Using RMC near normal water level

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PADDY

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OK....using PV coated RMC within 9" of NWL (normal water level) in a municipal-outdoor tank, below grade. Tough install, plus the fact that it's Class 1-Div-2 location.

Problem: These conduits are mounted under a concrete walkway with 10"
above raceway that will flood under abnormal operation. Key here is it will happen ! NEC really has no methods to relate to for the submergence of raceways. The design team is insistent on this method of install. Fate, or should I say warranty, will have us back on site making repairs from a boat once tank is put on line (during a real cold snap).

In the past (similar installs) we've installed classified drains....but in cold MN, the damn things freeze up.....then problems occur.

Can anybody offer a road to success for this application, I find no support from the NEC.
 
The NEC won't give you any design assistance. It's function is to ensure a safe installation.

What exactly will fail with respect to your scope of work ? That wasn't clear in your post. The PVC coated RGS (Ocal I assume) should withstand the exposure to water and if you are using THHN / THWN conductors, they should not sustain any damage, so I'm not clear what the exposure is here.

Maybe a little more info ...........

Oh, and welcome to the forum.
 
I agree on the NEC vs design.....as many of us turn to the NEC at times to support some logic in applications....my battle is design intent, but looking for any NEC support to battle back.

Failure is the install...conduits split..wiring shorted.

You're correct OCAL and THHN.

Last install utilized about a 1/4" drop per 20 feet...towards a sealoff with drain. Sure wished I'd saved the picture ! Picture a rack of 7 conduits supported horizantally under a concrete walkway in a huge tank. Conduits are suspended 9" above normal water level and 12" under walkway deck.

Condensation or sewage (from high level), accumalated at drain end and split the conduits. As the freeze and submergence continued.....wiring failed (shorted to ground) as wiring appeared to be pushed out at the split in conduit during proceess described. Seeing is believing.

My best guess is condensation accumalated in conduit (over time) being within 9" of warm sewage during the cold snaps.....

One item I'm searching out is, do threaded couplings have UL listing for rain tight...water tight or any tight...haven't found anything yet.

Thanks for the welcome
 
Is aluminum or stainless rigid an option ? RMC when made up with threads is wet location approved, but that doesn't seem to be the problem you are having.

How about heat tracing the conduits ? That would help with the freeze / thaw problem. There are heat tracing products that are suited to corrosive environments.
 
PADDY,
Conduit couplings are straight thread and will leak. If you used plumbers couplings (this would be a code violation) you could eliminate the leakage, but you would still have water in the raceway as a result of condensation. If you can't get the water to drain out, I don't think that there is any raceway that will stand up to this installation. Can you use tray cable and cable tray to eliminate the problem?
Don
 
Conduit type not a factor. PVC-rigid specified for highly corrosive area.

Confirmed on the wet location rating......for RMC, but even tightly made coupling leak while submerged....and the most likely cause...condensation affect.......must be enough heat transfer to cause this.

Heat trace application was thought of until design engineer found out about possible submergence......also plays havoc on insulation wrap.
Had classified rating..tuff stuff, but not for submergence in sewage water.

Also redesigned layout application at handrail height.....but to many intersections of walkway where would have to go up and overhead..and down again.

Still trying to convince the powers to embed in concrete walkway......but they reply not structualy possible. Not much to work with here..stubborn type.

I brought up the UL rating issue for couplings on RMC because I remember something about EMT not to long ago.....Also what is it about the "wet Location" listing by UL. Do you know where they define this ??

still searching
 
Don

Boy....cable tray in a Class1-Div 2 location????

I know I can use hard usage cable for Power & Control...but the kicker is the I&C shielded cable..... haven't found any yet. I'll have to research the tray issue. Would be great just to eliminate the conduit !
 
:D Paddy, Welcome to the forum. :D

It is a long shot, but is there any chance of using compressed air on conduits when temperature drops below freezing. I am not sure they make a listed fitting for conduit, but they make them for electrical panels. If you had a few pounds of air pressure, I think it would blow the moisture out through the drain before it could freeze.
 
Paddy,
Boy....cable tray in a Class1-Div 2 location????
Click to expand...
Tray cable in tray is permitted for the power circuits and PLTC for the control circuits. All of the standard shielded cables are availabe in PLTC (power limited tray cable).
Don
 
When conduit is run from a low temp area to a higher temp area you are required to seal off the conduits.

examples: pipes entering a walk-in freezer. 300.7(a)
 
What about some sort of heat trace cable INSIDE of the conduit? You would probably have to use the self regulating type, so that no section would get hotter than the conductor insulation would allow. Maybe even couple this with the already mentioned compressed air to keep the drains clear.

Also, I remember it being mentioned a few years ago here or on the ECN forums about a "gel" you can inject into conduits that won't freeze and displaces the water. I believe that they make it for fiber optics, where they can get cracked due to ice movement.....OK, found it, it's made by Poly Water. See http://www.polywater.com/icefree.html

mike
 
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