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I have a garage and a house. They are connected by a breezeway. The garage is now considered attached. I want to put a 400 amp panel in the garage. I want to make the 200 amp house panel a sub panel. The garage has no water pipes. The house main panel is bonded to the water pipes. I plan to drive two electrodes at the new 400 amp service in the garage, and pipe a four wire new run to the house panel. Do I have to run a bonding (water pipe) conductor to the house? If so, can I connect the bonding conductor to the isolated ground (bonded and grounded) in the 200 amp sub panel? There is an electrode at the house that is now my sub panel. The neutral and ground will be seperated.
If I must run a (water pipe) bonding wire to the house, can I run it in the conduit with the other four conductors?
Also, Should I size the bonding conductor for 200 amp or 400 amp. I appreciate any comments or suggestions.....John

Why do you want a 400A panel in the garage, and will your local POCO even let you put one that big on your garage?

I see no reason why you need more than the 4 wire feeder. The water pipe is bonded to the ground rods via the EGC in the feeder.

I agree. I did exactly the same install on my house a couple years ago - just with a 200A main and 100A existing in the house. That satisfied the inspector.

That is why I posted the question. I too assumed that the EGC and EMT would allow for a four wire feeder to the 200 amp sub panel. But I am getting conflicting advise on this matter. Several members at the Mike Holt forum have indicated that I need a bonding conductor seperate from the feeder to the sub panel, to go all the way to the house unbroken and connected to the water line at the house. Just as if the new service was at the house. I have also been advised that I can go with a three wire feeder since I am using EMT. I am going to contact the inspector and see what he requires. thanks for your feedback fellow forum members.......John

ps....400 amp service is needed for the new testing facility that will be in the garage.

The way I see it the main panel at the garage will need a GEC going unbroken back to the water meter.

Its going to bubble down to what the inspector wants to see.

What my inspector said was he is no longer considering the water supply as a ground. It simply needs to be bonded along with the gas supply.

That said, I do not have water or gas in the garage area. They are contained to the house where it is served by the existing panel. So that may be a factor.

Like LK said if it's an attached garage.

I suppose it gets down to what 250.64(C) exception means
.

It doesn't say "busbars in the same enclosure"

Unbroken, OR spliced via busbars or exothermic welds. Routing the GEC through the existing panel's busbar is acceptable per 250.64. In this case, the ground between the two panels is an GEC and must be a discrete wire sized IAW 250.66.

That said, the GEC is still required to be as short and straight as possible*, so the AHJ may feel routing through the old panel is unnecessarily circuitous and may require a more direct connection between the new panel and ground. In this case, the connection between the two panels is no longer a GEC, but merely an EGC, and EMT is acceptable if properly bonded.

* 250.4(A)(1) became stricter about this in NEC 2008.