Generator sizing 3p 480/277 or 208/120

R

Rdel6937

Member
Location
New Jersey
Occupation
Electrician
Evening all,
Need some help with a generator sizing.
Incoming service is 600A 3P 480/277 4wire.
That feeds a sub panel with a main breaker at 150 3P480/277v 4 wire.
Off that panel is a 30KVA 3phase step down transformer 208/120Y secondary which feeds a 100A 3phase 208/120 panel. That transformer if I did my math correctly outputs 83.3 Amps.
Each leg of that panel is 6A, 17A, 31A.

They want a generator to handle (8) 120V circuits from various panels which feed CCTV, door swipe etc systems. I do not know the actual draw of each of these circuits but likely not much.

Question
What size generator and ATS can be supported from that 120/208v 3phase panel. They wanted a 20KW generator which I think would require 100A and that would be to big.

2nd question
Option B is coming off that 150A 277/480 panel but I think then I need another 30KVA 208/120 transformer or larger to support the 20KW generator and (8) 120V loads which will be placed in a separate panel connected to the ATS.

I'm stumped on how best to achieve the end goal of having these (8) 120A loads protected with the generator.
The 20KW request was room for growth but also based by the customer on a 120/240v single phase system.
He wasn't aware is a 3 phase service.

Any assistance would be greatly appreciated
 
Rdel6937 said:
Question
What size generator and ATS can be supported from that 120/208v 3phase panel. They wanted a 20KW generator which I think would require 100A and that would be to big.
Click to expand...
You don't "support" a generator; it's a source, not a load.

Not a problem for the generator to be "too big" other than the cost.

Rdel6937 said:
2nd question
Option B is coming off that 150A 277/480 panel but I think then I need another 30KVA 208/120 transformer or larger to support the 20KW generator and (8) 120V loads which will be placed in a separate panel connected to the ATS.0v gear c
Click to expand...
Again, wrong thinking. Pus, 480v gear is more expensive.

Have you ever installed generators and ATSs before?

Rdel6937 said:
I'm stumped on how best to achieve the end goal of having these (8) 120A loads protected with the generator.
The 20KW request was room for growth but also based by the customer on a 120/240v single phase system.
He wasn't aware is a 3 phase service.
Click to expand...
3ph transformer - 3ph ATS - 3ph panel.

Added: I don't know whether a 1ph 120/240v ATS will wprk on 120/208v.
 
Last edited:
I've installed residential generators...
I'm just not sure what is required for 3ph equipment.... to support the end state loads.
But thanks for your comments
 
If all they need to backup is 8–120v circuits, a single phase critical circuit panel would probably be the easiest solution.

Is this is 700 or 701 system, or do they have flexibility on the unit?
 
Nothing has been purchased yet...
I'm just having trouble with the layout and engineering.
Need to feed the ATS...I only have 3p power...
Is either out of the 480/277 panel or ott of the 208/120 panel.
That 208/120 panel has a 3P main breaker at 100A.
 
If you use a 10 circuit critical circuit panel, you run the individual branch circuits to it.

Single phase generator is all you need. If it’s not a 700 or 701 system, you could probably use a smaller air cooled, like a Kohler or Generac 14 kW.

First thing you’ll have to do is get the actual amp draw on the circuits they want backed up.
 
If you want to backup the entire 208/120 panel, you’d use a 208/120 generator and ATS

If you want to back up the entire facility, either 480/277 or 208/120 genny and ats could be used, but it would require some ground work to determine which.

If you want to just back up individual 120V branch circuits, a single phase 120/240 gen and ATS would work.
 
I came here to say this. If the system is 120/208 then use a 120/208 generator and transfer switch.

I have seen way too many 208 systems backed up by a 240 single phase generator and it just leads to all sorts of weirdness.

People just seem afraid to special order the generator and ATS they actually need, it is not that hard.
 
