IS IT ALTERNATOR OR ENGINE USED FOR ELECTRICAL DISTRIBUTION DESIGN

[Ernie567]

Generator sizing programs provide engine and alternator sizes separately? I have three questions.

1). Where does the NEC or any other standard say we should use engine size (kW) for electrical distribution purposes?
2). Doesnt it make sense to use the alternator size the for the downstream distribution network since final output characteristics is from the alternator?
3). Does anyone know any white papers written about this?

 

[JoeStillman]

1. NEC 700.4 requires "adequate capacity and rating" for all loads. It's up to the designer to estimate the power factor of the loads and get the right size genset. Steady-state load is not necessarily the limiting factor. Fire pumps with high starting currents, for example, have strict voltage regulation requirements that sometimes drive the generator selection size to be larger than the steady-state load. Nearly all gensets have an engine and alternator paired with a P/S ratio of 0.8. That way, the engine will never have enough power to over-drive the alternator for loads with more than 0.8 power factor.
2. The alternator can't deliver power to more load than the engine can support.
3. Check out the Cummins Web Page for lots of helpful material.

 

[winnie]

The alternator limits the kVA which can be served, mostly because alternator heating depends upon current flow.

The alternator also limits the transient peaks (eg. motor starting) which can be supported.

The engine limits the true power which can be delivered.

Both impact your design.

-Jon