Conduit types

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steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
For underground conduits, our standard spec. has separate lists for:

1. More than 5' from foundation wall.
2. Within 5' from foundation wall.
3. In or under slab on grade.

Why would there be a difference? Is there any reason I wouldn't want PVC in any of the above??

Also, I am never really sure when to spec Schedule 40 PVC or Schedule 80. Is there any concern that the concrete could squish the schedule 40? Or for under a parking lot, is there a chance that heavy trucks could squish the thinwall? Even when there is a asphalt or concrete surface on the parking lot?

Thanks again;
Steve
 
The typical reason to prohibit PVC near a structure is due to settling.

If I run underground in through the side of a foundation with PVC and the soil settles later the PVC will be sheared off.

The typical spec I see has using RMC at least 10' from the building penetration.
 
steve66 said:
Also, I am never really sure when to spec Schedule 40 PVC or Schedule 80. Is there any concern that the concrete could squish the schedule 40?
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When allowed we will use 20 in cement pours, its eggshell thin but it is more than strong enough to hold it's shape in concrete till it sets.

Or for under a parking lot, is there a chance that heavy trucks could squish the thinwall? Even when there is a asphalt or concrete surface on the parking lot?
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I would say no chance assuming proper depth and soil compaction.
 
JohnConnolly said:
BAN it!
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That's kind a ridiculous, don't you think? :rolleyes:

But it does have that nasty habit of rotting out when buried. I am reminded of this on a weekly basis every time I drive by the rusty pile of it they dug up from the highway lights that are being relocated and replaced.
 
One of the worst cases for corrosion of rigid conduit is where there is a concrete to earth junction. If you want this installation to last very long you need to use additional corrosion protection on the conduit at this point.
From the UL Guide for rigiid ferrous metal conduit (DYIX)
Wherever ferrous metal conduit runs directly from concrete encasement to soil burial, severe corrosive effects are likely to occur on the metal in contact with the soil.
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Don
 
with the current drought here in the southeastern US we have resorted to using flowable fill wherever compaction is required.The red clay soil here turns to dust if you whacker pack it.A plus we have found with this is almost no settling at all.As a matter of fact the fill proctors out at 110% to 125% of the surrounding area and you can dig in it just like dirt unlike concrete or gravel (doesn't cave in like gravel).havn't run the #'s but the cost savings in labor more than likely offset the cost of the fill.Especially on deep excavations
 
don_resqcapt19 said:
One of the worst cases for corrosion of rigid conduit is where there is a concrete to earth junction. If you want this installation to last very long you need to use additional corrosion protection on the conduit at this point.
From the UL Guide for rigiid ferrous metal conduit (DYIX)

Don
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Sounds like RMC in a PVC sleeve would be a good solution for that.

Steve
 
They do make PVC coated RMC...


Bob, schedule 20? I haven't seen any, and am sure it's not stocked locally. So is that stuff like smurf tube thin? Cut with dull knife, bends on its own, etc.
 
e57 said:
Bob, schedule 20? I haven't seen any, and am sure it's not stocked locally.
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I bet it is. You may know it as "communications duct". It's got another slang term that's more popular, but it escapes me at the moment.
 
iwire said:
When allowed we will use 20 in cement pours, its eggshell thin but it is more than strong enough to hold it's shape in concrete till it sets.
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Anyone, When would this be allowed?
 
chris kennedy said:
Anyone, When would this be allowed?
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When you're sleeving conductors or cable that would otherwise be permitted to be buried without a raceway. URD and semi-con MV cable are some examples. Subbing sitework for a utility is another example. Phone and CATV is yet another.
 
JohnConnolly said:
To install something you KNOW will fail is ridiculous.

BAN it I say!!:wink:
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It's banned underground in San Diego County. Does that make you feel better? :)
 
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