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
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!
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.
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))
Re: Isolated ground receptacle#76122 12/15/0007:17 AM12/15/0007:17 AM
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