Generator Calculations and sizing

[wfr]

Could you explain the proper way to size a generator for a new installation and how to size one for an existing installation?

 

[George Stolz]

Assuming this is an optional standby system (a typical system that's not required, but is simply installed at the desire of the owner) we look at 702.5:

702.5 Capacity and Rating. An optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. Optional standby system equipment shall be suitable for the maximum available fault current at its terminals. The user of the optional standby system shall be permitted to select the load connected to the system.

In essence, it is simply the load that the designer wishes to supply with it. The last sentence of the section allows the manual selection by the user, relying upon them to shut off unwanted circuits before using the system.

This will change in the 2008 slightly, in that automatic transfer switches will either need to shed the unwanted load automatically, or carry the calculated load of the whole.

 

[ghostbuster]

You may want to check out this link from Cummins
http://www.cumminspower.com/www/literature/technicalpapers/F-XXXX-SizingGensets.pdf