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rossfingal 05-05-2011 05:30 PM

Underground run to a garage
Need some advice.
We're doing some excavating around our building and we're going to uncover
an underground connection to the detached garage.
Had problems with the wiring in the garage (that will be dealt with) -
the line going into the garage is "Romex", 12 - 2, no bare copper wire.
Don't know yet, if the buried wire is "UF".
We'll find out what it is tomorrow.
Question is - we're going to replace it -
run PVC, Sch. 40 - then Romex (12 -3) - or can we pull 12 - 3 wires inside
the PVC?
Any thoughts?



Jim Port 05-05-2011 06:02 PM

If you are going to run conduit you would install individual THWN conductors in the proper sizes and colors.

Type NM-B cable cannot be used outside, even if in conduit.

rossfingal 05-05-2011 06:19 PM

Jim Port
Would PVC be considered adequate (as conduit) - or metal?
Already know about grades of wire.


McSteve 05-05-2011 10:14 PM

Check with your local inspectors to be sure, but by the NEC you may use Sch 40 PVC conduit underground; it must be buried deep enough to have 18" of cover over it. Where the pipe is exposed to physical damage (where it comes out of the ground to enter the side of the structure) you must use Sch 80 PVC.

WillK 05-05-2011 10:18 PM

Some locals might say otherwise but where I am the way my AHJ reads NEC the riser is subject to physical damage. So that means that my choices for conduit for the riser are schedule 80 PVC or intermediate metal conduit or rigid metal conduit.

Schedule 40 PVC would be allowed for the run. There are tables for the depth at which it has to be buried, and somebody will correct me if I remember wrong. I think you have to bury schedule 40 18", you can direct bury UF cable at 24" and rigid metal conduit you can bury as little as 6".

Missouri Bound 05-05-2011 10:25 PM

Are you planning to use the garage as a workshop? If so consider putting a panel in the garage for expansion purposes. Then you could put in larger conduit and perhaps install a 60 to 100 amp service in the garage. You will need a few more things, such as a panel and grounding rod with proper attachment, but it will give you flexibility in the future. Schedule 80 will be fine, and the depth will be determined by it's location....that is it would be buried deeper if under a driveway.

McSteve 05-05-2011 10:27 PM

Good point. I would say that if you're only going to run a 20A MWBC, run large enough conduit that you can upgrade to at least a 60A feeder later without digging it all up again.

rossfingal 05-06-2011 04:52 AM

Thanks all, for the advice!
At least I have something to show the boss!
I'll find out today what the old wire is.
Probably going to use 1 1/2 inch PVC, with suitably rated 12-3 Romex -
1 1/2 inch conduit out of the ground.
(Digging sure is fun! - ha, ha!)
Thanks again!


rjniles 05-06-2011 05:39 AM


Originally Posted by rossfingal (Post 642984)
Thanks all, for the advice!

Probably going to use 1 1/2 inch PVC, with suitably rated 12-3 Romex -


NIX on the Romex, not rated for wet locations like the conduit.

rossfingal 05-06-2011 10:32 AM


Yeah, just talked to a friend of mine (licensed elect. Cook County,Ill. - that's
where we're at) - moisture/water problems.
Probably, go for direct bury - 2 ft. down.
Thanks for the "heads up" advice!
(Don't think it's going to be shop/home-shop)

McSteve 05-06-2011 10:39 AM

I'm not sure what you mean by "moisture/water problems" but in my opinion, THWN wire in PVC conduit, or even UF cable in PVC conduit, is a far superior installation to UF cable direct-buried in the ground, for future upgradability and reliability. UF cable in the ground will eventually fail. Without conduit, any repair or upgrade down the road is going to require trenching again.

Personally, I will use UF cable to feed a lightpost or post-mounted outlet, but always conduit between buildings.

That's just my advice though, you can take or leave it. Direct burying UF cable for this installation is also perfectly legal.

rossfingal 05-06-2011 12:35 PM


I'm here to get some guidance.
As far as I'm concerned, I don't think 1 1/2 PVC, as an underground run,
would not be unacceptable - at least, it can be upgradeable.
What we have is 12-2 Romex, not even a bare-copper ground.
No markings - no stamps - nothing.
(I haven't seen that too much!)
It's 30 - 40 - 50, years old (Don't know).
If we're going to replace it: I just want it to be - somewhat, "up to snuff".
Thanks for the input!



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