How do I install an isolated ground circuit?


Old 05-21-05, 08:02 PM
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How do I install an isolated ground circuit?

I'm making a couple isolated ground circuits for the computer in my office and the TV watching equipment in the living room. I bought the receptacle from Lowes, it's an orange one with the triangle costing about $10. I'm doing the living room now and I've pulled 12/3 NM cable directly from outlet to main circuit box (home run) and want to use a plastic retrofit (old work) box. I know where to put the black and white wires, but I don't know where to put the ground wires.

I'll be using the red wire for the insulated ground. Normally I'd put the one end of the ground wire on the green grounding screw on the outlet and the other end on the same bar as my nuetrals in my main circuit box. My box doesn't have a seperate bar for grounds and nuetrals. Is that correct for the isolated ground circuit?

That also leaves me with the regular bare copper wire that's normally the ground. Where is it supposed to go?

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Old 05-21-05, 08:07 PM
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Here's the first thing you do. Take that Black and Decker wiring book you have and cross out all references to isolated grounding circuits. Cut them out if necessary and throw them away.

No only are the instructions in that book a violation of the U.S. National Electrical Code for residential applications, isolated grounding circuits in a home are completely worthless. Just wire an ordinary circuit. Leave the red wire unused. Whatever you do, do not use the red wire for ground as the Black and Decker book says!!! Do not, do not, do not!

How do I know you have a Black and Decker book when you didn't say so? Because it's the only book ever written to advise something so incredibly stupid.
Old 05-21-05, 08:43 PM
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As John said, take those orange receptacles back and get your money back. You wasted good money on something that will gain you nothing.
Old 05-21-05, 10:43 PM
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Thanks for the replies.

Yea I've been using the Black and Decker book. It didn't really explain the isolated ground circuits except to use them for your computer.

My house was built in 1954 and used non grounded plugs. The circuits are really spread thin. All the kitchen (which also has the clothes washing machine) receptacles and lights were on the same circuit... except the stove and dryer. Part of the lliving room was wired on the same circuit as the hallway, smallest bedroom and bathroom. The two largest bedrooms, one which is my office, are on the same circuit. The space heater in our bedroom tripped the breaker this winter and parts of my computer were fried.

I've rewired the kitchen and I am working on the living room/dining room combo now.... it's one room. I put 4 15A recepticals on a 12/2 20amp circuit. I'll put the dining room chandelier, living room ceiling fan, outside light, foyer light, and hallway light on a 12/2 20A circuit. The plug I'm using for the DVD player, digital cable box and TV and maybe a future surround sound system, I'm going to put on it's own 20A circuit. I was thinking a isolated ground circuit using 12/3 nm wire.

Because I bought a 250' roll of 12/3 wire and only used it on the 3 way switched fixture in the kitchen, I have a lot of excess that I'd like to use. If don't use it for isolated ground circuit, it'll be wasted since everything else only needs 12/2 wire.

It's good to know that isolated ground circuits for residential isn't necessary and overkill. I appreciate the advice, but at this point I'd like to do the isolated ground circuit if I can. It may be a situation where I can't do this correctly since my in my circuit box doesn't have a bar for the ground that is seperated from the nuetral. But I've got the 12/3 wire and the isolated ground receptacle, so I might as well use them if I can. Any advice on how to do this would be great!

Also, what's wrong with using the red wire as a ground if it's marked as a ground?
Old 05-21-05, 11:35 PM
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It is a serious violation of the National Electrical Code to use a red wire for ground in a residential circuit. Don't do it!

This isn't merely a case of "isn't necessary and overkill". It's a case of "bad". Don't do it!
Old 05-22-05, 06:33 AM
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Originally Posted by assemblage
It may be a situation where I can't do this correctly since my in my circuit box doesn't have a bar for the ground that is seperated from the nuetral.
This is not an issue. Even in a commercial application if the circuit is run from a main panel the grounds and neutrals are terminated on the same bars.
It has to do with the environment in a home and the fact that to effectively run an IG circuit you need redundant grounding. This cannot be accomplished with 12/3 NM, or ANY NM for that matter.

If you really are set on using this $10 receptacle, which can easily be returned, get you some 12/2 AC with the green sheathing. This cable has a sheathing which provides a ground as well as an insulated ground inside.
I also assume you want to use the 12/3 you have so this may also be a non-issue.
Old 05-22-05, 07:00 AM
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OMG.....I had one of my students come into our NEC class I was giving about 6 months ago with one of those B & D books.......I used it as a reference for all the wrong things to do in electrical...yes...BURN that book and B & D needs to stick to drills ( hopefully with double insulated cords )

Yes, If you are intent on using the 12/3 w/G then I would simply install a 2 gange box and have (2) isolated circuits at the location since you already have the wire run and use standard recepts...But again do not use that B & D book in reference to multi-circuit wiring......

