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#96465 - 11/30/05 12:19 AM One electrode for two buildings
e57 Offline
Member
Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Ok... I'm attempting to describe this...

200a Service at a detached garage and 200a feeder tapped from there to house with EGC run with the feeder and disconnect in thast panel. (Neutral bond at main panel only.)

House is suppimented with water, (required) and Garage is supplimented with a rod. (Required - Garage has several circuits from main dist. panel)

Ufer is in house structure and has GEC run to both garage, and panel in the house.

The Ufer is run in EMT and RMC in the same path as the feeder conduits, as well as a set of branch circuits, and CATV and phone conduits all in metalic conduits. No water connection between the two buildings.

Anyone see any problems with this?
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Mark Heller
"Well - I oughta....." -Jackie Gleason
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#96466 - 11/30/05 03:18 AM Re: One electrode for two buildings
shortcircuit Offline
Member
Registered: 06/27/04
Posts: 608
Loc: massachusetts
e57...250.58 requires the electrode at the building to be used to ground equipment. But I don't see in the code anything that prohibits the interconnection of electrodes at seperate structures as you have described...

And 225.30 allows only one feeder or branch circuit to supply a seperate building.

shortcircuit
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#96467 - 11/30/05 03:38 AM Re: One electrode for two buildings
winnie Offline
Member
Registered: 09/15/03
Posts: 652
Loc: boston, ma
So the house panel has a direct connection via a GEC to the house ufer? And the detached garage has its own ground rod(s)?

But in addition there is a 'GEC' going from the garage panel to the ufer, going through the same conduit as the feeder to the house?

I can see only one potential problem: is the GEC bonded to the EMT or RMC at _both_ ends of every run? GECs can be subject to extremely high _unbalanced_ currents, and the inductance caused by the steel tube is enough to cause a serious impedance. The GEC needs to be bonded at _both_ ends of the tube in order to permit current to flow in the tube itself. See 250.64(E)

-Jon
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#96468 - 11/30/05 04:59 PM Re: One electrode for two buildings
e57 Offline
Member
Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
I was wrong! Checked again and the other wire from the ufer goes to the water. Panel in the house has no GEC, and the ufer/water GEC only goes to the main, located at the detached garage. And, the garage does not have a rod, just a wire dangling out of the panel where it used to be...

Goes to show, when you think something is messed up, look again, it might be worse than you thought!
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Mark Heller
"Well - I oughta....." -Jackie Gleason
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#96469 - 01/02/06 09:03 PM Re: One electrode for two buildings
tdhorne Offline
Member
Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
Quote:
I was wrong! Checked again and the other wire from the Ufer goes to the water. Panel in the house has no GEC, and the Ufer/water GEC only goes to the main, located at the detached garage. And, the garage does not have a rod, just a wire dangling out of the panel where it used to be...

Goes to show, when you think something is messed up, look again, it might be worse than you thought!
Is the only thing that is missing here the GEC connections to the feeder supplied panel at the house? By NEC alone is the Concrete Encased Electrode and the water line located at the house enough to ground the service neutral at the garage? How long is the GEC between the two buildings? Would anyone, besides me, see some virtue in running the GEC as bare number two in the trench between the house and garage?

Call me a fanatic but I would want to drive a stacked rod at the garage and add sections until I got under 25 ohms if the GEC did not show an impedance of 25 ohms or less when run to the electrodes at the house only. What's more I would make the GEC connection from those stacked rods at the service head if the service is supplied by an overhead drop. The idea of a lightning induced surge trying to find earth via the GEC run to the house would make me nervous.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

[This message has been edited by tdhorne (edited 01-03-2006).]
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Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
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#96470 - 01/03/06 01:31 AM Re: One electrode for two buildings
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Quote:
Call me a fanatic but I would want to drive a stacked rod at the garage and add sections until I got under 25 ohms if the GEC did not show an impedance of 25 ohms or less when run to the electrodes at the house only.


I would not call you a fanatic only concerned with your work.

I will ask Why?

What is the magic in 25 ohms?

The NEC does not require 25 ohms.

Here is part of a post from a Florida electrical instructor who is also spends a lot of time studying lighting protection systems. His info generally comes from here.

http://www.lightning.ece.ufl.edu/

Quote:
If you have a normal soil resistivity of something around 4,000 to 50,000 ohm-centimeters, a ground resistance of anything around 15 to 200 ohms will be sufficient for basic lightning protection. Deep rods are of almost no use as lightning typical drives no more than 2 feet into the earth before extending out laterally from its strike point.


His entire post can be seen here.

http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=7;t=001 802;p=1#000003

Bob

[This message has been edited by iwire (edited 01-03-2006).]
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96471 - 01/05/06 02:07 AM Re: One electrode for two buildings
e57 Offline
Member
Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Stacked rod? Que?

Oh by the way... A tree fell on the drop that fed the garage, ripped whole service off building, and being replaced new en toto. (With new ground)
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Mark Heller
"Well - I oughta....." -Jackie Gleason
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#96472 - 01/05/06 04:54 AM Re: One electrode for two buildings
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I think he means a sectional rod. I have installed 20' sectional ground rods.

The one I did came in two 10' sections that screwed together.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96473 - 01/12/06 08:37 AM Re: One electrode for two buildings
XtheEdgeX Offline
Member
Registered: 04/17/05
Posts: 127
Loc: Florida
NEC 250.56 seems to want 25ohms or less.
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#96474 - 01/12/06 03:05 PM Re: One electrode for two buildings
George Little Offline
Member
Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Quote:
What is the magic in 25 ohms?

The NEC does not require 25 ohms.


What am I missing Bob? I read 250.56 ('02 NEC) and it says 25 ohms or less?
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George Little
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