Seperately Derived Systems

[Proteus]

My question deals mainly with the NEC.

When you are stepping down from 480 volts to 120/208 volts
This is down stream from the building service equipment.
I was taught that you ground the neutral (XO) at the tranfomer connetions. The grounding electrode conductor is attached to building steel (generally) and then connected to the neutral (XO) lug in the transformer.

Then the secondaries of the transformer are feeding a new 120/208 volt panel. And in this panel the equipment grounds are seperated from the neutral conductors. Two different terminal bars. That is how I was taught and I belive this to be the correct method.

However, I recently transferred to a new project where some of the experienced personnel see things differently.
They say that the neutrals and the equip. grounds should be tied together. Their reasoning is that it is a "new service" hence neutrals and grounds go together at the first means of disconnect(service). However, I don't think it should be considered a service, I belive it is a seperatedly derived system.

I would like some clearification on this matter, with code references to help me understand. Thanks, Chris J. Henson















new service

 

[reese]

From your description I will assume that the separately derived system does in fact have to be grounded. Article 250-30(1) allows the bonding jumper to be installed "...at any point on the separately derived system from the source (secondary) to the first system disconnecting means or overcurrent devices. ..."

Exception No. 1 allows bonding at both the source and disconnecting means where doing so does not establish a parallel path for the circuit conductor. ...

To be honest I have no experience with the exception and can not picture the hardware unless it is some sort of packaged transformer and disconnect.

My personal preference is to bond the neutral of the secondary to the steel and local water piping at the transformer and leave the bonding jumper out of the panel. The opinion that counts the most is that of the local authority.

 

[busbar]

I believe that the core issue is satisfying 99NEC 250-2(d). “The fault current path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it, and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.”

So far as the Code is concerned, the bonding jumper in either location will satisfy these requirements.

 

[stevenal]

From Art 100
Separately Derived System. A premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Certainly it is not a service. The definition of separately derived is confusing to me, but the NEC handbook indicates it applies to this situation.

It would be unsafe and not to code if bonded at two locations. Is this what your colleagues were advocating?