malachi constant
Senior Member
- Location
- Minneapolis
Question. When we ask for existing utility data, the utility in our area (Xcel Energy) reports max KW on a monthly basis. For instance on one facility that opened in 2011 with a 1600A 208V service we have a max reading in July 2012 of 138KW.
I am sizing a generator to back up this service. As it is has only been in operation through one summer I am being very conservative and sizing the generator at 200kW. One engineering colleague of mine says that the 138kW is only the max AVERAGE taken over a fifteen minute span, and that the actual instantaneous loads on the system during that fifteen minutes are almost certainly larger, maybe sustained for 5-10 minutes a time. He recommends that I size the generator even larger than I have it, or at least tie the air conditioning controls to the transfer switch such that the cooling is kicked off whenever the generator comes on.
I had never considered that peak demand was not actually peak. As it relates to electrical equipment (switchboards, feeders, etc) I am not concerned - code clearly allows you to take 125% of max meter readings and use that as a starting point on existing installations. But what about generators? How much larger could the real peak be than the metered peak?
I know, I know, I should get out the generator sizing program and plug in all my motors and lights and equipment. The problem with that is you don't know what the diversity factors are - how often the lights are on, how often the motors start, how long they run, etc - and there are a lot of loads to consider. When available, the peak load is, in my opinion, very strongly preferred.
All that to say...any thoughts/experience as metered peak load relates to actual peak load? Is it something I can ignore and not lose any sleep?
Thanks!!
I am sizing a generator to back up this service. As it is has only been in operation through one summer I am being very conservative and sizing the generator at 200kW. One engineering colleague of mine says that the 138kW is only the max AVERAGE taken over a fifteen minute span, and that the actual instantaneous loads on the system during that fifteen minutes are almost certainly larger, maybe sustained for 5-10 minutes a time. He recommends that I size the generator even larger than I have it, or at least tie the air conditioning controls to the transfer switch such that the cooling is kicked off whenever the generator comes on.
I had never considered that peak demand was not actually peak. As it relates to electrical equipment (switchboards, feeders, etc) I am not concerned - code clearly allows you to take 125% of max meter readings and use that as a starting point on existing installations. But what about generators? How much larger could the real peak be than the metered peak?
I know, I know, I should get out the generator sizing program and plug in all my motors and lights and equipment. The problem with that is you don't know what the diversity factors are - how often the lights are on, how often the motors start, how long they run, etc - and there are a lot of loads to consider. When available, the peak load is, in my opinion, very strongly preferred.
All that to say...any thoughts/experience as metered peak load relates to actual peak load? Is it something I can ignore and not lose any sleep?
Thanks!!