Generator and utility voltages

[MikeOlson]

I have a new building with the incoming voltage from the utility at 208V. This is a very small new building and will be part of an existing building complex with other buildings fed from 480V. The complex is also backed up from a 480Y/277V standby/emergency power generator.

For emergency/standby power for this new small building, since the utility and generator voltages do not match, am I allowed to provide a transformer ahead of the building ATS' so I can derive 208V from the generator and provide 208V from the utility? Or do I have to tell the utility to provide a 480V transformer for the building to match the generator?

Looking in the NEC, I do not see anything that specifically tells me that the generator voltage must match the utility voltage. However, Article 705.14 Output Characteristics states, "The output of a generator or other power production source operating in parallel with an electric supply system shall be compatible with the voltage, wave shape, and frequency of the system to which it is connected." Also, a FPN states, "The term compatible does not necessarily mean matching the primary source wave shape."

Do I make the utility and generator voltages COMPATIBLE by utilitizing the transformer on the primary side of the ATS?

Thanks,
Mike

 

[jtester]

Mike

I don't see a problem with a transformer stepping down to the service voltage. I would caution you to verify if this is an emergency generator vs an optional standby generator, and then approach the installation from that perspective.

Jim T

 

[MikeOlson]

The generator will be used for both emergency and standby. This 208V facility would receive at the minimum one transfer switch for the emergency branch, and perhaps a second for the standby power branch. So I could have 2 transformers and 2 ATS'.

 

[jim dungar]

The compatibility issue is only when the sources are being paralleled (i.e. co-generation).

Also, if the voltages can be matched through the use of a simple transformer then I would consider them to be compatible. This concept is what allows a 480V generator to be used at a facility that purchases medium voltage power as is done in most large industrial plants and hospitals.