ECN Forum Forums General Topics of the Trade General Discussion Area Parallel Isolated Ground

A job that I am bidding is calling for a 600A service with 300MCM paralleled in 3" conduit. The service disconnect is on the outside of the building and there is a MDP inside so this will be a subfeed.

They just decided to add an isolated ground for 2 pieces of equipment that are on 20A 120V circuits.

I have never had to parallel an isolated ground before so...

Can I just install a an extra 1/0 copper in one conduit in addition to the 1/0 grounding conductor in each conduit.

Or, do I have to install an isolated 1/0 in both conduits. If I did this I would have (4) #300MCM and (2) 1/0 in each conduit. This would seem like overkill for two 20A 120V circuits.

Thanks
Brian

My journeyman installed 3 duplex receptacles in 3 separate 1900 boxes. 1 of the receptacles was grounded in the common fashion, and 2 of the receptacles had to have an isolated ground. All 3 were linked together by EMT. So what we did was pull a common ground and bond all the boxes and 1 receptacle with it. And we pulled a separate ground and grounded ONLY the 2 receptacles that required an isolated ground. Hope this helps...not sure if applicable.

It might indeed seem like overkill, and I might ask again if that's what they are reaaly needing, just to make sure. But if it is, then install the wires. You'll also need insulated isolated ground busses (so identified) for any panels the IG goes thru as well.

I'd make both parallel feeders look exactly alike, and of course of equal lengths.

Radar

Is the MDP the first overcurrent protection from your new service? If so then the isolated ground bond to the neutral and regular ground at that service.
Do not have my code book with me so I can't quote the section but Isolated grounds are bonded at the service or at the first point ao a seperatly derived system, if memory serves me correctly.

There is a fused disconnect on the outside of the building which is the service disconnect. The MDP is MLO. I have to get an isolated ground back to the disconnect.

It may pay to find out _why_ isolated ground receptacles are required. Often times there is no good reason for the isolated ground receptacles other than someone thought 'computer needs it'.

Also, do you need to run the isolated ground circuits from the MDP? Could you run a separate 1" conduit to a panel simply for quiet ground use?

-Jon

jon


For me it is usually 'because the engineer said so'.


No, the NEC requires the that EGCs (including IGs) run with the circuit conductors.

This is a interesting question and IMO the answer is the following.

The IGs will be required in each of the parallel conduits but they need no be larger than 250.122 requires for any of the outlets served by the IG.

In my experience none of the IG receptacles will be larger than 20 amps meaning a 12 AWG in each conduit back to the MDP will sufficient.

Of course than we run into 250.122(B) which may require the 20 amp IGs to be 300 kcmil as are the phase conductors.

I find this post very interesting and amusing. For the IGC to be even remotely effective, the IG EGC should originate at the service N-G bond point. That would mean from the service disconnect device you would need two EGC circuits and two ground bars in every panel down stream to accommodate the IG EGC. I agree with Bob (iwire), the IG EGC circuit could be done with 12 AWG.

Here is the funny part. There is no NEC requirement to do this, NONE... If you wanted to do this cheaply, you could simply originate the IG EGC at the panel that serves the receptacle by attaching the green/yellow IG EGC right to the EGC bus in the sub-panel. IMO according to the NEC you could go even cheaper by simply bonding the IG EGC pin of the receptacle to the box the yoke is mounted in.

Enough nonsense, here is my recommendation as an engineer in the know (sorry Bob, couldn’t help myself): Instead of running IGR, run dedicated 20-amp circuits to the locations. If you really wanted to make a PQ circuit, oversize the grounded circuit conductor or even both the N & L conductors. Some electronic equipment is sensitive to N-G voltages. By over sizing the neutral conductor you minimize the N-G voltage to less than 2-volts peak to peak (IEEE spec).


[This message has been edited by dereckbc (edited 02-05-2006).]

If I set the "way back" machine to when IBM wanted IGs (maybe before some of our readers were born) all we wanted was a wire that originated at the MDP and did not land in any other load side panel. Typically these were simply spliced with a wirenut in intermediate panels and it could be a single wire sized to the largest downstream EGC until it split out to the various branch circuits. As long as it was a "tree" radiating from the ground bus it was OK with us.
Over a quarter century ago IBM decided it was not worth the money and stopped "officially" recomending it. The legend still lives on tho.

I was suggesting a separate feeder for a separate panel. Sorry that I wasn't clear.

-Jon