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Joined: Apr 2001
Posts: 83
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donles Offline OP
Member
Need advice, I have not done this type of upgrade before. Customer is having an addition to home. Current loadcenter is is 100 amp, does not have enough available circuits. 150 Amp panel would be sufficient for finished dwelling. The utility service drop is overhead to a customer-owned pole. Meter socket and a enclosure mounted 150A 2 Pole OCPD mounted on pole feeding rigid conduit (3 conductors) running underground (100') to a 2 pole fused disconnect in the basement. Then from the disco to existing breaker panel.
Does the NEC require 3 conds. & a grounded conductor from pole mounted OCPD to disco in basement? I think existing underground conductors are sized properly for 150 amp but I'm thinking that there aren't enough of 'em.
I'm visiting site tomorrow to fill in missing details.
Don

Joined: Apr 2001
Posts: 83
D
donles Offline OP
Member
In original post, change grounded conductor to grounding conductor in conduit from pole to disconnect in house. IOW, conduit now has 2 phase conductors and grounded conductor. Because the main OCPD is at meter, are seperate grounding and grounded conductors required a la subpanel rules?
Sorry for confusion.
Don

Joined: Apr 2002
Posts: 914
E
Member
I'm not sure if I followed you completely, but here's the scoop.

You must bond the grounded conductor to the grounding conductor at the first disco. If that is in the basement(not code, but if so), then that is where the first ground needs to be. You may also need to have a ground rod for the meter base. You really need the disco to be outside the building in this case. I don't know if this answered your question, but maybe it helped.

Joined: Apr 2001
Posts: 83
D
donles Offline OP
Member
First disco is at the pole in the form of 2 pole 150A breaker. POCO requires gnds at the pole.
My issue is whether or not I need 3 cond & gnd. from pole-mounted OCPD to disco in house. I think that I do but am looking for confirmation.
Don

Joined: Sep 2003
Posts: 38
K
Member
It is a sub-panel. You would need a 4th
wire.

Joined: Dec 2000
Posts: 4,287
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If the rigid conduit you mention is metallic rigid, then it should suffice as your grounding conductor...S

Joined: Apr 2001
Posts: 83
D
donles Offline OP
Member
So then, opinions are that a grounding conductor is called for and that the existing RMC will serve well as the grounding conductor.
Thank you, Don

Joined: Sep 2003
Posts: 650
W
Member
I am going to be contrary.

The NEC does not require 4 conductors to a subpanel in the following circumstance:
A detached structure with no other metallic paths between the structures.

The house is clearly a structure detached from the pole, so the main panel in the house _could_ be fed with a three wire circuit, with ground and neutral bonded in the house.

That said, there is quite likely a metallic path between the structures, specifically the RMC. With the metallic path, you are required to use a standard 4 wire feed to the subpanel. I would not want to trust the RMC as the ground return path, however. We are talking about 100 feet of steel buried in soil essentially as the main ground bond for the entire house. Is there any evidence of corrosion of the RMC?

IMHO there is also a very real chance that the RMC is _not_ continuous, with short lengths simply used to provide protection for direct buried cables.

-Jon

Joined: Jan 2003
Posts: 4,391
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Quote
I am going to be contrary.

I don't think so, I think you stated it pretty well. [Linked Image]

If you can get past the no metallic path requirement IMO it may be better to run a 3 wire feed. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Apr 2001
Posts: 83
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donles Offline OP
Member
Judging by the type of insulation on the conductors - cloth, I would guess that the RMC is continuous. I hate surprises so I'm going to ask the inspector his opinion ASAP.
Thanks for the input.

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