The issue I'm having is the feeder to the ATS.
If I come out of the 120/208 3p panel, what size feeder needs to be used to the ATS from that panel. If the MBR in that 120/208 panel is 100A... the transformer feeding it is 30KVA which is 83.3 A... and the highest current load on that existing 120/208 panel is 31A... what is the max size i can use to the ATS. That will determine the gen size. I think.
Having trouble with what size ATS generator and feeder to the ATS.
 
Seven-Delta-FortyOne said:
Brain fart, possibly caused by Mezcal intake. Forgot he wants to go automatic.

In that case, I would just back up the entire 208/120 panel.
Click to expand...
I'm installing a separate panel for these 8 120v circuits as they currently reside in 4 different panels throughout the building.
I'll bring them all back to the equipment room, into their own panel, and connect that to the ATS.
Now I need to feed the ATS and connect the generator to the ATS.
Trying to figure What is the max size feeder, ATS and generator that can be used.

The ATS is just a switch and it has to be sized based on the max size I can take out of that 100A 120/208 panel. So if I use a 20KvA 120/208 generator, that's I belive 100A. I need to feed the ATS with the same. I think I need to use a smaller generator and ATS.
I likely am only protecting 20-30 load amps.
 
Rdel6937 said:
The issue I'm having is the feeder to the ATS.
If I come out of the 120/208 3p panel, what size feeder needs to be used to the ATS from that panel. If the MBR in that 120/208 panel is 100A... the transformer feeding it is 30KVA which is 83.3 A... and the highest current load on that existing 120/208 panel is 31A... what is the max size i can use to the ATS. That will determine the gen size. I think.
Having trouble with what size ATS generator and feeder to the ATS.
Click to expand...
Generally you would use a 100A ATS if the normal feeder is a 100 amp circuit. You may not have actual 100 amps of load on that feeder and certainly can have a generator sized for the actual load, or even a load that has been reduced by load shedding methods. For no more than what you say is of interest to be powered here you possibly could have a generator with output capability of say only 30 amps to back up this panel though you possibly would need to use load shedding methods to assure you don't end up drawing too much and end up tripping the generator feeder device.

The other thing to do if only the items you mentioned are all that is necessary to back up is install a sub panel that only supplies those items and put the ATS in the feeder to that panel. For no more than what those items appear to be, it too could likely only be a 30 amp feeder circuit or even less.
 
Within reason the generator could not be "too big". It si not going to force more current than the load requires.
You mentioned a 20kw and 100 amps. Note on a 3 phase system that would be closer to 55 amps per phase,
For simplicity sake I would install a 100 amp ATS on the existing 100 amp panel and use the 20kw generator the customer desires.
If you elect to go smaller changes are they will decide to add more load than currently anticipated and , with the 20kw, you will be prepared.
 
electrofelon said:
But he has a three phase system. Is there a transfer switch that will be ok with single phase 120/208 and a 120/240 gen?
Click to expand...
Depends on how the voltage monitoring is set up. since it is a single phase unit it won't have a "phase monitor" so to speak and will only be looking at single phase voltage. If it is looking for 120 to neutral on each line it won't know or care if line to line is 208 vs 240. If it is only looking line to line for voltage, then it may depend on what minimum trigger level is and whether it can be adjusted. In my neck of the woods often 208 nominal will tend to run 212 to 215 volts. That is within 10% of 230 volts but pushing it if you are looking to be within 10% of 240.
 
augie47 said:
Within reason the generator could not be "too big". It si not going to force more current than the load requires.
You mentioned a 20kw and 100 amps. Note on a 3 phase system that would be closer to 55 amps per phase,
For simplicity sake I would install a 100 amp ATS on the existing 100 amp panel and use the 20kw generator the customer desires.
If you elect to go smaller changes are they will decide to add more load than currently anticipated and , with the 20kw, you will be prepared.
Click to expand...
So then it is OK to install a 100A MB in the existing 208/120V panel and feed the ATS.
Use A 20KW 208/120 3p generator, install a 208/120v 3ph panel off the ATS and put the 8 120v loads in that panel.

I was concerned that taking 100A to the ATS would be too much. It would be equal to the MBR of the existing 100A 3P 120/208v panel.
 
You must log in or register to reply here.
Top