Sizing equipment for normal and standby load are not equal

[anbm]

If the building has one standby generator/ATS to back up the whole building load (or main service panel load),
can the standby generator be sized less than normal service panel size? i.e. We had a project, we calculated total
total building load is around 400kW. We planned to size the standby gen. set at 400kW load as well as the building has only one
ATS.

The client said based on historical data from similar building they want 200kW generator instead, the ATS is still sized
to support 400kW load, will this 200kW generator be code compliant? Thanks.

 

[Rock86]

If the building has one standby generator/ATS to back up the whole building load (or main service panel load),
can the standby generator be sized less than normal service panel size? i.e. We had a project, we calculated total
total building load is around 400kW. We planned to size the standby gen. set at 400kW load as well as the building has only one
ATS.

The client said based on historical data from similar building they want 200kW generator instead, the ATS is still sized
to support 400kW load, will this 200kW generator be code compliant? Thanks.

Has the client performed load calculations in accordance with required Codes? Is the client qualified to make the decision? If you can get out of it? Could you have them sign off that you have provided your recommendations based off of calculations performed by a qualified person and the client has chosen to go with an alternative option, making you exempt from any future issues?

 

[anbm]

Has the client performed load calculations in accordance with required Codes? Is the client qualified to make the decision? If you can get out of it? Could you have them sign off that you have provided your recommendations based off of calculations performed by a qualified person and the client has chosen to go with an alternative option, making you exempt from any future issues?

No, they are based off peak utility bills of similar building. No calculation but they sent an email and confirmed they want a smaller generator size.

 

[gadfly56]

No, they are based off peak utility bills of similar building. No calculation but they sent an email and confirmed they want a smaller generator size.

You might explain to the customer that the peak billing power might be fine for sizing the service to the building based on similar experience elsewhere, but utility power is very "stiff" and can handle the inrush loads without too much trouble. If a lot of the connected loads are motors, they may have a real problem if several of them cycle on and off near the same time. Even a generator sized to the capacity of the panel might not keep them running smoothly.

 

[anbm]

My actual question is: will this design comply with code? i.e. calculated load and generator size are not same even we knew the actual load may work. Will this be flagged by plan reviewer, building inspector even this is only a standby generator?

 

[don_resqcapt19]

See 702.4(B)(2). If you are using an automatic transfer switch, then the generator must be able to supply the full calculated load or have auto load shedding. 220.87 would permit actual load data to be used in lieu of the Article 220 calculation, but I would not accept load data from a "similar" site as being compliant with the requirements of 220.87.

 

[anbm]

See 702.4(B)(2). If you are using an automatic transfer switch, then the generator must be able to supply the full calculated load or have auto load shedding. 220.87 would permit actual load data to be used in lieu of the Article 220 calculation, but I would not accept load data from a "similar" site as being compliant with the requirements of 220.87.

Thank you!