Last edited by ElectricalMan; 05-22-05 at 07:27 AM.
Old 05-22-05, 07:16 AM
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In certain situations, an isolated ground provides a 'quiet' ground that is beneficial for computer use. These are generally commercial situations where metallic wireway or armored cable is used, and because of bonding requirements, loops in the ground circuit may be present. These loops can pick up electrical noise. The isolated ground is a separate, _insulated_ equipment grounding conductor that goes right back to the main ground bus, with no connections to other bonded metal. An isolated ground provides for the benefits of star single point equipment grounding in an environment with multiply connected equipment grounding paths.

As such, a _radial_ circuit wired using non-metallic cable to a non-metallic box using a normal receptacle _is_ an isolated ground receptacle. Just wire a single branch circuit from your panel to a single location, using normal 12/2 NM, and you have your isolated ground. Repeating what John was telling you: wiring this 'isolated ground' circuit offers exactly _zero_ benefit, in addition to being a code violation.

If you really want to use your 12/3 cable, then spend some time learning about 'multi-wire branch circuits', sometimes called Edison circuits or 'shared neutral' circuits. These are harder to understand, and not generally recommended in this forum. But the offer the benefit of running _two_ circuits in a single cable, offering twice the power at the destination.

Old 05-22-05, 04:25 PM
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Is isolated ground useful? Maybe. Maybe not.

An isolated ground is useful in some cases. Sensitive equipment that can be damaged by a line to ground fault on the circuit is one of those cases. That includes computers. And many newer computer driven home entertainment equipment has a similar risk.

The problem is that with external connections like telephone or cable, there is a multiple ground path. Normally this is not a problem. A fault on another circuit in the main also won't be a problem if your ground at the panel is good. But a fault from line to ground on the same circuit, or on any circuit in a subpanel shared with the circuit in question, can create a surge that can damage equipment. It is the high frequency portions of the fault

So how does an isolated ground help? It effectively makes a separate circuit out of things like the metal box. If there is a fault from line to box, if that box is on a separate ground, little if any surge would approach the computer.

But does this justify an actual isolated ground circuit? Not really. If you mitigate many of the risks, you won't really need an isolated ground circuit. The more of the following you can do, the better off you are.

1. Don't use metal conduit. If you were, you could have just run THHN single wires and gotten all the correct colors anyway, and not needed to (illegally) use a red wire. If you must use conduit, use non-metallic if that is allowed by your AHJ.

2. Don't use a metal box. Use a non-metallic one instead. Now there is less for a line wire to fault/short to. If you have a non-metallic box, an isolated ground receptacle is virtually pointless.

3. Use a dedicated circuit. With a dedicated circuit going only to the sensitive equipment and nothing else, you reduce the already low exposure to faults. The fewer things on the same circuit, the less exposure to problems.

4. Avoid subpanels for sensitive equipment. Run the dedicated circuit for sensitive equipment all the way to the main panel. Note that if this is a detached building with a subpanel where the grounding conductor is earthed, that is acceptable in place of a main panel (don't even think of trying to make a separate run between buildings for such a circuit, as that will just make things worse).

5. Use an arc-fault breaker. This breaker can quick shut off series arcs or high impedance arcs that can still cause high frequency surges that can damage sensitive equipment.

6. Use only quality surge protection equipment.

These things won't eliminate all risks. But then, nothing ever really can.
Old 05-22-05, 07:17 PM
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Please try and read the National Electrical Code, Section 210.

Your plan to add lighting to a 20 amp circuit with the dining room receptacles is also a Code violation. There are specific rules for receptacles serving bathrooms, kitchens and other areas. You really should try to understand these before you run wires willy-nilly around the house.
Old 05-22-05, 08:30 PM
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In a home if you want an IG circuit run a 12/2 wg romex from the main entrance panel to a duplex in a plastic box and its a done deal. Simple as that.
Old 05-22-05, 08:33 PM
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How do you define a dining room? My dad's new house has one big single "great room" that essentially has everything but the bedroms, bath, laundry, and garage. One side and corner has kitchen cabinets and counter. 2 GFCI protected 20-amp circuits with 5-20R receptacles are on the countertops. The rest of the room has various family interaction space, entertainment center (not that centered), and lots of places for eating, watching TV, playing cards, surfing the web, etc. Is that considered part of what 210.52(B)(1) refers to? Other receptacles are all around, but they are 15-amp and non-GFCI.
Old 05-23-05, 05:18 PM
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Thanks for all the replies with info and advice about the isolated ground circuit.

I took the receptacle back and just used a regular 15A receptacle for that dedicated circuit. I wired the four others using 15A duplex receptacles in a circuit. Both are on a 20A slim breaker. I'll do the lights on another circuit.

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