Isolated ground circuits

[Alwayslearningelec]

Do they have two grounding conductors? So one circuit would contain a hot/ground/iso ground? Then what if you picked up a second circuit? WOuld it be (2) hots, 1 G & (2) Iso Grounds?

Thanks.

 

[charlie b]

You left of the neutral, or "cold," to go with your "hots." A single, single-phase, 120 volt isolated ground circuit would consist of 4 wires: ungrounded, grounded, EGC, and isolated ground. Three would originate within the panel that feeds this load. The fourth, the isolated ground wire, would have to be brought back to the main panel (or the first panel after a transformer that establishes a separately derived system), without being connected to any ground or neutral bar of any intervening panel.

If you have two isolated ground circuits in the same conduit, I would expect to see either 6 or 7 wires, depending on whether the neutral were to be shared. You can share the EGC, but I would think the isolated ground wires would have to be brought back to the main panel individually.

 

[LarryFine]

In a nutshell, the isolated-ground conductor is wired like an additional circuit conductor, in that it is insulated and not 'grounded' anywhere except the main-disco neutral-bonding point.

This conductor is not used for grounding anything except the isolated-ground terminal on the receptacles. There must also be a normal EGC for the conduits/boxes/equipment/etc.

Whether separate circuits can share neutrals or isolated-ground conductors is up to the system designer. Barring that, I'd share the IG anywhere the neutral can be shared.

 

[Smart $]

... the isolated ground wire, would have to be brought back to the main panel (or the first panel after a transformer that establishes a separately derived system), without being connected to any ground or neutral bar of any intervening panel.

...

An alternate arrangement is an isolated grounding bar in subpanels with one large iso-grounding conductor run back to the main panel... but its a design decision.

 

[jmellc]

We got subbed out to do some IG's, among other things. The EC provided 12/3 w/g MC cable. I was showing my helper how to do it with this cable. Used black & white as normal, green for IG and stripped out the red to ground the box. EC heard this and said that's not the way he did it. He'd already been on my back all night about 1 thing or another. I got in his face and said that was the only way to do it if he didn't have specific IG MC or AC cable. He backed off and left me alone, never told how he would have done it. Anyone know another way with the cable provided? Only other variation I know would be using red hot if a red ckt, and stripping out black for box ground.

 

[iwire]

I was showing my helper how to do it with this cable. Used black & white as normal, green for IG and stripped out the red to ground the box.

IMO 250.119(B) can be used to fail that method.

250.119(B) Multiconductor Cable.

Where the conditions of maintenance and supervision ensure that only qualified persons service the installation, one or more insulated conductors in a multiconductor cable, at the time of installation, shall be permitted to be permanently identified as equipment grounding conductors at each end and at every point where the conductors are accessible by one of the following means:

(1) Stripping the insulation from the entire exposed length

(2) Coloring the exposed insulation green

(3) Marking the exposed insulation with green tape or green adhesive labels

If the inspector does not feel the 'conditions of maintenance and supervision ensure that only qualified persons service the installation' they can fail striping the red for the EGC.



We buy IG MC which has a green and a green with yellow along with the white and black.

Or we buy 12/2 hospital grade AC which has an armor that serves as the dirty ground and an insulted green condutor for the IG.

 

[infinity]

We buy IG MC which has a green and a green with yellow along with the white and black.

Or we buy 12/2 hospital grade AC which has an armor that serves as the dirty ground and an insulted green condutor for the IG.

I would prefer either of those cables to the red stripping method. Typically on all jobs now we use MC-IG with an additional EGC.

 

[Alwayslearningelec]

That was all really helpful and insightful. Thanks and have a great and ssafe weekend. Off to Fire Island shortly with the fiancee and some friends.