Question About Using Direct Bury Electrical Cable in a Conduit

[Dennis McDonaugh]

I have an extra circuit in my workshop and want to use it to supply power to an outlet in our greenhouse. It's about a 40' run and I bought 12/2 feeder cable which can be direct buried. I plan to put it in 3/4 plastic conduit to protect it because it pretty much goes right through a planting bed. I couldn't find anything that said that wasn't allowed. Is that okay?

[Ian Moone]

I am no licensed electrician.

If it were me?

It would be buried MORE than 2 feet down! The electrical conduit would be the orange "danger" marked one. I'd bury a row of bricks on TOP of the conduit, and on Top of the bricks would be a blaze orange danger buried power line roll of that warning tape buried face side up!.

That's how we would do it here... but our 240AC will definitely kill you - there's no maybes about it.

No idea what your local laws say - best get a qualified licensed electrician to advise.

[Dan Friedrichs]

Unless the cable is rated for use in conduit, then no, you aren't allowed to do that. Just like "Romex" is not rated to be run in conduit. The rationale for this is that the NEC has specific requirements for how much "fill" can be in a conduit, and sheathed cables add thermal insulation and could result in overheating (in very adverse situations).

But....is it unsafe? No. Not the way you're proposing it.

[Steve Peterson]

I am not an expert, but that is probably what I would do. Maybe upsize the pipe to 1" just in case. Thread the pipe over the wire and glue it as you go. 40' doesn't sound like much, but stiff direct bury wire might be really difficult to push through later.

Steve

[Dennis McDonaugh]

Thanks for the advice. I didn't think it would be a big deal. I'll bury it 18-24" so hitting it with anything short of a trencher or backhoe shouldn't be an issue.

[Anthony Whitesell]

Per the NEC, NM-B can be run in conduit. But at a cost. You end up "derating" the fill. Per the NEC, 14g and 12g NM-B require that you use the cross sectional area for a 1/2" diameter wire (0.196 sq in), which is far more than the actual oval shaped cable occupies.

But this post is not on NM-B but direct burial service feeder.

Normally, I would comment that I thought you would have to below the frost line for burial depth. But the frost line in Texas is probably only 6". LOL. If you are running it through an area like a garden that you know will get dug and tilled, I would rent a trencher and go as deep as it will go. Then place the buried wire marker on top with a second 6" or so above the first (perhaps even a third if you are still 2+ feet down).

[Stan Krupowies]

Thread the pipe over the wire and glue it as you go. 40' doesn't sound like much, but stiff direct bury wire might be really difficult to push through later.

Steve

You're not supposed to do that as the solvent in the glue for the pipes can eat through the outer sheath of the cable. It's best to run a pull string and then pull it through. If it's tough to pull you can get cable lube to help it along.

[Julie Moriarty]

Any questions one has about meeting local codes can be best answered by the local inspector. As far as direct burial cable in conduit, I wouldn't do it and I don't know any electrician who would. As far as PVC pipe being used to protect the wires, I've seen a shovel cut PVC pipe in half, even SCH 40. When installing direct burial and PVC, local codes usually require burying them much deeper than GRC, usually at least 18" but more often 24". If you're digging the proper depth trench, why mess with tried and tested methods? Install either PVC with separate conductors or direct burial cable.

[Eric Anderson]

In Central Texas (where Salado is, about 40 minutes north of Austin), there isn't a frost line. There are some years in Austin where I don't even turn my heat on, much less worry about the couple of hours a year it might get below 30 degrees. Now summers, yea we got that covered

[Dennis McDonaugh]

Thanks for the replies. As Eric said, frost really isn't a concern here as we don't get any sustained freezing weather. Due to some concerns about the size of the conduit I used 1" so I don't think overheating is an issue. The 100 amp electrical service for my workshop goes under the concrete drive in 2" conduit so I don't see the issue with this one. It's about a 35' run and is buried at 18". I won't be digging that deep in the flower bed and I sure can't cut through this PVC with a shovel in one wack. It's not too far from the propane line running from the tank to the house so no one is going to be digging with a back hoe in the area.

