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Townhouses (one common wall) - special ground considerations?

Townhouses (one common wall) - special ground considerations?

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  #1  
Old 02-27-13, 08:17 PM
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Townhouses (one common wall) - special ground considerations?

Several concerns were raised in my "Zinsco panel replacement project" thread. If it is OK with the moderators, I would like to tackle them one at a time in separate threads. The 1st issue has to do with the GEC and grounding.
 
I did not make clear at the beginning of my other thread that this is a two-owners/two-townhouses situation.

Nashkat1 said "The GEC should be bonded to the utility neutral in the outside disconnect box. The conductor bonded to the cold water inlet should go directly there without passing through the distribution panel. The conductor between the bond in the outside disconnect and the distribution panel should enter with the other three feeders and should be green insulated #6 AWG copper."
 
Currently there is one large outside panel with TWO meters and TWO main disconnect breakers for TWO townhouses. One of the townhouses (one common wall) is mine. The owner of the other townhouse rents out his unit. He will cooperate but does not want to invest more $$ in his unit.
 
From this large (double) disconnect panel, there are TWO RMCs (one for each townhouse), each carrying #2 AL feeders and one ground (#10 CU) to TWO Zinsco-compatible distribution panels. One green wire (AWG #6 CU) goes from the panel ground bar in each townhouse to a cold water inlet somewhere in the unit.
 
How should the grounding be corrected?

Thanks in advance,
Lynnx
 
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  #2  
Old 02-27-13, 09:51 PM
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By replacing the #10 AWG conductor with a #6 AWG conductor, as stated in the other thread. Bond the grounds and isolate the neutrals in your new panel.

Is there a divider between the two disconnects outside?
 
  #3  
Old 02-28-13, 06:12 PM
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Since the conduits connecting the subpanel from the main breaker are metallic, I believe you could use the conduit as the code approved ground path provided each end is fitted with bonding busings and the bushings are properly bonded to the grounded neutral bus at the main breaker and the the ground bar in the subpanel although this is not common in today's residential work. If you were to run a grounding conductor through the conduit, since this is a 100 amp service, I think either #8 copper or #6 aluminum would meet the code requirement. The ground to your water service should terminate at the main breaker though and not in the subpanel. I see no need for the #10 conductor. Your existing installation is probably at least 25 to 30 years old. It's anyone's guess what the local AHJ required when your units were built.
 
  #4  
Old 02-28-13, 07:07 PM
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CJ, I'm used to installing bonding bushings on each end of a piece of Greenfield, pulling a ground, and using the lug on the bushing to bond that pulled ground. Are you suggesting that the bond bushing on each end of the rigid could be bonded to the ground on its end and just let the pipe be the low-resistance path?

I've never done that, but it sounds interesting. I wonder if our inspectors would accept that. I might ask.
 
  #5  
Old 02-28-13, 08:35 PM
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Good guess, CJ. This installation is ~32 years old.

So, if this meets code (if you would ask, Nashkat1?) and if I implement it, I would:

(1) Pull a #8 CU through the RMC (via the #10 wire) from my subpanel to the grounded neutral in the main breaker panel;
(2) Replace the RMC's plastic bushing in the main panel with a bonding bushing and bond to the grounded neutral;
(3) In my distribution panel, remove the cold water inlet #4 CU from the ground bar and connect it to the #6 pulled in (1);
(4) Replace the plastic bushing in my subpanel with a bonding bushing and bond to the ground bar???
 
  #6  
Old 02-28-13, 09:18 PM
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I discovered that my cold water inlet connection does not qualify as part of the GEC because it is far from within 5' of entry.

So I must modify step (3) in the previous post to:
(3) Pull the unconnected end of the #8 CU in my subpanel (pulled in (1)) over to, and bond to, the cold water inlet where it enters the structure.
 
  #7  
Old 03-01-13, 03:08 PM
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I discovered that my cold water inlet connection does not qualify as part of the GEC because it is far from within 5' of entry.
The bond to the cold water inlet must be made within 5' of the point at which your water supply pipe comes in through the wall or floor. You should have access to all of that.
 
  #8  
Old 03-01-13, 06:11 PM
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The bond to the cold water inlet must be made within 5' of the point at which your water supply pipe comes in through the wall or floor. You should have access to all of that.
This is the way it is by today's code, but this was installed 32 years ago and was compliant back then. That being said, I would upgrade to today's code.
 
  #9  
Old 03-01-13, 06:30 PM
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CJ, I'm used to installing bonding bushings on each end of a piece of Greenfield, pulling a ground, and using the lug on the bushing to bond that pulled ground. Are you suggesting that the bond bushing on each end of the rigid could be bonded to the ground on its end and just let the pipe be the low-resistance path?

I've never done that, but it sounds interesting. I wonder if our inspectors would accept that. I might ask.
Of course, we all know that methods will vary by area, but the way it's done here is like this. For example, consider a simple 200 amp service. Any time the conduit between the meter socket and main panel is metallic and the conduit terminates within a concentric knockout, there must be a bonding bushing on each end of the conduit. In the meter socket, a #6 copper ground wire from the ground rod enters the socket, loops through the lug on the bushing and terminates at the ground lug on the load side neutral. In the service panel, the #4 copper or #2 aluminum ground wire enters the panel from the water service, loops through the lug on the bonding bushing and terminates on the neutral bus. IF there are no concentric knockouts, such as with a panelboard where we punch our own knockout, we use a grounding locknut and an insulating plastic bushing. The metallic conduit becomes the grounding path solidly bonding the meter socket to the panel.

In the case such as the OP has, with a main panel outside and subpanel inside, the metallic conduit can carry the ground for the subpanel feeder utilizing bonding bushings as I just described, but because it is a subpanel, there would be a bonding conductor from the lug on the bushing to the ground bar and the neutral is isolated. The water service ground wire goes to the main panel neutral bus outside. It's really not much different than using pipe ground on a branch circuit in conduit.
 
  #10  
Old 03-01-13, 09:40 PM
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(1) Pull a #8 CU through the RMC (via the #10 wire) from my subpanel to the grounded neutral in the main breaker panel;
I'm liking Joe's logic. I would pull a ground conductor through the RMC iff either end of the conduit entered a box through a concentric knockout. Otherwise I would just bond ground to a bushing on each end of the RMC.

(2) Replace the RMC's plastic bushing in the main panel with a bonding bushing and bond to the grounded neutral;
If you mean tie it into the existing ground/neutral bond, then yes.

(3) In my distribution panel, remove the cold water inlet #4 CU from the ground bar and connect it to the #6 pulled in (1);
No. In your distribution subpanel all grounds must be bonded together and the enclosure must be bonded to them. The neutrals must be bonded together and isolated from all grounds including the enclosure.

(4) Replace the plastic bushing in my subpanel with a bonding bushing and bond to the ground bar?
Yep.
 
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