Pump Station

[cds9044]

Bidding on existing pump station project that requires changing out overhead service with new underground service and changing out existing switchboard. The loads are 3(Ea) 335 HP pumps(249.91KVA each), 2(Ea) panelboards(186.1KVA) for total of 935.83KVA or 749KW.
The problem is that there is an existing 650KVA(520KW) standby generator with ATS feeding entire load. Not sure that all pumps will ever run at same time or not. What is proper way to calculate load? I'm thinking NEC 08, 702.5(2)(a) will dictate resizing existing generator. Any thoughts? I also posted this topic in the electrical calculations/engineering forum.

 

[tom baker]

Largest motor at 125%, all others at 100%, then add up the other loads per standard load calcs.
If the load is bigger than the generator can supply, then the NEC allows some type of interlock circuit so only two pumps can run at a time to not overload generator.

If all pumps need to run at the same time is determined by someone else. If they do and the total load exceeds your generator, ...watch the power factor, then the 08 NEC requires the generator to be able to carry the load.

Are you sure the panelboards are 186 KVA each?

 

[cds9044]

Are you sure the panelboards are 186 KVA each?[/QUOTE]

Total of panelboards is 186KVA. Thanks for your input.

 

[yeemartin]

Shouldn't the starting current of the motor also be considered when sizing generator? OP didn't mention the motor starting detail though.

Largest motor at 125%, all others at 100%, then add up the other loads per standard load calcs.
If the load is bigger than the generator can supply, then the NEC allows some type of interlock circuit so only two pumps can run at a time to not overload generator.

If all pumps need to run at the same time is determined by someone else. If they do and the total load exceeds your generator, ...watch the power factor, then the 08 NEC requires the generator to be able to carry the load.

Are you sure the panelboards are 186 KVA each?

 

[jbt260]

How the motors start will play a big roll in sizing the generator (across the line, soft start, etc). Also the start sequence of the motors will affect the sizing of the generator. With the current generator and listed loads, you would probably only be able to run one motor at a time.