Generator sizing

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hhsting

Senior Member
Location
Glen bunie, md, us
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Junior plan reviewer
I have switchboard replacement in outpatient hospital like setting that has essential branch. In the time existing switchboard disconnected and new switchboard install they are powering the hospital thru temporary generator.

However they need to determine existing loads so they can size the temporary generator accurately. Can they use NEC 2017 Article 220.87 for sizing the temporary generator in my setting
 
Many commercial occupancies will have a demand meter. Check to see if that supplying utility has that information.
 
hhsting said:
Yes I meant that they say they have utility bills data that shows only kW not kVA for one year. So still in that condition one year period of utility data of kw be used?
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You need the 15 minute demand data....if may not be on the bill, and you may have to ask for it.
You can also use the 15 minute kWH values...take the largest for the past year, and multiply by 4 to get a 15 minute demand number. Again that will not be on the bill, however you may be able to log in and download that data.
 
don_resqcapt19 said:
You need the 15 minute demand data....if may not be on the bill, and you may have to ask for it.
You can also use the 15 minute kWH values...take the largest for the past year, and multiply by 4 to get a 15 minute demand number. Again that will not be on the bill, however you may be able to log in and download that data.
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I dont follow utility bill has monthly kwh for 12 months. Can I select the largest one and use that?


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I'm certain you can get kWh and kW data from the POCO. Smart meter billing can be looking at 256 samples per second in some cases. They catch everything. The bill will show maximum kW demand. They get hit with that. kWh really does not matter. Energy kWh and Demand kW are totally different.
 
hhsting said:
I dont follow utility bill has monthly kwh for 12 months. Can I select the largest one and use that?


Sent from my iPhone using Tapatalk
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No...there is no way to get a peak demand from that and you need the peak demand to size a generator.
 
In my area the max kW is on every bill. Under "actual demand", as well as kVAR, kVA etc. If it's not on the bill, I can assure they have the data. How could they bill demand charges if they didn't ?
 
herding_cats said:
In my area the max kW is on every bill. Under "actual demand", as well as kVAR, kVA etc. If it's not on the bill, I can assure they have the data. How could they bill demand charges if they didn't ?
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I thought so too, but I recently did a FOIA request for one of our local schools because I thought they wasted money on replacing an existing single phase 600 amp 120/240 volt service with a new 1200 amp 3 phase 208Y/120 volt service.
The transformer that fed the original service was a100kVA and the new service is supplied by three 25 kVA transformers connected in a wye secondary. There is no reason that they needed the new service, but I wanted utility documents to talk with the school board.
 
don_resqcapt19 said:
I thought so too, but I recently did a FOIA request for one of our local schools because I thought they wasted money on replacing an existing single phase 600 amp 120/240 volt service with a new 1200 amp 3 phase 208Y/120 volt service.
The transformer that fed the original service was a100kVA and the new service is supplied by three 25 kVA transformers connected in a wye secondary. There is no reason that they needed the new service, but I wanted utility documents to talk with the school board.
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They just said they dont have meters on service switchboard. They can use NEC 2017 Article 220.87(1) exception minimum 30 days metering to size temp standalone generator. However I am not sure if temporary standalone generator that would feed the service switchboard can be sized using minimum 30 days metering or not under my situation?
 
Reading the entire thread again-
I don't think 220.87 will suffice because it requires at least 30 days of data, which doesn't seem to be available. However, you're trying to size the supply, not to add more loads - you can always add the main breakers of all the connected panels and have a very high but usable figure to size a generator (three 3-ph 208/120v panels w/ 200 amp mains, that's 600 amps per phase and just over a 200kva generator). Anything after that would get into an actual load survey or putting a data logger on the feeder for 30 days and analyzing that...are those worth the trouble?

Is the generator going to be the main or backup supply? Generally generators are de-rated maybe 20% when used as the primary supply.
 
zbang said:
Reading the entire thread again-
I don't think 220.87 will suffice because it requires at least 30 days of data, which doesn't seem to be available. However, you're trying to size the supply, not to add more loads - you can always add the main breakers of all the connected panels and have a very high but usable figure to size a generator (three 3-ph 208/120v panels w/ 200 amp mains, that's 600 amps per phase and just over a 200kva generator). Anything after that would get into an actual load survey or putting a data logger on the feeder for 30 days and analyzing that...are those worth the trouble?

Is the generator going to be the main or backup supply? Generally generators are de-rated maybe 20% when used as the primary supply.
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They said they will put meter for 30 days to be in compliant with NEC 2017 Article 220.87(1) exception for measurement. They don’t have 1 years worth of data. The generator would be standalone inly power source until new service switchboard arrives for replacement. I am not sure if minimum 30 days of metering data in compliance with NEC 2017 Article 220.87(1) exception be used or not for sizing the temp generator supply service switchboard in my situation?
 
hhsting said:
The generator would be standalone inly power source until new service switchboard arrives for replacement.
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It sounds like it would be manually started. In that case they can be permitted to manually manage their own loads manually since they already are aware they are on generator. 702.4(A)(1)
 
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