PVC in slab

[dema]

We have a PVC conduit laid in slab which is taking on water. The owners are all het up because the conduit is to be used for power to feed an expensive piece of machinery and they think the vapor and fumes will cause long term problems. My thought is, "Why are you talking to me about it?" I'm electrical. The structural engineer and/or civil engineer should have foreseen an issue. The water table is apparently inches below the surface. We didn't pour the slab. We were requested to put the conduits there.

So, am I being a crabby ostrich or is this the case?

 

[bphgravity]

Re: PVC in slab

How is it that plumbers can keep the water in, but electricians can't keep the water out?

[ July 29, 2005, 03:45 PM: Message edited by: bphgravity ]

 

[charlie b]

Re: PVC in slab

It's a civil engineering problem, not an electrical engineering problem. I don't know if there is an electrical solution. Does anyone think that the conduit can be sealed, using materials normally reserved for Classified (hazardous) areas? I'm not suggesting that the conduit system be converted to a Class 1, Div 1 installation. That might not be possible anymore. But can the stub up from the underground into the equipment be sealled in a similar fashion?

 

[celtic]

Re: PVC in slab

Originally posted by bphgravity:
How is it that plumbers can keep the water in, but electricians can't keep the water out?

That's easy...glue and primer! (let's not forget a recent thread: PVC Glueing )

Originally posted by celtic:
....plumbers prime to help keep water IN, electricians prime to help kep water OUT.

LOL


But to answer dema's concerns:
PVC in a slab is allowed by NEC...make the connections as sound as possible...the owner's "thoughts" must be based on same "fact" - what kind of place is the installation in?


Whether the conduits were PVC or RGS would make little difference - as long as they were made up correctly (Incorrect thought is: We are not putting water in them ~ Correct thought is: We are keeping water out).

If the owners are willing to foot the bill for overhead runs - so be it....but w/o a signed change order and accompanied feeder/BC details, the PVC stays and WILL be used.

 

[bphgravity]

Re: PVC in slab

Originally posted by charlie b:
It's a civil engineering problem, not an electrical engineering problem.

I once read a joke somewhere that says, "What's the difference between electrical and civil engineers?" The response, "Electrical engineers make weapons, civil engineers make targets."

I always liked that.

 

[jimwalker]

Re: PVC in slab

"That's easy...glue and primer! (let's not forget a recent thread: PVC Glueing )"
Yes and i seen more of it today.Real shame when you see a green helper being told only glue the cut end this is on a job that has thousands of feet of 3/4 to 3 inch pvc.And many of them have pulled lose.I am at the point of i no longer care.What good does it do when only 1 or 2 guys know how to glue pvc.
And if your getting water in your pipe then its time to start learning how to glue.When doing plumbing i almost never have leaks,but then that using cleaner and plenty of glue.Ever get the idea they are buying the glue ?

Plumbers are forced to show a pressure test,maybe its time electricians do too.

[ July 29, 2005, 04:41 PM: Message edited by: jimwalker ]

 

[celtic]

Re: PVC in slab

Originally posted by jimwalker:


Plumbers are forced to show a pressure test,maybe its time electricians do too.

Technically, we are too..as per manufac.s spec sheet ~ PVC Glueing ~

Originally posted by celtic:

Cementing PVC Conduit for Submerged Areas Requiring Air or Water Tightness:

2. Test workmanship by
conducting a low pressure air
(3.0 - 5.0 psi) test after system
is installed and cemented

...and I'm sure that happens with any regularity.

The italicized portion is copied from Carlton

 

[thinfool]

Re: PVC in slab

I have rarely seen a dry underground conduit, regardless of the care given during installation. All unsealed conduit breathes, the ground temperature causes the moisture in the air to condense on the walls and conductors, and it tends to stay there....Over time it can fill the conduit...at least to the point where air movement thru the conduit is stopped.

 

[redfish]

Re: PVC in slab

How is the conduit taking on water when it is in the slab? I would think that inside a slab is fairly waterproof.When I am gluing my pipe, I always keep in mind that I am going to run wire in it.I can run plenty of pipe in good time and still glue it right. I know that when you are installing thousands of feet of PVC, it gets old, but always keep in mind the wire installation. Take a little more time and save a lot of time.

 

[peter d]

Re: PVC in slab

As others have said, trying to keep water out of inground or slabbed raceways is a losing battle. That is why we are required to use wet rated conductors in these applications.

 

[celtic]

Re: PVC in slab

...and we haven't even talked out the conduits that somehow end up getting filled with cement.

 

[dema]

Re: PVC in slab

Where does it say that wet location conductors are required in slab? I've been specifying THWN, but I don't see that it's required.

 

[peter d]

Re: PVC in slab

Originally posted by dema:
Where does it say that wet location conductors are required in slab? I've been specifying THWN, but I don't see that it's required.

True, a slab is not an underground location. I think you are correct. That's what I get for not using my code book.

 

[macmikeman]

Re: PVC in slab

Seems like whenever I have to run pvc conduit outside its going to be raining. Its like Murphy's law or something. It stops as soon as I move inside. Use duct seal to seal up the conduit at each end . This should shut up the owner's for awhile.

 

[augie47]

Re: PVC in slab

Where does it say that wet location conductors are required in slab? I've been specifying THWN, but I don't see that it's required.

Try NEC Art 100: "Location, Wet........
concrete slabs...in direct contact with the earth...."

am I misreading?

I agree with thinfool.
Over the years I've seem slab PVC glued various ways, and taped and ever "tarred"...never seen any wire pulled out that wasn't wet.

 

[allenwayne]

Re: PVC in slab

Ok a true vote,who has primer and cleaner on the truck whan sent to do an underslab