[Brice Rogers]

I'm not an electrician either. But I've run a bunch of wires in conduits. including one to my greenhouse Some advice:
Don't use 3/4 or even 1" pipe. It is far easier to pull wires through bigger pipe, especially if you are pulling a cable with a sheath on it.
Use the gray electrical PVC
Use the big electrical sweeps (90 degree fittings with a large radius) - - don't even try to use water fittings as they are a PITA to pull the wires through
40 feet isn't that much of a pull, but if you have any issues pulling the cable, get a bottle of the pulling lubricant. Liquid dish soap or KY may work in a pinch
Use a pull cord to pull your wires. Pushing them usually doesn't work - - similar to pushing a chain. To get the pull cord through the pipe, I find it easiest to put a vacuum cleaner at one end and tie to pull cord to something like a small wadded-up piece of poly grocery bag. Some people use compressed air to force the "mouse" or ribbon through the pipe, but I find it MUCH easier and much faster to use a vacuum.
For safety purposes, use a GFCI to feed the greenhouse circuit

[Ian Moone]

What I used here that met the local code was 1 inch orange electrical PVC Conduit - you can't use this above ground coz it's not UV stabilized (unless you paint it) so above ground you use the 1 inch Grey PVC Conduit.
Yes the large radius corners to pull the cable thru and yes we needed cable lubricant.
The cable in my case was 6mm^2 copper, 3 phase (4 plus earth) carrying 415 volts.
Thats what I used to run the 3 phase from the meter box thru the roof and down underground to the workshop... the conduit being for all the external and underground run.
The conduit, cable and bricks and warning tape is coz you REALLY don't want to sever 415 volts with anything like a trencher / mini digger backhoe etc coz it will ruin your week and curl your hair and toes at the same time!. No need for funeral - you'll be cremated on the spot. I have several 415 volt, 3 phase, 63 amp outlets and a few 415V 3 phase 20 amp outlets running off this single run in, plus all my single phase 15 & 10 amp 240 V GPO's (General Purpose Outlets) and the 8 or 9 double 4 ft batten flouro single phase 240 volt lights - all distributed thru a sub mains box with earth leakage trip switches!. The run was 60 meters in total from memory, with about 15 meters of this underground.
YMMV

[Dennis McDonaugh]

I'm not an electrician either. But I've run a bunch of wires in conduits. including one to my greenhouse Some advice:
Don't use 3/4 or even 1" pipe. It is far easier to pull wires through bigger pipe, especially if you are pulling a cable with a sheath on it.
Use the gray electrical PVC
Use the big electrical sweeps (90 degree fittings with a large radius) - - don't even try to use water fittings as they are a PITA to pull the wires through
40 feet isn't that much of a pull, but if you have any issues pulling the cable, get a bottle of the pulling lubricant. Liquid dish soap or KY may work in a pinch
Use a pull cord to pull your wires. Pushing them usually doesn't work - - similar to pushing a chain. To get the pull cord through the pipe, I find it easiest to put a vacuum cleaner at one end and tie to pull cord to something like a small wadded-up piece of poly grocery bag. Some people use compressed air to force the "mouse" or ribbon through the pipe, but I find it MUCH easier and much faster to use a vacuum.
For safety purposes, use a GFCI to feed the greenhouse circuit

It's up and running. I used a GFCI breaker and boy are they expensive!

[John Stankus]

In Central Texas (where Salado is, about 40 minutes north of Austin), there isn't a frost line. There are some years in Austin where I don't even turn my heat on, much less worry about the couple of hours a year it might get below 30 degrees. Now summers, yea we got that covered

No frost line... but the limestone line can be pretty shallow. When I was in high school in the Austin area, I made some extra money helping put yards and gardens in. My grandfather's coal mining tools did come in handy, pulling out sheets of limestone 8-16 inches thick.

John

[Dennis McDonaugh]

No frost line... but the limestone line can be pretty shallow. When I was in high school in the Austin area, I made some extra money helping put yards and gardens in. My grandfather's coal mining tools did come in handy, pulling out sheets of limestone 8-16 inches thick.

John

We've got pretty deep soil where I'm at. I hand dug 200 feet of water line and didn't need a breaker bar! I have mostly black dirt with fist size rocks mixed in it. There are some people near me that have limestone right at the surface.