Service Entrance Rated ATS or not.

[montericci]

Hello, I have a general question about standby generators and ATS?s.
I specify generators and ATS?s for water and wastewater projects where they are considered as ?Legally Required? systems.
I typically provide enough power with a generator to completely operate the plant, lift station, etc.
All my projects are low voltage, three phase and but the voltages very: 208Y, 240 delta, and 480 (straight B phase) or 480/277V.
My questions are:
Do I always have to specify a SE rated ATS for these projects or are there criteria that would allow me to use a non-SE rated ATS?
What gets bonded and where?
Do I always have to switch the neutral?
The cost and size difference is significant and I?d like to save money and space if possible.

 

[augie47]

"service rated" ATS have a service disconnecting means.
If your facility has a "service" main OCP device ahead of the ATS then there is no reason to use a service rated ATS.
There are engineering reasons for switching the neutral with the ATS. I rarely see one with a switched neutral.
The bonding issue is a bit more complicated and varies as to a "switched neutral" or not. If the neutral is not switched then there is no bonding of the neutral at the generator and an equipment ground is required to ground the generator.

 

[Keri_WW]

Where your system is ground fault protected you may want to go with a 3-pole with overlapping neutral contacts or a 4-pole to help mitigate any issues that may arise. I don't fully understand the issue completely, but there is quite a bit of literature on the web in reference to GF problems and ATS's.

As for the SE rating, we typically design our generators to have integral circuit breakers and all of the service entrance connections to happen there as opposed to providing a SE ATS. That's just we've been doing for the past few years, whether that is industry standard or not, i dont know.

Keri

 

[masterinbama]

On new construction most of the engineers I work with,have the ATS built into the switchgear. Makes for a much neater installation.

 

[hillbilly1]

It depends a lot on whether its a new installation or retrofit, in a retrofit, if it is a large service that requires minimal downtime, a service rated switch may be the best option, new conduits are put in place to the existing transformer, and if a service lateral, conduits ready to intercept the old service. Most of this can be done with minimal downtime when switchover is ready to be completed. I've probably put in more four pole switches than three pole due to the larger generators and services we install. Generally we can swap over an existing 2000 amp 277/480 wye service with 8 hours down time on average.

 

[JoeStillman]

I only use switched neutrals when there are multiple ATS's and each one backs up a different grounded-wye transformer. In that situation, solid neutrals would allow "objectionable neutral currents" (see 250.6) to flow between the two transformers. The generator needs to be a separately-derived source when the neutrals are switched.

When you are only backing up one transformer or utility source, the generator shouldn't be separately-derived (no bonding jumper at neutral) and you use a solid neutral bus in the ATS.