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Isolated ground... are you kidding me?

Posted by pharkus (My Page) on
Tue, Jun 12, 12 at 11:22

We are an inbound sales call center. We recently moved into an old mill building that has been entirely renovated. The former tenant, a government agency, had an IG panel installed for all of their computer equipment. We have decided to use this for ours as well. Our layout is completely different, and I have thus had to rearrange/relocate most of the circuits. Easy enough.

Now here's the challenge: we're expanding - there aren't any more circuits to rearrange (however, there is plenty of space in the panel, which is a modern 3-phase QO) so we actually have to add more.

I thought this should be simple, and I still think it should be, except that materials - specifically the cable - is simply nonexistant! Two local electrical supply distributors have told me "Yes, we have what you need," but then looked in their computers and told me "but we don't have any... and can't order it."

The existing cable is MC, 12/2 with two grounds... One of the electrical places called it "12/2/2" which I have found in NM-B at $225 for a 250-foot roll... we need about 75 feet of it... and I have no idea if our local code for this building requires MC or not, but since every single piece of cable up there IS MC, I'm assuming so.


So uhh... where do I get this obviously-existant-but-apparently-impossible-to-find 12/2 MC with two grounds?


Follow-Up Postings:

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RE: Isolated ground... are you kidding me?

I wired a computer center with isolated grounds back in the day. We used individual wires in liquidtite conduit under the raised floor. Frankly, I think isolated ground is pretty overrated. I've never seen it in a computer center since.


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RE: Isolated ground... are you kidding me?

Frankly, I'm not even sure WHAT good isolated ground does if you are going to use NM (especially if you use plastic boxes). What is connected to the "non isolated" ground in your branch circuit?

We used the isolated ground to get the equipment ground separate from the conduit/metal box.


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RE: Isolated ground... are you kidding me?

use 12/3 and tape the red green at the panel and at the plug.


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RE: Isolated ground... are you kidding me?

Southwire does make an type MC cable with an isolated ground.

You can call them at 770-832-4242 and they should be able to help you find a distributor in your area.

Here is a link that might be useful: Southwire


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RE: Isolated ground... are you kidding me?

"use 12/3 and tape the red green at the panel and at the plug."

this is horrible advice and illegal since you can't "re-phase" conductors that small.

I'm more concerned with why you (pharkus) are doing work in a commercial building when you've mentioned numerous times that you are unlicensed.

Under the table un-permitted scab work?

(here's where you come back with attacks and insults about how that isn't what you asked and how educated you are)


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RE: Isolated ground... are you kidding me?

"Southwire does make an type MC cable with an isolated ground."

The actual ground conductor is often superior for electro-magnetic interference than using the armor.


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RE: Isolated ground... are you kidding me?

"this is horrible advice and illegal since you can't "re-phase" conductors that small"

Read 250.119.B

It is allowed in multiconductor cable where serviced by only qualified persons.


Why not just use HCF-AC cable? Use the jacket for your ground and the insulated green conductor for your I.G.


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RE: Isolated ground... are you kidding me?

MC is what I expected to find. Nobody had it. Only one could order it, if I wanted a 1000-foot spool. I needed 75 feet of cable.

Legally? The right names are on the paperwork and the job is about as legal as it's ever going to get.

It's also complete, working, and in use.

12/3 MC with the red conductor relabeled to green is what we wound up using, per the input from this forum and seconded by the advice of the person whose name is on the papers...


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