underground conduit & frost heaves

[JoelG]

I'm curious which way the majority of people deal with buried cable in the Northeast, where we're subject to frost heaves and the frost line can easily be several feet deep (too deep to easily dig that deep).

So consider an existing outdoor outlet attached to a house, and say you want to extend the circuit underground for 15' or so and then come up to a new outlet in the yard somewhere. Say the outlet is supported directly on two PVC schedule-80.

Choice #1: PVC all the way. Expansion coupling on the vertical run down from the existing outlet because of potential frost heaves. Schedule 80 on the two vertical runs, Schedule 40 on horizontal run.

Choice #2. Same as above, but Carflex on the buried horizontal run.

I've always done choice #1 and never really had a problem. I've heard that some people have trouble with the PVC cracking in winter, so that's why they go to Carflex on the buried run. (there's no way I take the time to dig deep enough to get below the frost line here).

Do most people do Carflex to avoid PVC cracking from frost heaves?

Thanks,

Joel

 

[mdshunk]

I always run PVC the whole way. I don't "take the time to dig", the backhoe operator does. I generally go a tad deeper than the NEC requires, but not down to frost depth. I have zero trouble with frost heave, as far as I know. What I do have trouble with is sinking backfill along buildings that pulls conduits downwards at meter cans and such. I can't recal ever having seen a frost heave issue related to PVC that I, or anyone else, has installed. The sinking can be accomodated by an expansion coupling, but unless you have a place to leave slack in the conductors too, it can be a wasted effort. You'd eliminate raceway damage and substitute conductor termination damage . It's a tough thing. Check out the "slack box" gadget. It's kinda interesting. http://www.rizzcon.com/index.html

 

[stickboy1375]

I have zero trouble with frost heave, as far as I know.

I do temporary wiring every year for the local fire department carnival, You should see what happened to a panel located at a pavilion with no slip fitting, I could barely get the cover off, because the bottom got pushed up so much from the TA.... If I get a chance I'll snap a picture...

 

[busman]

Marc,

Agree completely. Have experienced quite a few underground services that have pulled the meter enclosure right off the building. You mentioned bad backfill. Do you know the specific cause of the problem. I would guess that the backfill is not compacted to the level of the undergroud cable before laying the cable.

Thanks,

Mark

PS: Forgot to mention that if it is the conductors pulling down, the insulators frequently crack and fail when the meter is removed. BE CAREFUL WITH THESE.