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#185725 - 03/25/09 02:16 PM adding a sub-panel to existing 10/2 with ground
carolina_elect Offline
New Member

Registered: 07/08/07
Posts: 5
Loc: sc usa
Just a little help please. A customer wants to use the existing wire that fed a irrigation pump to install a sub panel to supply 110 and 220 to new boat dock.
Question. How do I make this code? Do I drive a ground rod at new sub-panel and seperate nutral or do I add a transformer to create new neutral?
any suggestions?

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#185726 - 03/25/09 02:22 PM Re: adding a sub-panel to existing 10/2 with ground [Re: carolina_elect]
dougwells Offline

Member

Registered: 05/03/03
Posts: 1228
Loc: kamloops BC Canada
how do you get 120/240 from a 2 wire cable

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#185728 - 03/25/09 02:30 PM Re: adding a sub-panel to existing 10/2 with ground [Re: dougwells]
carolina_elect Offline
New Member

Registered: 07/08/07
Posts: 5
Loc: sc usa
The irrigation pump is currently runing on 240. He wants to add receptacles. Thats my question? Do I need to install a transformer so the neutral will be correct. I dont want to use the bare ground as the neutral from the 10/2.

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#185736 - 03/25/09 04:40 PM Re: adding a sub-panel to existing 10/2 with ground [Re: carolina_elect]
carolina_elect Offline
New Member

Registered: 07/08/07
Posts: 5
Loc: sc usa
Just checking, does anyone have an answer for this. The inspector says that he will let me know if this is ok. he also says that the ground wire has to be an insulated conductor all the way from the source if used around water. The wire if 10/2 UF.

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#185740 - 03/25/09 07:28 PM Re: adding a sub-panel to existing 10/2 with groun [Re: carolina_elect]
EV607797 Offline
Member

Registered: 10/25/06
Posts: 756
Loc: Fredericksburg, VA, USA
I just don't know of any legal way that you can do this. If you had three insulated conductors and there was no other metallic connection (water pipes, etc.) between the feed location and the structure where the sub panel is located, there could be some exceptions. A grounding conductor in a piece of 10/2 would not be considered to be an insulated conductor.

I don't even see how you could do this with a transformer while still maintaining the original circuit to the irrigation pump.

Sorry for the bad news, but it looks like the original UF is going to have to be replaced.
_________________________
---Ed---

"But the guy at Home Depot said it would work."

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#185745 - 03/25/09 09:20 PM Re: adding a sub-panel to existing 10/2 with groun [Re: EV607797]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
This is one of those classic "but it works" death traps that the untrained handyman types come up with. The short version:

There is NO substitute for good design, especially near water!

I run into this sort of thing all the time ... guy wants to cheap out, run as little wire as possible ... 'all I need is 220' they swear. Then. before the sod takes root, they wnat to 'make' 120 for 'just alight,' etc.

NO. NO. And again- NO. What part of 'NO'can they not understand?

Your 'service' is where your meter is. Period. Let the PoCo bring in just three wires.

YOU, on the other hand, get to run FOUR wires. A sub is a sub is a sub. Period. Plus, since you're adding a panel, you ALSO get to bang in a ground rod.

Which brings us to issue #2: Just what size panel? I can't say I've ever seen a 20 amp panel before. 20 amps is all you'll get, over any distance, with #10. Maybe even less.

Chances are, that UF isn't buried 2ft down either. So - do it right, or pass it by. Dig deep, run big pipe, and four wires. Expensive? Well, you get what you pay for. Had the guy designed well in the beginning, he would not be crying today.

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#185747 - 03/25/09 10:16 PM Re: adding a sub-panel to existing 10/2 with groun [Re: renosteinke]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
What does he want to supply there? If this is just a lift and some lights, do it with 240. A am sure you can get a 240v GFCI and that is the only way I could sleep at night.
My guess is it could all be on 120v, small pumps can usually be switched, but you probably run out of amps.
_________________________
Greg Fretwell

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#185755 - 03/26/09 05:50 AM Re: adding a sub-panel to existing 10/2 with groun [Re: gfretwell]
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Pardon if I misunderstand, but the problem is not that this UF lacks a ground, but that it lacks a neutral. I don't see any issue with feeding an isolation transformer from the existing cable, and using the isolation transformer to feed a new 30A panel.

The tricky question is not whether to drive a new ground rod, but whether it's safer to bond the neutral or leave it floating, which would reduce shock hazard. In this case, I don't think code gives us an option; the neutral must be bonded. As it's a separately derived system, there is no issue with bonding the new neutral and existing #10/2 UF ground wire to the new ground rod.

Edit: oops, missed the statement that the inspector wants the ground wire to be insulated the entire way back to the source if used by water. Unbroken is a good point, that's a safety issue... but why insulated? What code requires the ground wire to be insulated, and why would UF (in which, the ground IS insulated) not be considered insulated? Driving a new ground rod and installing an isolation transformer seems to make both these objections rather moot.


Edited by SteveFehr (03/26/09 05:56 AM)

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#185760 - 03/26/09 08:57 AM Re: adding a sub-panel to existing 10/2 with groun [Re: SteveFehr]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Steve I would disagree with you on the point that the bare wire in 10/2wg UF cable is considered an insulated conductor. Article 200 makes it quite clear that a wire of this gauge used as a neutral must be colored white or grey the entire length and the bare conductor don't get it. As for the xformer approach that could work. Under the '05 NEC you can still run a feeder to a "Structure" and not include a neutral and an EGC but under the '08 that would be an issue. So depending on your definition of a "Structure" his only choice is a xformer.
_________________________
George Little

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#185761 - 03/26/09 09:06 AM Re: adding a sub-panel to existing 10/2 with groun [Re: SteveFehr]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Isolation transformer? I might go along with that for the control circuit for the motor controls, but that's it.

If nothing else, he simply doesn't have the power available.

True, we're not on-site, and cannot know all the details. Yet, as my earlier post suggests, I've seen this scenario all too many times. I suspect that many borderline compromises were necessary for the OP to get painted into this corner - and there's a world of difference between 'code compliant' and 'good design.' Those poor choices have finally caught up, and are no longer adequate.

If he does anything - panel, transformer, whatever - to make the location have more than one circuit, he has a separate structure that requires a 4-wire feed. Especially with the revisions in the 2008 NEC, that's what he needs. He also needs a ground rod.

Off on a tangent ..... one of my 'wish list' items is a nice, little, very quite generator. Just enough for a tool and a light. Premium models sell for about $800. If this guy's needs are truly that little, such a generator will solve his problem, and for a lot less than installing a transformer, replacing his feed, etc.

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