SWITCHED ATS PER NEC HANDBOOK EXHIBIT 250.13

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Thomas Underwood

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Is it permissible to switch from a 4 wire Genset (including neutral) through the switched neutral ATS to a 3 wire delta fed transformer?
2014 NEC Handbook Exhibit 250.13, pg. 179 indicates in the picture that the transformer is 4 wire feeding a neutral from the other side of the ATS. Nowhere does this exhibit or text in these two pages indicate in writing that the transformer must feed a neutral using a wye from the other side of the ATS. It is only the picture in the exhibit that indicates this.
 
Switced Neutral ATS

Switced Neutral ATS

I seen where you are confused. I should have said Delta secondary on the transformer so you would have know it is the secondary of the transformer I am concerned about for this service entrance. The fact that the transformer is delta-delta should indicate self derived I think. Mike Holt wrote an article that said the electrical conductors of a transformer must make contact between the primary and transformer of a transformer, or the transformer is considered self derived. Only the neutral could make connection between a primary and secondary on a transformer and this delta-delta cant do this, so its self-derived as I interpret Mike.

I mention this self-derived situation because maybe the issue of a self derived transformer service entrance has something to do with this situation?
 
I'm hoping the utility delta-delta transformer is grounded in some way.

If the utility source will not have a neutral, then bond the N-G at the generator and do not bring a neutral from the gen to the ATS. The neutral lugs at the ATS will not have any wires connected.

Double check with the ATS folks that a 4W ATS can be programmed to not look for a neutral voltage.
 
Sorry, but I am greatly confused.
Thomas Underwood said:
2014 NEC Handbook Exhibit 250.13, pg. 179 indicates in the picture that the transformer is 4 wire feeding a neutral from the other side of the ATS.
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I have that picture open on my desk. There are no transformers in that picture.
Thomas Underwood said:
I mention this self-derived situation because maybe the issue of a self derived transformer service entrance has something to do with this situation?
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The phrase "self-derived" does not appear in the NEC. I suspect you are talking about "separately derived," but I still don't understand what you are asking.



 
Switched Neutral ATS

Switched Neutral ATS

Sorry, I meant to say separately derived.

We have an ATS connected to some motor loads (for a sewage lift station). Our generator is wye and we must send a neutral to the ATS to meet federal government regulations. The transformer feeds the ATS a delta because only motor loads exist at this site.

Can we connect a neutral from the Genset to the ATS and only connect 3 phase wires (non-grounded conductors, i.e. phases A,B,C) to that ATS from the delta-delta transformer.

Our motors at the sewage lift station do not need a neutral. It is well known thought that the neutral is used to send fault currents back to the source which is a safety issue that was recently added to the NEC to prevent electrocution.

The neutrals are isolated because the ATS is switched from the Genset to the Transformer. The load side of the ATS is the sewage lift station pumps (motors).

A previous moderator indicated the ATS electronics may get confused with a neutral connected from the genset and no neutral connected from the delta transformer.

If we don't connect the netural to the motors, the ATS should not even know it is there.

We have to follow the UFCs, Unified Criteria Codes issued by the federal government that say we need to install a separately derived genset and a 4 pole ATS. In order to have a separately derived Genset, the ATS must be a switched neutral, 4 pole ATS.
 
Thomas Underwood said:
Sorry, I meant to say separately derived.

We have an ATS connected to some motor loads (for a sewage lift station). Our generator is wye and we must send a neutral to the ATS to meet federal government regulations. The transformer feeds the ATS a delta because only motor loads exist at this site.

Can we connect a neutral from the Genset to the ATS and only connect 3 phase wires (non-grounded conductors, i.e. phases A,B,C) to that ATS from the delta-delta transformer?....
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Yes. That is no different than bringing the neutral from the power company to your service disconnect or ATS and then on to the transformer.
 
Thomas Underwood said:
We have an ATS connected to some motor loads (for a sewage lift station). Our generator is wye and we must send a neutral to the ATS to meet federal government regulations. The transformer feeds the ATS a delta because only motor loads exist at this site.
Click to expand...

Don't run a neutral from the generator to the ATS. You will have a separately derived system with a 3 pole ATS.

Why not use a Delta generator?
 
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david luchini said:
You have a Delta service and a Wye generator and they are the same voltage?
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I wonder how the Delta service is referenced to ground. Is it truly ungrounded (what kind of monitoring is being used) or is it solidly grounded (like a Grounded B-phase). What about any loads fed by the ATS are they intentionally grounded?
 
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