separately derived system

[mpd]

came across a job today that has a transformer
feeding a disconnect that is feeding multi meter packs for a commercial building, between the transformer and 800 amp disconnect they ran grounded conductor & equipment ground, bonded together at both ends,and the electrode is connected at the transformer, the 800 amp disconnect is rated service equipment only and is factory bonded to the enclosure, in this situation would the electrode have to be at the disconnect and the bonding jumper be removed at the transformer and remove the equipment ground on the secondary side, because I do not think you can isolate the neutral between the disconnect & meter packs, to prevent parallel paths, these meters are not part of the main service to the building

 

[George Stolz]

Re: separately derived system

In this case, the neutral and the grounding conductors are essentially two grounded conductors run in parallel. So long as they adhere to 310.4, are the same size, same length and are terminated in the same manner, there's no problem. Otherwise, that grounding conductor is going to cook.

Why did they go to all the trouble of running the extra conductor?

Edit: 250.24(C) requires the neutral to be bonded to the service enclosure, you cannot separate the bond at the service.

[ May 07, 2005, 05:25 PM: Message edited by: georgestolz ]

 

[mpd]

Re: separately derived system

these meter packs are not part of the service equipment, they are located thru out the building
for metering tenant space. and the transformers
are fed from the main switchgear in the building.

 

[iwire]

Re: separately derived system

If I understand you correctly it sounds like you can make this code compliant by simply lifting the EGC at each end providing there are no other metallic paths between the transformer and disconnect.

You need to look at 250.30(A) and in particular this exception.

Exception No. 1: A bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded circuit conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

If there are other metallic paths the bonding has to be lifted at one or the other keeping in mind the GEC must land at the bonding point.

 

[mpd]

Re: separately derived system

iwire, there is EMT between the transformer and
secondary disconnect, I guess my question is if the grounded conductor is factory bonded at the disconnect & meter packs, the bonding jumper and
GEC would have to be done at the disconnect and
the bonding jumper removed at the transformer to
prevent parallel paths.

 

[iwire]

Re: separately derived system

Then you have to lose the extra bonds.

You have bonds at the meters, disconnect and transformer with an EGC between all of them.

As you stated this is all on the load side of the service disconnect you have a direct violation of 250.24(A)(5).

The only bond allowed due to the EMT will be one at either the transformer or the disconnect.

[ May 07, 2005, 09:23 PM: Message edited by: iwire ]