Situation: Proposed new 120/208V 225A MB load center with an isolated ground, fed from a transformer (480V primary). Can the isolated grounding conductor be terminated at the transformer or should it be run back to the service equipment? The service switchgear is located in the same room as the proposed transformer. Is it sized the same as the equipment grounding conductor for the feeder?

It should be terminated at the main bonding jumper for the transformer secondary. The minimum size is based on the size of the OCPD on the secondary side of the transformer and Table 250.122
Don

[This message has been edited by resqcapt19 (edited 07-16-2002).]

Be sure to abide by 250.30(A)(1), thru (6).

Glenn

Don,

So are you saying the grounding conductor should run from the isolating, step down transformer, main bonding jumper, to the nearest building steel? I am just trying to get this straight in my head. I see a situation like this coming up.

Caper

Don's right.
Go to the XO terminal of the xfrmr, & no further.

Are you folks describing the general practice or a code requirement? My understanding is there is no exact code description as to how far back the isolated ground conductor must go, and it depends on specifications.
Could someone help with a reference.

I haven't seen any code requirements that the isolated ground MUST originate at a specified point. However it seems that 250.146(D) assumes that you will be running it back to the service (or APPLICABLE derived system) ground: ". . .this grounding conductor shall be permitted to pass through one or more panelboards. . .so as to terminate directly at an EGC terminal of the applicable derived system or service." Often times however, I have seen isolated ground receptacles fed from panels where the equipment ground is the pipe feeding the panel. I just wonder if that defeats the purpose.

I was interested in a reference to be sure that the requirement that I set for installations does not conflict. I always specify that an isolated ground panelboard have its equip. grounding conductor run as normal to its upstream source. The isolated equipment ground conductor gets run back to the main point of the greounding electrode system (bus bar at the water pipe, etc).

Treat the Isolated Grounding Conductor feeding the IG bus as if it was an EGC that is used for a typical subpanel - only that it lands on an isolated grounding bus and does not bond the subpanel plus subfeed conduit to the enclosure.

Alternately, the concept of how a Common Grounded Conductor is installed will make more sense in regards to how the "DESIGN INTENT" of an IG system is set up.

Using this train of thought, apply it towards a system which is straight 208Y120 from the PoCo. The EGC's will land on the Grounded Conductor's bus in the main service section of the gear, or the EGC bus will be directly bonded to the Grounded Conductor's bus. From there, it runs directly to the subpanel.

For your Isolated Transformer / SDS [480 x 208Y120], land the IG feeder to the X0 terminal, or to a bus which connects all the Grounding stuff for that Transformer's Secondary [all the EGCs plus the Grounding Electrode Conductor, along with the main bonding jumper to the X0 terminal].

This is the "Intended" method and is by far the better installation choice.

The NEC is only concerned in establishing a solid ground bond for this IG stuff, so to simply satisfy NEC minimums, one could attach the IG termination from a Receptacle directly to a Grounded 4s box - exactly the way any other Receptacle is Grounded - and pass NEC's requirements with flying colors!
That's not the DESIGN INTENTION for an IG system, though.

Also, the IG conductor size per the NEC is what's listed in 250.122 [or 250-95 1996 NEC]. Simply if the highest trip rating for an IG circuit is 30 amps [not including Voltage Drop or other derating factors], the IG conductor need not be larger than #10 cu.

Once again, DESIGN INTENTION will push more towards using a #8, a #6 and even as far as #4 for the IG subfeed conductor [going from the main grounding point to the IG bus].

This is a subject which may require you to do some information searching prior to designing the complete system.

To do it the simple way, just run a #8 cu [insulated, of course!] from the X0 terminal at the transformer, to an Isolated Grounding bus in the subpanel. If there are any branch circuits exceeding 60 amps which will use an IG, then step up to #6 cu.
For your 20 amp branch circuits, run a #12 cu IG conductor for each 20 amp circuit [dedicate it for outlets on that particular circuit]. One IG conductor will do, but shoot towards one per circuit.
Include the normal EGC's and terminate them to a typical Grounding bus in the panel, as would normally be done. Just do not re-ground the IG bus in the subpanel or elsewhere.

Good luck

Scott S.E.T.

From the secondary of the transformer, I agree with an earlier post that I have to run at least a #4 IG for 225A OCPD. Any equipment related fault will return on this path. I'll run another #4 to bond the raceway and enclosure. I'll probably start the job next week, so I'm grateful for all the responses to this topic!