2 buildings, one utility transformer, 2 meters, common g

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sailboat

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Customer wants to connect a single generator to two different buildings (house and detached garage). Both buildings are fed from a common utility transformer but each building is metered seperately (and have individual service entrance disconnects after the meters).

Right now each building has the grounded conductor (neutral) bonded to the ground bus (grounding electrode)at the service entrance disconnect.

The intent is to conect the generator output to both buildings (via seperate transfer switches)

The generator is not a seperatly derived source. (the frame is connected to grounding electrode but there is not a bonding jumper between the neutral and ground)

The neutral in the transfer switches are "unswitched" therefore, if I connect the generator neutral to both buildings (via a bus bar in a j-box), the neutrals for both buildings are then connected.

Should I disconnect the main bonding jumper (between the grounded conductor and the grounding electrode) in ONE of the service entrance disconnects? (i.e. - isolated the neutral in one panel)

The plan would be to tie all the equipment grounding conductors (both service entrance panels and generator frame) together.
 
Re: 2 buildings, one utility transformer, 2 meters, common g

sailboat,
Should I disconnect the main bonding jumper (between the grounded conductor and the grounding electrode) in ONE of the service entrance disconnects? (i.e. - isolated the neutral in one panel)
Click to expand...
That would be a violation of 250.24(B)(5). I think that you need transfer switches that switch the grounded conductor for this application.
don
 
Re: 2 buildings, one utility transformer, 2 meters, common g

The Generator is not configured as a seperately derived source which I believe would be a requirement if the neutral were switched?

Would maybe it work to connect the grounded conductor to the grounding electrode system at the utility transformer?
 
Re: 2 buildings, one utility transformer, 2 meters, common g

Originally posted by sailboat:
The Generator is not configured as a seperately derived source which I believe would be a requirement if the neutral were switched?

Would maybe it work to connect the grounded conductor to the grounding electrode system at the utility transformer?
Click to expand...
The only way I can think of to remedy your problem is to bond the X0 of the generator to the frame or to bond the neutral to ground on the generator side of each transfer switch as permitted by 250.30(A)(1).

250.30 Grounding Separately Derived Alternating-Current Systems.
(A) Grounded Systems. A separately derived ac system that is grounded shall comply with 250.30(A)(1) through (6).
(1) Bonding Jumper. A bonding jumper in compliance with 250.28(A) through (D) that is sized for the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. Except as permitted by 250.24(A)(3), this connection shall be made at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices. The point of connection shall be the same as the grounding electrode conductor as required in 250.30(A)(2). (copyright 2002 National fire Protection Association)

Running an Equipment Grounding Conductor by a separate route to the X0 of the utility transformer would not comly with 300.3.

300.3 Conductors.
B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (4). (copyright 2002 National fire Protection Association)

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Tom Horne

[ May 19, 2004, 01:57 PM: Message edited by: hornetd ]
 
Re: 2 buildings, one utility transformer, 2 meters, common g

Tom,
Unless the transfer switch switches the neutral, the generator cannot be SDS and 250.30 would not apply. The only time that you can bond XO at the generator is when you are using the generator as SDS.
With two seperate services, I see no code compliant method other than setting the generator up as SDS and using a transfer switch that switches both the ungrounded and grounded conductor.
Don
 
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