[LHB_Texas]

I'm running 4/0, 4/0, 2/0 Al plus #6 Cu about 220' from main to sub-panel on a metal building. The cable will be buried. I was planning to run through Sch 40 gray carlon, but because of needing to work under/around tree roots and possibly having some (minor) bends along the way, I was thinking it would be much easier to snake some polyethylene pipe through the trench than having to slide long sections of PVC. Also save lots of gluing as I'd have one continuous conduit; no joints. I'd transition to PVC where the cable came above grade. Some additional information: This is out in the country. No code/inspection issues to deal with. Also, the conduit will not be buried in areas with any heavy traffic running over it, so no issues with crushing. If I need to go under a location where there may be heavy traffic, I'll run the poly tubing through a larger diameter sch 40 or sch 80 pipe to provide greater protection. Planning to bury about 18" - 20" deep. Again, this is on land in the country, and I'll know where it is, so I'm not concerned with it being cut or damaged at that depth by other activity. Just need to know if there is any reason you can't pull these cables through poly pipe. I'd be pulling all 4 cables/wires through 2" conduit. Based on prior research, I think this will allow for sufficient heat management. Thanks.

 

[rjniles]

No reason in the world other than the pipe is non compliant with code and the burial depth in insufficient.

 

[LHB_Texas]

Thanks. Is the code issue related to crush resistance or what? What is 'required' depth per code?

 

[gregzoll]

A proper trencher or backhoe can get right through those roots. You may have to go out as an angle to get a clear line towards the out building, then come back from a long sweep to head towards the building.

If you know someone with a directional boring machine. They can easily get the proper HDPE conduit pulled to the building.

 

[rjniles]

Thanks. Is the code issue related to crush resistance or what? What is 'required' depth per code?

Code requires that materials used in electrical installations be approved for that use. There are approved flexible conduits that could be used for your installation.

Burial depth is 24 inches above the conduit.

You can dig a lot of trench cheaper than you can hire a directional boring machine.

 

[Jim Port]

I didn't think the Navy could use directional boring that much. Grin.

 

[Oso954]

While flexible conduit could be used, my question would be why. It will cost more than pvc and it is generally more difficult to pull thru. The tree roots can be cut out, if the ditch witch or backhoe doesn't handle them.

I would go with the PVC. I would probably upsize it to 2 1/2 or 3 inch, just to make pulling easier.

 

[LHB_Texas]

I am doing much of the trenching by hand. Mechanical means such as trencher or mini-ex will do too much damage to roots putting trees at risk. I have to run close to several large, old post oaks. Luckily the ground is sand or sandy loam so digging is relatively easy. I may use mechanical for open areas, but I have more time than money, so don't mind the digging too much. Gets me some exercise too.

I'm trying to run as straight as possible to minimize distance from power pole. Again, the issue is trying to get the conduit beneath large roots and avoiding having to glue up PVC while it's down in a trench. If not for the roots, I'd just glue up on at ground level and drop into the trench, but roots won't allow. I haven't made a final decision on how I'm going to do it. Just wondered if there were any issues using polyethylene pipe other than those already mentioned (code); especially any sort of chemical reactivity that might occur over time, say between the conduit and the sheathing on the cable. Actually, I think the sheathing may be polyethylene, so if that's the case, there shouldn't be any compatibility issues. I think flex PVC would be too expensive. Isn't URD cable rated for direct burial? I'm just using conduit for extra protection.

 

[rjniles]

I have never seen URD cable in this configuration

"I'm running 4/0, 4/0, 2/0 Al plus #6 Cu"

Good luck on pulling that thru poly pipe.

 

[LHB_Texas]

Probably using wrong terminology. Maybe it's SEU like Southwire Sweetbriar. The #6 Cu is separate for ground from main to sub. Still don't know why I need a separate ground from the main since it's bonded to neutral there. Seems like I should just be able to split at the sub-panel with a separately installed ground rod.

 

[Oso954]

Why are you using a 3wire cable plus a separate ground wire instead of a 4wire cable ? With the right 4wire cable assembly, you could direct bury it.

But having said that, I would install the PVC conduit and use 4 individual conductors

 

[Jim Port]

Can you glue it up and slide it under the roots?

I would not try to pull a tri or quadplexed cable like URD into a conduit. Individual conductors would be easier.

 

[rjniles]

Probably using wrong terminology. Maybe it's SEU like Southwire Sweetbriar. The #6 Cu is separate for ground from main to sub. Still don't know why I need a separate ground from the main since it's bonded to neutral there. Seems like I should just be able to split at the sub-panel with a separately installed ground rod.

SEU and Sweetbriar are 2 vastly different cables. SEU is a service entrance cable for above ground use. It is a sheathed cable. Sweetbriar is an unsheathed triplex cable for underground use. If you want to use a direct burial cable you should go with a quadplex.

Code requires a 4 wire feed which includes a separate ground. You are only allowed to bond the neutral and ground at the first point of service disconnect.