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#76115 - 12/13/00 02:25 PM Isolated ground receptacle  
cinkerf  Offline
Joined: Dec 2000
Posts: 84
Pittsburgh, Pennsysvania. ...
A 120 volt 20 amp isolated ground receptacle is needed for computer equipment. The service panelboard is approx. 50 feet from computer equipment. A 12/3 metal clad cable is intended to be used. The green insulated wire will be used to connect directly to the green terminal of the isolated ground receptacle. The red insulated wire will be stripped bare and used to properly ground the metallic box. At the panelboard the red insulated conductor will be totally stripped bare and connected to the bonded neutral / grounding bar. There are no junction boxes between panelboard and isolated ground receptacle outlet box. Does anybody object to this installation? (Frank) cinkerf

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#76116 - 12/13/00 05:37 PM Re: Isolated ground receptacle  
rmiell  Offline
Joined: Nov 2000
Posts: 242
La Junta, Co. USA
Besides using the red wire for grounding? The main use of a isolated ground receptacle is to keep stray noise from getting onto the ground, and into the computer equipment. Run a 12/2 mc cable,with an insulated equipment grounding conductor, use fittings approved for grounding,attach the equipment grounding conductor to the metal box, run cable from load back to service panel, and call it good.

This will help by not allowing a path for noise to get onto the equipment grounding conductor.

Do not, like I have seen in some cases, add a seperate ground rod just for the computer equipment!

Good Luck!

Rick Miell

#76117 - 12/13/00 06:38 PM Re: Isolated ground receptacle  
Bill Addiss  Offline
Joined: Oct 2000
Posts: 3,875

You're assuming that the jacket of the MC cable is approved for grounding by itself. Is this true? I know that it was not at one time.

#76118 - 12/13/00 07:21 PM Re: Isolated ground receptacle  
pip  Offline
Joined: Dec 2000
Posts: 15
Little Elm,Tx
he is not using the jacket for a his post again. bill, would you explain the reasoning behind not running the seperate ground rod. I've ran many job where it was spec'd that way.

#76119 - 12/13/00 07:55 PM Re: Isolated ground receptacle  
Bill Addiss  Offline
Joined: Oct 2000
Posts: 3,875

you're right Pip!
I thought Rick was still talking about Isolated Grounding Receptacles.

As to your question, there are others here better able to explain it than I, but I have also been involved in jobs where someone mistakenly specified different grounding schemes to eliminate noise. And, all I can say is that besides being against the code, they just didn't work. You need a good solid ground for several reasons. My experiences involved bonding of Transformers for Computer rooms in several Major retail chain stores where there was a difference in potential of about 29 volts Neutral to ground - (this gave them some problems with Cash registers also) After several weeks of work running new wiring (insulated grounds) and balancing circuits better the difference was down in the single digits, (would change somewhat with connected loads) but not gone. Finally they allowed me to bond the Transformer properly and the voltages almost vanished. So, just because it's specified doesn't mean that it's right or even beneficial.

#76120 - 12/15/00 12:14 AM Re: Isolated ground receptacle  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Sounds good, but instead of stripping the red, ID it with green tape [unless you are using non-metallic outlet boxes, but in that case, you would not need an equipment ground bond].

Attach the ground that will be for equipment bonding to the panel's enclosure. Isolate the other one until you get to the Grounding Electrode System for that particular system.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#76121 - 12/15/00 01:28 AM Re: Isolated ground receptacle  
gpowellpec  Offline
Joined: Dec 2000
Posts: 127
Irving, Tx USA
Scott, maybe it should be pointed out that stripping, taping, and coloring a conductor in a multiconductor cable to mark it as an EGC is allowed only under conditions where it can be ensured that qualified persons will be doing the maintenance. (250-119(b))

#76122 - 12/15/00 08:17 AM Re: Isolated ground receptacle  
cinkerf  Offline
Joined: Dec 2000
Posts: 84
Pittsburgh, Pennsysvania. ...
In my original post I should of included that qualified persons will be doing the maintenance. After reading Scott's post I tend to agree with him. Instead of stripping the red conductor, ID with green tape would be better. (Frank) cinkerf

#76123 - 12/15/00 05:43 PM Re: Isolated ground receptacle  
sparky  Offline
Joined: Oct 2000
Posts: 5,303
sounds like an "isolated circuit" would do fine.....

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