Hey y'all. Been gone a long time. Hope you can help me:
A residential client wants an isolated ground for optimal performance in his audio/visual entertainment system. I plan to run 12-3 from isolated-ground type recept, marking the red wire as green ("clean" ground) and attaching it to isolated groundscrew on device, then running the 12-3 into branch circuit panel, but with the (red) isolated ground wire continuing on to the service entrance for bonding (i.e., no bonding at branch circiut panel). My question is: what do I do with the other ground ("dirty", uninsulated copper)? As this is a residential job, all boxes are plastic and there is no conduit. At the panel I will of course terminate this bare copper on the equipment grounding bar; but what do I do at the recept? Since on an isolated-ground recept the yoke is isolated from the device's grounging screw, can I (now this is gonna sound like a real hack, but...)..can I wrap the bare copper around the screw that attaches the yoke to the plastic box? Is there a better (i.e., more proper) way?

Bad idea.

An isolated ground is simply a ground wire that runs straight from the device to the panel, without connection to other ground wires along the way. Since you're running a dedicated circuit, the usual ground wire will do just fine.
Heck, with plastic boxes, you don't even need a fancy IG receptacle. The yoke is never in contact with anything that's also grounded. Just connect the ground wire to the screw on the receptacle.

Thanks for the quick reply. But still I have these concerns: NEC 250.146(D) "Isolated Receptacles" requires "an insulated equipment grounding conductor" which I won't have with 12-2 romex since the copper is bare. The same article requires that this isolated ground terminate only at the service, i.e., not in the branch circuit distribution panel. Further thoughts?

In Canada the days of taping an identified conductor green are gone , we are not allowed to tape the red wire green anymore.

What John is saying is you don't need IG hardware if you run a dedicated circuit in a plastic box. The ground is "isolated" simply because there are no other connections.

The NEC is simply recognizing that IG is really designed for places that use metal raceways that are also incidentally grounded a number of places in the building, creating ground loops.
IBM decided in the late 80s that IG was snake oil and removed the requirement from the physical planning manual. Unfortunately a lot of people who should know better did not get the word.

Use MC

~Matt

Matt,

Why use MC cable?

1. It's going to have to use MC-HCF (for Health Care Facilities) or some other form of IG cable anyway. The armor of MC cable doesn't meet the requirements for grounding. It's both the insulated ground in conjunction with the armor that's required for grounding path. An additional ground wire (present in the IG cable) is needed for the IG circuit.

2. It's going to have to have a metal box, now that you're using MC cable. Plastic boxes can't be used with MC cable

3. It will require the use of an IG receptacle.


Reno's method is just fine.

Just keep in mind if you do decide to run IG the green insulated wire will start at ground unity, ( the ground rod ) and run isolated from there to any device on the IG circuit. True IG installs are expensive.

The IG should terminate on the bus where the main bonding jumper lands according to the old standards IBM used. Then it passes through all cabinets on the load side without connection to the EGC or enclosure. You can create a "tree" system on insulated bus bars radiating from the MBJ bar.

All that said, IBM decided there really wasn't much to be gained by it and removed the recommendation from our books.

Well, yah, if you try to build a network using RS-232 or using some sort of coax-cable signaling that relies on the shield being a common ground, you're pretty much toast if you have ground noise or ground differentials.

Modern signaling systems like Ethernet use coupling transformers with minimum 2000V isolation. Ethernet doesn't really care if you have ground problems.

I note that the guys at Xerox PARC found about a 90V ground differential between one end of the building and the other when they installed their first experimental Ethernet network.

The IBM 3270s did not ground the signals in the terminal, just for that reason. The coax was only tied to frame ground in the controller. The terminal end had the front end of the signal receiver floating off of the EGC.

When we did have ground shift problems we found bonding was a lot more effective than IG. It was exactly the opposite methodology. We ran a fat "drain" wire between machines creating that dreaded ground loop. It was always black, never green and we never used the "G" word, so as to avoid article 250 questions. It simply bonded the frame ground of the machines together. It was how we fixed the lightning problems in a lot of Florida installations.
We never saw any signal related problems and our line driver/receiver cards stopped expelling the magic smoke every afternoon.

Tripp,
Getting back to the question that you asked (we got a little bit off track there). If you and your client still want an isolated ground circuit, you must use cable with an insulated ground wire. Unfortunately the code does not allow taping the red wire green if it is #12 AWG. If you are not getting the work inspected, I would certainly tape the red wire.

