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#48502 - 02/13/05 09:50 PM PVC vs Rigid pipe underground
aldav53 Offline

Registered: 08/22/01
Posts: 547
Loc: Chandler, AZ USA
I have come across more metal ridgid pipe underground (even wrapped with 20ml tape) that was rusted out. I've come to the conclusion that PVC is the way to go. Schedule 80 is so thick, it is probably almost as strong as metal ridgid. It should be legal to use at a 6" depth like metal ridgid, think PVC is 18" now.
The schedule 80 PVC also says ridgid on it, is it legal at 6"?
Any thoughts?
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#48503 - 02/14/05 12:16 AM Re: PVC vs Rigid pipe underground
e57 Offline

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
PVC isn't nearly as strong as RMC when it comes to shovels, pick axes, and drilling. Which is why it requires deeper burial. Always dig deeper that the landscaper would, someone once told me. Imagine a home owner or landscaper putting a pick axe through the side of service conductors.

Even a ditch-witch! From experiance I can tell you larger RMC makes a distinctive pinging sound when the teeth first hit it. Fair warning to stop. PVC you just see it come up, and hope its only a water line, as opposed to anything else, like gas or electric, or worse both! Even if you call before you dig, there is alway a chance of hitting something.

They unfortunately haven't found anything that will stand up to back-hoe abuse. Or the pavement saw.
Mark Heller
"Well - I oughta....." -Jackie Gleason

#48504 - 02/14/05 03:17 AM Re: PVC vs Rigid pipe underground
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I agree with Mark (e57) RNC (Rigid Non Metallic Conduit) even schedule 80, does not come close the the strength of RMC (Rigid Metallic Conduit)

That said we use plastic almost exclusively underground for the same reason you brought up. Metal conduits rust out and even when they do not rust through they rust enough to make the conductors stick to the conduit so replacement can be tough.

I do not believe you will see the burial depths for RNC reduced. Take a look at Table 300.5

The burial depths for RNC range from 0' under a building to 24" under a parking lot.

You said;

It should be legal to use at a 6" depth like metal rigid,

That depends on the location, the burial depths for RMC range again from 0" under a building to 24" under a parking lot or road.

As a matter of fact there are only two places in the table where the burial depth differs between RNC and RMC.

1)In trench below 50-mm (2-in.) thick concrete or equivalent

RMC 6"

RNC 12"

2)The row titled "All locations not specified below"

RMC 6"

RNC 18"

Keep in mind very few locations are "not specified below" in the table.

You really need to study the table, if you are in an area with tough digging you can choose to run the RNC at 12" with 2" of concrete on top of it.

We do mostly commercial jobs and many times the specifications call for RNC at 24" depth with concrete encasement.

Of course a machine is doing the digging.

Bob Badger
Construction & Maintenance Electrician


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