Automatic Transfer Switch Question

A

alldayheadache

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Location
United States
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apprentice
Hello,
I wanted to ask the opinions of people who are more knowledgeable than me on this topic.
I am trying to set up some automatic transfer switches for critical loads that are currently residing within 2 subpanels. The critical load circuits aren't mapped out, so I don't know how many of these critical loads reside in subpanel 1 and how many reside in subpanel 2. Each sub is 100 amps, being fed by a 400 amp main. The load calculations for the critical loads ask for 100 amps of power, so i need a 100 amp ATS. Now, since each subpanel is already 100 amps, and already has several circuits, I can't use a 100 amp ATS. My plan was to install 2 50 amp ATS's, each being powered by 50 amp breakers, i.e. 50A Breaker Subpanel 1 goes to ATS 1 & 50A breaker subpanel 2 goes to ATS 2. I will attach a diagram to illustrate.
System is 3 phase 120/208
ATS is single phase and will be fed with 2 legs to make 208 (I've already researched the ATS, it registers 208 for the transfer back to utility)
Additional information. The ATS generator side will be a portable genny run on natural gas, approximately 21kw of power with natural gas. Genny has 2 50 amp outlets and I plan on using one outlet for each ATS.
Please dont ask why I can't just put the ATS on the 400A main and transfer everything to that, that is not an option.
Does my math add up or should I just become a painter?

https://imgur.com/a/Ynx34Yz
 
Way to many possible issues.

The first is the ATS must be fully rated for the normal source.
For example, if each panel is normally 3 phase and intended to draw 100A, your ATS needs to be able to provide it.
 
If your generator is 208 single phase, you are not feeding any 120 volt loads. It most likely is 120/240 single phase. The transferswitch is most likely 240 volt too, if it is automatic, the pick up and dropout voltages may need to be adjustable, the cheaper switches like Generac uses tend not to be as sensitive to the input voltages.
 
hillbilly1 said:
If your generator is 208 single phase, you are not feeding any 120 volt loads. It most likely is 120/240 single phase. The transferswitch is most likely 240 volt too, if it is automatic, the pick up and dropout voltages may need to be adjustable, the cheaper switches like Generac uses tend not to be as sensitive to the input voltages.
Click to expand...
Correct, the ats being used is adjustable for the pickup/drop out voltages
also correct, the generator is 120/240v single phase
also, all the critical loads are 120v
 
jim dungar said:
Way to many possible issues.

The first is the ATS must be fully rated for the normal source.
For example, if each panel is normally 3 phase and intended to draw 100A, your ATS needs to be able to provide it.
Click to expand...
Apologies, I wasn't clear. The ats will house the critical circuits, as it has 16 spaces for circuit breakers. Once utility power fails, the ats will only power the circuits that are on it.
 
alldayheadache said:
Apologies, I wasn't clear. The ats will house the critical circuits, as it has 16 spaces for circuit breakers. Once utility power fails, the ats will only power the circuits that are on it.
Click to expand...
Another issue is are there any 208 single phase loads in that panel? Such as HVAC for a computer room? Such loads must be rated for 208/240 volts. If all 120 volt loads, then no problem (assuming those panels are single phase)
 
hillbilly1 said:
Another issue is are there any 208 single phase loads in that panel? Such as HVAC for a computer room? Such loads must be rated for 208/240 volts. If all 120 volt loads, then no problem (assuming those panels are single phase)
Click to expand...
All the critical loads that will be powered by the ATS are 120volts. There are 208 single phase loads in the existing subpanels 1 and 2, but the ATS will not be providing generator power to them.
 
Little Bill said:
What's to prevent the two subpanels from backfeeding the 400A main and back to the grid?
Click to expand...
The ATS? Once utility comes back on, it will switch the power from the genny to utility. Again the ATS has the breakers for the critical loads inside, and will only power them with the generator when utility fails.

Just to note, the ATS being used is a Natures Generator 50A ATS.

https://imgur.com/a/nw1kGLg
 
Not familiar with that brand, don’t know how well it will play with 208 utility and 240 generator power. It may see the 240 as over voltage and not transfer, if as you say it’s set for 208.
 
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