Stand-By kW vs Utility transfomer kVA

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Sparky2791

Sparky2791

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I am having brain fog today. Must be the weather around here today.
If a building by calculation of the loads requires a 3000kVA transfomer for the service would it be a 3000kVA generator to back up the building properly?
By standards .8pf is used for stand-by gen sets which makes that 3000kVA generator 2400kW.

Math tells me
3000kVA = 3605A
2400kW = 2884A

In stand-by gen sets should kW size equal the utility transformer kVA.
I am missing something simple here so I hate asking but I can not wrap my head around this today. Likely the 12hours I worked yesterday to get a project done for a client.

Thanks for looking
 
Don’t ever go by the utility transformer.
For that load I MIGHT go as high as 2MVA, probably going to go with 1500 and watch my metering…
We size by experience, and generally a 50-60% load factor.
A utility transformer will double for a time based on ambient temps.
A generator won’t do much more (if any) than the nameplate.
Size it by calculated loads and motor starting
 
Hv&Lv said:
Don’t ever go by the utility transformer.
For that load I MIGHT go as high as 2MVA, probably going to go with 1500 and watch my metering…
We size by experience, and generally a 50-60% load factor.
A utility transformer will double for a time based on ambient temps.
A generator won’t do much more (if any) than the nameplate.
Size it by calculated loads and motor starting
Click to expand...
The transfomer size indicated was not sized by the utility company because they do there own engineering to determine a transfomer size and typically have no issues loading them to 125% for brief periods.
We designed a building for a client where they have a 15kV service loop so the buildings utility transfomer size was based off of our calculations 3000kVA is what we established. Now the client wants to add a gen set to back up the building but wants approx costs first before engineering. I was wondering when I ask the vendor for cost estimates of the gen set do I tell him 3000kW gen set or 3000kVA gen set
 
I see now..
For one that big I would ask for 3000 kW. Stay with the .8 pf
The generator loading isn’t determined by the generator, but rather by the load. What is your calculated pf of the building?
The last thing you would want is for the power to go out with full loads and the lights pulse or dim as motors start and stop.
For a million dollars you don’t want to get it wrong…
 
Last edited:
LibertyEngineering said:
I am having brain fog today. Must be the weather around here today.
If a building by calculation of the loads requires a 3000kVA transfomer for the service would it be a 3000kVA generator to back up the building properly?
By standards .8pf is used for stand-by gen sets which makes that 3000kVA generator 2400kW.

Math tells me
3000kVA = 3605A
2400kW = 2884A

In stand-by gen sets should kW size equal the utility transformer kVA.
I am missing something simple here so I hate asking but I can not wrap my head around this today. Likely the 12hours I worked yesterday to get a project done for a client.

Thanks for looking
Click to expand...
What voltage is it? Just curious.
 
Hv&Lv said:
I see now..
For one that big I would ask for 3000 kW. Stay with the .8 pf
The generator loading isn’t determined by the generator, but rather by the load. What is your calculated pf of the building?
The last thing you would want is for the power to go out with full loads and the lights pulse or dim as motors start and stop.
For a million dollars you don’t want to get it wrong…
Click to expand...
.8pf was used for all motors. I agree use the 3000kW.
 
Sahib said:
Motor starting pf would be much lower and to be taken into account for sizing of alternator. For engine sizing, starting kw and not running kw to be taken into account.
Click to expand...
If this helps, starting is typically is about 0.25 pf and starting current is six times FLC. The motor in this case is 1750 HP.
 
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