ECN Forum Forums Code Related Discussion NEC & other Code issues adding a sub-panel to existing 10/2 with ground

George,
I was not intending for him to use the ground as a neutral, only as a ground. You don't need a neutral to an isolation transformer; a new neutral is derived on the transformer secondary.

Also... you can't do it with Romex, but isn't the ground in UF encased in gray insulator? Looks to me like it meets the letter of the law! (j/k!)

Reno,
If the existing run were 480V 1-phase, and he wanted to step it down to install a 240/120V panel, would any of us have any issues at all feeding a panel off off this circuit? An isolation transformer is no different. Shoot, carolina_elect might have to step this up at the house to 480V anyhow just for voltage drop. A 480V feeder would let him feed a 60A 240V panel from this #10 wire, too.

I see this as an economic/design choice, not a code issue. An isolation transformer is safe and acceptable to feed a panel if it's uneconomical to replace the feeder cable.

A 5 KW isolation (not an auto transformer) transformer 240 volt primary and 120/240 secondary center tapped and ground the centerpoint should be compliant. The supply bare wire is a bonding conductor not a ground wire. A new ground at the second service. The new neutral has a bonding jumper installed so the service and new system are bonded together. I don't know why the inspector would need the bond wire to be insulated.

Caveat My answer is based on the CEC but the NEC requirements should pretty much match. Technically this works and is safe.

Steve I guess I was thinking of the options and got them mixed up. Hey- It happens. Thanks for making that neutral/ground thingee clear.

I'm reading in the NEC at 340.108 and 340.112 and 340.116 of the '05 edition and I get the impression that the bare conductor is not considered as insulated but only as contained in the sheath which doesn't qualify as an insulated as described in 340.112. IMHO I know there is something in the UL white book about the insulation on NM cable conductors and the sheath but I can't really find anything along those lines for UF cable.

As for approaching this as a xformer application, with a 240v. supply to the primary and a 120/240v. secondary he could use a 7.2KVA xformer if they made such a thing and it may or may not be adequate. He would need a grounding electrode (probably a rod or two) and sub panel with a main in it.

Thanks Guys for the input. This was my first post. I know there are sub panels every where that only have a three wire feed (two insulated conductors and a bare ground).
I also know that code now requires a forth wire or seperate ground. I was asked for the most economical way to use this existing conductor. The transformer idea was the only way I could think of to create a neutral and still have the seperate grounding conductor. Just wanted to see how this post thing works.
Reno: Not all people are handymen tring for the cheap way out. Some are people trying to think out of the box, keep the price down, make money and do it safe.
The inspector also called back and said he had no problem using the transformer as long as article 682 guidelines where followed. I am still concerned that the feeder grounding wire is bare and not insulated and by this wire being in uf cable does this make it insulated?

I think I found the answer to my last question about the 10/2 w/ground. look at article 250.32 2008 code and then 250:118. Am I correct that this bare wire can be used to feed another structure. or transformer.

Was an interesting question for a first post
I believe the "insulated wire" requirement is for a grounded conductor, AKA neutral. In this case, it will not be used for a neutral, and will be used only to bond the two grounding systems together. I can see no reason why it would matter if this conductor is grounded or not.

I know I made a tongue-in-cheek comment about it, but I would not consider the bare copper wire in UF to be insulated.