Electrical Code, buried cable question

[Rob Russell]

You can use a three-wire feed if there aren't any other metal pathways between the buildings (water line, cable tv, phone, etc). If there are any other metal pathways (or you intend to install them) then a four wire feed including hot, hot, neutral, and ground is required. You'll need a driven ground for a subpanel on an accessory building. If your area has adopted the 2008 NEC, a four wire feed would be required.

Ernie is correct about possibly being able to use a 3-wire feed. One thing that he doesn't say is that the conductor used for the neutral must be insulated. If the 3rd conductor in the UF is not insulated (just bare copper in the jacket/casing), you can't use it as the neutral.

Addy protocol: unlicensed, homeowner electrician

[Rob Russell]

I recently ran 200 feet of # 2, four wire underground from my home to my new workshop. Since this is an extremely rocky area, its necessary to use conduit around e- wire to prevent eventual degrading of the insulation around your underground cable. I used schedule # 40 conduit from Lowes ($3.18 for a 10 foot section) and lay the cable/conduit at 24" depth. You have to use UL rated (Underground rated) cable even if its sealed inside conduit. I had to cross a gravel driveway so I elected to use galvanized 1 1/2" conduit where the conduit would be subjected to vehicle traffic. Seal the joints between sections after installation and use wire lubricant to put the cable inside the conduit. .Extra expense but well worth the extra cost.

FYI, running Schedule 40 conduit for most of the run is fine. Using the galvanized under the driveway was a good idea. Still, where the PVC went vertical and came up - that technically needed to be Schedule 80.

Rob
Addy protocol: unlicensed, homeowner electrician

[Rob Russell]

Yes they do make split conduit.

http://www.innerduct.com/products/split_pvc.php

Interesting product, but it only comes in Schedule 40 and you need Schedule 80 to sleeve the UF where it's "subject to damage" (which is above ground). See 300.5(D)(4)

Residential direct buried is 18".

Only if it's under a driveway or parking area. All other installations require 24" cover. (Table 300.5)

It's only 12" if it's 120v or less, 20amp max. with GFCI protection.

The UF cable only needs protection down to 18" below grade. I would see if you can find enough slack in the cable and use the correct fittings to do it right.

I agree. I need to edit my first post about the 24". The sleeving requirement is limited to 18" in 300.5(D)(1).

Rob
Addy protocol: unlicensed, homeowner electrician