If the work is to be inspected, the first solution to that conundrum (is that really a word?) is to use 1/2" flex with two green ground conductors and a metal cut in box or metal 4" square box if in new construction. You could then put a band of yellow tape on the green isolated ground wire, leaving the green exposed to comply with the NEC.

One trick that you may have used with flex is to pre-install the conductors in the flex (somebody will probably point out that the NEC forbids this). You can then install the flex like cable and strap every 4-1/2 feet, if possible.

As previously indicated special IG cable is available, but quite expensive.

Best of luck,
Larry

Homer,
That is sure a hack way to do things.
I'm surprised you even bring suggestions like that to this board.
If no inspection is called for, that's no reason to do things in a "less than" manner.

I still agree with John. 2 wire wg Romex to a dedicated outlet in a plastic box IS an isolated ground.
If you want a code compliant way to get to an IG receptacle with a green wire, think smurf tube (type ENT)

The job is being over engineered.

Tell the client that his solution is to go to a 'dedicated ground'.

Avoid any worries about IG engineering.

Go with Reno's solution.

The results will be fine.

You can still use an IG receptacle just for looks, no harm there.

Better still go to a novel color like grey with a totally conventional receptacle.

Or you can use a Decora style receptacle for this one circuit in a house that is conventional otherwise, etc.

Regular 12-2 or 14-2 Romex will do you fine.

Larry, thanks for bringing the topic back around. I have to admit that the IBM stuff was somewhat going over my head. All the same, perhaps I am a better person for it (but don't count on it).

And thanks to everyone for their inputs. I get so much out of these discussions.

I have really vascillated between (1)going the three-wire route (taping the red to green - yes, it violates code, but in a residential setting there is little chance of confusing a subsequent electrician as long as I tape it at all junction points - and I believe if there is to be a "noise" problem, having the ground insulated can only help), or (2) sticking with 12-2 and the bare copper ground. One remaining hesitation I have with the second method is that I would really want to bring the ground all the way through the distribution panel and into the service disconnect enclosure, without any bonding in the distrib. panel. But to do this would likely subject this bare ground to at least minimal contact with the "grounded" enclosure of the distrib. panel. Did I lose anybody?

With all that said, there is one remaining factor that is strongly influencing my decision. And that is that before the client said anything about isolated ground, I had already fished 12-2 into the basement crawlspace, where I will splice it into a j-box before fishing it up to the panel. Now I know most of you will say, 'oh, well, if you've already gone that far, why not just finish it with 12-2?" Thing is, I was wondering if I should refish the wall with 12-3. As I write this I can already hear the bombarding of answers, so I guess I can say that I will finish the job with 12-2, tell my client a "dedicated ground" is as good, and be done with it. Sigh.

Get some green shrink tube and tube the bare ground through the MDP or simply splice some green wire on the ground conductor and green shrink/<green>tape up the bare part.

I can't resist jumping in here!

Run a 3 wire cable and:

a) strip the red everywhere it's exposed and use it as a normal ground. Is there a rule against that?

b) shrink or tape the bare ground green and use it as an isolated ground.

Question:

Where is the "normal ground" to be terminated?

Plastic boxes have no place to terminate the normal ground, and IG receptacles have no place to terminate the ground wire to the yoke of the device.

I really think some folks are over-engineering this ... much as the guy who wears both a belt, and suspenders.

The first question to ask is: what are we trying to accomplish? A good, solid ground, with no connections to either introduce resistance or noise?

In that case, you simply can't improve on the ground in the dedicated cable that terminates on the device, in a plastic box.
Are you worried that the ground wire might touch another within the panel before it gets terminated on the bus? My first thought is "so what?' ... after all, the box is metal, and will therefor contain any radio noise thus made. If it makes you feel better, though, wrap the exposed portion in the panel. A penny's worth of tape, and you're done.

You want it to go all the way back to the service? Well, good luck. As Greg has mentioned, IBM -who got all this nonsense started in the first place- has backed off, admitting that their concerns were largely unfounded. (It seems that they were being bedeviled by poor wiring practices in general ... rather than any esoteric grounding theory.)

About the only 'improvement' you can make is to up-size the romex. If the orange jacket makes the customer happy, so much the better.

If this is just A/V equipment the biggest factor is to just be sure everything is plugged in the same place so all of the equipment is using the same power and ground. Then bond the frames of all equipment together. High end equipment usually has a bonding lug to do this with. Then you won't have that nagging 60hz ground shift and hum.
You start having troubles when you have a couple power sources and cables strung around the house.

Electure has a good point about the IG receptacle. In a plastic box, just use a regular one