Backup Generator for a backup generator

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spsnyder

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I have a project where a building has an emergency generator (2M). We are making provisions for a portable generator (1M) to be parked adjacent to the building as a backup to the existing generator. The existing configuration is the generator feeds a distribution panel with two breakers. One 3000 amp feds switchgear. One 200 amp feeds a fire pump. The fire pump is locked out as the street pressure and supply has been determined to be more than adequite for the 3 story building. The switchgear feeds several 4 pole ATSs. ie. the generator is a SDS with the generator N-G bonded. Here's my questions (at long last...)

In sizing the backup to the backup, I used historical demand data and 220.87 for additional future loads.

"220.87 Determining Existing Loads
The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under all of the following conditions:
(1) The maximum demand data is available for a 1-year period."

1. Is this acceptable and of sound practice?

2. Am I correct in using a 3 pole ATS (not 4) prior to the emergency switchgear? The return path for a ground fault would be back to the existing generator N-G bond, through the neutral of the new ATS to the new generator.

3. Is it acceptable to connect the new ATS on the load side of the existing generator 3000 amp breaker? Given the fire pump is not used/needed, and nothing requires a backup to the backup for a fire pump.

Thank you for your time and help.
 
benaround said:
spsnyder,
Do you see a problem with regrounding the neutral between the generators ?
Click to expand...

It appears that the "backup" generator would not have a N-G bond.
This is no different than a typical 3P install (existing gen is the "utility")

If the thought is that the backup gen would only be brought in if the first gen smoked, go with a manual transfer switch. (Many ways to do this depending on what level of backup you are looking for....Cadillac or Pinto?)

If the thought is that the backup gen would be brought in if the first gen was to undergo maintenance, or taken offline for whatever reason, and the second gen would be brought in to be "ready and waiting", then the ATS is justified.... (Once again, Many ways to do this depending on what level of backup you are looking for....)
 
Thank you both. Generator No. 2 will be fire up if Generator No. 1 failed for any reason. The Generator will be parked at the site, but can be taken down the road if another mission critical site required a generator.

The ATS will be a 3 pole. Generator 2 will not have a n-g connection. This treats the Gen no. 1 and Normal power each as SDSs. Does this sound correct?

Thanks again.
 
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