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#161719 04/06/07 11:17 PM
Joined: Mar 2005
Posts: 240
this is a can of worms i am beginning to think, but here goes.
a home owner is building his own music studio designed by an accomplished designer.
who has specked out isolated grounds for five of nine circuits in studio. seems simple enough....uggghhh i didn't say simple did i? first i should say, this studio is an addition to his existing house(glorified remodel) the existing panel only has three circuits available so the studio will require its own sub panel. which seems perfect in the fact that i can isolate ground easy enough since neutral will be floating. but in order to be a true isolated ground(my interpitation) it would require the three ground rods with a delta configuration using a #4 cu. insulated conductor back to the sub panel which wouldnt be bonded to the sub panel at all.....not electrically a good idea but true to the word isolated ground. so here we have a isolated ground bar not bonded and an equipment grounding bar which is bonded to the sub panel.....sound right so far?
but it seems moot since the boxes i will use will be plastic and there wont be any raceways(emt or otherwise) whats the point?
i use a three conductor tape red conductor green and dump bare ground back into box for have an insulated conductor ground my recording equipment? which is supposed to reduce noise, yet creates bigger and more serious problems electrically speaking. and if i bond iso ground to sub panel havent i just created a short-cut for fault current?
what am i missing? what can or do you guys suggest? i am not an expert by any means only remember from tech school very little.
thanx in advance.

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Joined: Jul 2004
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An IG only means it is not connected to the building steel or any other grounding path other than the bus where the main bonding jumper resides. In computer rooms we would pass an insulated green wire unconnected through all the sub panels and attach it in the service disconnect enclosure ground/neutral bus.
A romex wired, dedicated circuit in a plastic box originating in the "main" panel with the service dosconnect IS an IG, even without that expensive orange receptacle and 4 wire circuit (hot, neutral, box ground and IG).
IBM gave up IGs in their specs in the 80s because they figured out it was usually a waste of money. Some data center managers (and "computer science engineers") still insist on it but they are just wasting their money.

Greg Fretwell
Joined: Jan 2003
Posts: 4,391
watersparkfalls, I have to run so let me just say this for now.

You can not install a truly isolated ground.

If you choose to install IG receptacles you must without question tie them into the grounding bus at the main panel.

To tie the IG into just electrodes in the dirt is both a code violation and is dangerous.

250.4 General Requirements for Grounding and Bonding.

(A)(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.

Bob Badger
Construction & Maintenance Electrician
Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Check out these IG Drawings + Schematics - they may help to explain proper designs of IG Circuitry.

Although the examples are built on Metallic Raceways + Outlet Boxes (therefore the Branch Circuits contain an EGC, or use the Raceway as an EGC), the concepts are still the same - just no need for a separate EGC when using Non-Metallic Boxes + NM-B Cable ("Romex").

as previously mentioned, a 2-Wire NM-B cable will be sufficient for a 120 Volt, 1Ø 2 Wire IG Branch Circuit.
The included bare EGC in the Cable is already Isolated by design - and if the Receptacles are in Non-Metallic boxes, the Equipment Bond is also Isolated by design.

Here are the drawings:

[Linked Image]

Fig. 1: Isolated Grounding Receptacle - Termiantions details.

[Linked Image]

Fig. 2: Panelboard + Branch Circuitry detail option

[Linked Image]

Fig. 3: Overall SDS detail with IG Branch Circuitry.

These items may be found in the Technical Reference section, at this location:

IG Details #1

Feel free to reply if you have more questions.

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

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