for discussion. Many times, foundations/slabs in industrial buildings, and restaraunts, will be cut to install or replace air, water, sewage, and drain lines. When this happens, the guy operating the saw will typically set his cut to just a little past the depth of the slab. When this happens, and the conduit is installed "per code," wires get cut. I propose that even burying the pipe an inch or two would make a big difference in how many repairs need to be made. Your thoughts?
Mr. John, I see your point, and can understand why you are concerned. The code, as you know, has various burial depths depending on the application (Note 300.5). You also know that sometimes we install floor plugs, which calls for conduit to be installed directly in the foundation. There's an application that may be cut through as well. In my opinion, the NEC is limited in what it can do. 90.1(A) says the practical safeguarding... So, maybe, anything more wouldn't be practical, and the code makers are simply not able to further protect people and property in this particular application. There's nothing more fun than saw cutting into a circuit or feeder. It keeps you on your toes.
Practically speaking, Doc
The Watt Doctor Altura Cogen Channelview, TX
Re: Pipe under Foundations#84168 03/13/0308:50 PM03/13/0308:50 PM
Well I install pipe that way and I repair pipe that gets cut.
One of our customers is now putting it in the specs to be down 6" and the saw guys contract says to not cut all the way through, he must finish with jack hammers this kind of defeats the purpose as one slip with the hammer and the PVC is done.
I have no problem doing it if it is in the spec. but I will not excavate for free just because they might saw cut the floor.
Bob Badger Construction & Maintenance Electrician Massachusetts
Re: Pipe under Foundations#84169 03/13/0309:01 PM03/13/0309:01 PM
John; I agree that it would be nice if the pipes were buried deeper. Whenever we get the chance to scratch them into the stone, we do. On most of our jobs we supply an "as built" with the approx. locations of underground included. On most of our service calls and additions, none exists. Go figure!
Typically when we go to saw cut a floor, (because we know what a joy it is to repair under-slab conduits and water lines)we bring our tracer and use our best guess to decide the cut. We seem to have good luck and we usually set the saw depth to 3". The rest is "Macho" time.
Re: Pipe under Foundations#84170 03/13/0310:19 PM03/13/0310:19 PM
I have sawed into slabs that were 30 or 40 years old,and sometimes the rigid is as much as 3 or 4" under the dirt. Was this a common practice back then? Also,when did pvc become the mat. of choice? Around here,up until the early 80's it was all rigid under a slab. Then pvc slowly began to catch on. Do any EE's still spec rigid?Haven't done much commercial in the last 10 years. Russell
Re: Pipe under Foundations#84171 03/13/0310:37 PM03/13/0310:37 PM
We usually dump it 4-6" under the slab pour, a few times it was deeper, as the trench guy got a little carried away. When we are the guys that the "cuts" are for, we get a laborer to "scratch" down 4".
Lately, PVC seem to be "king" We had 1 job with RGC as a spec in the floor in the last couple of years. We do the "as-built" thing, and we do the tracer trick if we are doing the cut, sometimes we use good ol' "lucky guess" & "dunb luck"
Best blast was a 200 amp feeder, 3 phase, 208. Had a section of slab that was 3", set the saw at 4" dead, brand new diamond blade, & you guessed it........lots of sparks, and a big BOOM. Maintenance guy said "no pipes" in this area, agreeing with the old "as builts" & the PE's best guess. John
Re: Pipe under Foundations#84172 03/13/0310:57 PM03/13/0310:57 PM
I’m with Bob. While it would be nice, this is a competitive industry where low bid is king. Unfortunately we can’t afford to do things that are not code or part of a job spec just because we are good guys. If it’s a requirement great, everyone is playing at the same level. If it’s not, in the bed of sand they go.
Re: Pipe under Foundations#84173 03/15/0302:17 PM03/15/0302:17 PM
I have had discusions with GC's about this. Sometimes contractors wanted to keep the PVC pipe in the concrete. Well most spec sheets say that if you put the pipe IN the concrete then you need written permission from the architect or engineer. They will have to be responsible for floors that crack. I wish that the NEC said to put PVC at least 2"-4" below the slab. I think that it would out of harms way. BTW what about sticking the PVC in the concrete when they are pouring curbs. I have seen it where some electrical contractors (EC) will just lay the PVC right in th trench, then install concrete and Belgium block on top of it. This forms the new curb in a parking lot and they pick up all of their site lights this way.
Re: Pipe under Foundations#84174 03/15/0302:26 PM03/15/0302:26 PM
In most situations, I would agree here, and prefer to get the Conduits at least 2" below the bottom of the concrete slab.
Sawcutting dudes are always talking about the sparks flying out of underslab conduit runs! Some have been shocked to an almost lethal extent! (if I was doing this work fulltime, the first nibble would be the end of my sawcutting career!).
The most common scenario of sawcutters vs. underslab conduits is in the pourstrips for Tilt-Up Buildings. Seen sawcutters blast through Subfeeders and the area lighting circuits (for wallpacks). The Subfeeds pop really hard (sawcutting personnel usually take a break and check their pants! ...) The HID wallpack circuits do nothing, until the timeclock kicks in the lighting contactor! That's when you hear the familiar roar!
For most underslab runs - used to feed Floor Boxes containing Receptacles + Comm/Data - I prefer to drop as low as possible. After inspection, backfill and compress, until back to proper level. Then drill and install Rebar "Pins" in the cut areas.
When Walker Duct (or similar) comes in to play, there is nothing else to do except place it in the finished (repoured) concrete slab. however the conduits feeding the pullboxes can be swept down below the slab bottoms.
Now for the exceptions!
In areas with Flammable Vapors seeping upwards from the Earth, the need to keep an unpenetrated Vapor Barrier just below the concrete slab is encountered. Vapor Barrier consists of heavy visqueen (sp?) plastic with Sand top cover and (I seem to remeber) fine peat gravel cushion layer underneath the plastic. it's been awhile, so don't quote this directly!
Working around this presents a new meaning of the word Tedious!
One certain area in Southern California has an extreme problem with Vapors seeping out of the ground and collecting in "Not So Preferred Places" (i.e. conduits and such! explosions waiting to happen!). The Huntington Beach / Huntington Seacliff area is prone to this. (Electure and Nick should know right away what I am getting at here!)
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!