Generator Sizing

[jojo]

Hello guys. I just want to know if there's a standard method of sizing a generator:
a) For non-motor loads, is it okay if the generator size is calculated based on the demand load? My understanding is we size it based on the demand load times 1.25 (the 80% maximum loading of generator). Please advise.
b) For combination of motor and non-motor loads, do we size it like the way we sized for the main breakers? (max FLA motor *2.5) + FLA of the remaining motors plus non-motor loads? Or do we size the highest motor load on the starting kVA then the remaining motors using its FLA plus the demand load of the non-motor loads? Please help. Thank you.

 

[MSU]

Best to ask the GenSet manufacturer about their motor starting capabilities. They change somewhat from manufacturer to manufacturer.

If Diesel, you must worry about duration of light load conditions. Some diesels tend to 'slobber' or 'wet stack' when run for long periods at light load relative to nameplate. Excessive unburned fuel goes through to the exhaust. Over time, this liquid can degrade turbo efficiency and therefore degrade the engines ability to respond to step loading.

As rule of thumb, I try to load between 30% and 80%, while maintaining reserve capacity to start the largest motor.

 

[Shoe]

Proper generator sizing is very load-dependent. The generator is not a stiff source like the utility, so this needs to be taken into account based on the criteria of your load. Large motors may increase the starting requirements and require thought into specifying the properly-sized alternator. UPS loads introduce harmonics that can overheat generator windings if not properly sized. Medical imaging or telecommunications loads may have strict voltage or frequency regulation criteria that will need to be taken into account.

However, if you're just serving basic loads that are fairly linear or small non-linear loads, then the exercise is much more basic.

 

[jojo]

Thank you MSU and Shoe.. I checked from a generator manufacturer's site and I learned that there are many things to consider in generator sizing. There are other things to consider like the sequence in starting.. I thought we have only to size it based on the demand load and motor starting kVA.