ECN Forum Forums Code Related Discussion NEC & other Code issues Isolated Grounding

The drawings I am working on show a #6 EGC run from the isolated ground bus of a 120/208V panel directly to the equipment grounding point of the supplying transformer. This is all fine and dandy with one exception. The transformer secondary consists of two conduit runs. There is only one isolated grounding conductor to run to the transformer. One pipe will have the 3/0 EGC and #6 isolated EGC. The other pipe will only have the second 3/0 EGC. The engineer does not see a problem. I think it’s a violation of 250.122(F). Am I wrong?

In my opinion, the engineer is wrong. The isolated equipment grounding conductors have to connect to the equipment grounding terminal of the derived system (250.146(d). This is in the panel, not the transformer. Technically speaking, the grounding terminal in the transformer (load side) is for the equipment bonding conductor, not the equipment grounding conductor. Equipment bonding conductors have to follow the same paralleling rules as any equipment grounding conductors. The equipment grounding conductor is sized per 250.122. The equipment bonding conductor is sized per 250.66.
I don't know how the engineer came up with the #6 size for the combined isolated 'equipment grounding conductor'.
From a third world nation point of view, there is no real problem with the way the engineer had it designed, except for noise radiated from the equipment supply conductors and grounding conductor during a fault. If they are run in the usual parallel way, the fields cancel and so the noise problem goes away. So running the isolated grounds in parallel would work..but still illegally!

I see no problem with the isolated grounding conductor being connected in the transformer, but I think it should be run in both conduits if the ungrounded conductors are is parallel.
I think that 250-146(d) permits this connection at any point up to and including in the transformer.
Don(resqcapt19)

I agree with Don, but believe it could be pulled through all the way to the service ground, insulated, if one wanted to.

If the power source for the panel is a transfomer, the isolated ground connection can't go past the XO terminal of the transformer. This is still a safety ground and must be able to clear a fault.
Don(resqcapt19)

Nick,

I can see how the EE is thinking on this issue. Since the IG Conductor is used on the Dedicated IG circuits, the largest OCPD value would be 20 amps [unless there are circuits of higher amperage...???], so even with a #6 cu it would be "Overkill" in a way.
Minimum NEC requirements would be the angle the EE is using when addressing only one IG bond for Parallel Feeders.
The minimum will allow only one IG bond run from the panel to the source.

It would be different if this was the EGC, but still it wouldn't hurt to include a #6 with both Feeder sets.

Elzappr,

I believe the system that Nick is describing would be an SDS [Separately Derived System].
The Transformer mentioned would be an Isolation type Step-Down Transformer - Like 480 VAC 3 phase Delta Primary x 208Y/120 3 phase 4 wire Wye Secondary.
In this case the proper point to land the IG conductor would be at the X0 Ground bonding point [where the EGCs and the GEC bond to the X0 termination].

The use of a #6 cu is a very common spec for IG bonds. I typically spec that size. It's larger than needed, but that's a good thing!

Back to Nick,

If the Subfeeds aren't really long, would it be easy enough to simply include a #6 IG bond in both Subfeed conduits?

This would satisfy everyone - you will not have the 250-122 item bugging you [it will be installed to your satisfaction], and you will not need to resolve EE / EC issues, nor wait around for RFIs to be answered and all that baloney!

Scott SET

Thanks for the reply's. I think I will just put the extra IG ground in to the transformer and be done with it. It's only about 20ft of wire and two termnations. These drawings are remarkably good for this day and age. Only a couple of small items like this to sort out.