# The Problem Using 220.87 to Size Generators and Other Equipment



## jar546 (Nov 11, 2018)

NFPA 70, aka NEC 220.87 allows you to use the maximum demand data for load sizing.  If you don't have a smart meter or access to this information from your POCO, you would have to do record for a 30 day period with data every 15 minutes and hopefully it was done during the highest usage month of the year.

All of this, rather than doing the calculations may not be the best way.  For one, I really don't know anyone other than a large company that would take the time to analyze their power for 30 days and two, usage does not take into consideration motor startup current which is definitely part of the requirement.

You may be drawing 30A when your AC is on and running but what about the 100A+ during startup that a generator has to handle for example.  So the question is, what do you accept for sizing loads for new install items such as a generator?


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## chris kennedy (Nov 11, 2018)

Either way you get your number you multiply by 1.25. That will cover motor start up. 

Take Williamsburg Landing for example. We have a Genny rated at 800A sized per 220.87 using the last years max demand as per POCO. Poco has a 150 kva tranny out side capable of delivering 400A continuous. 

Most of 220 is way off base IMO.


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## jar546 (Nov 11, 2018)

chris kennedy said:


> Either way you get your number you multiply by 1.25. That will cover motor start up.
> 
> Take Williamsburg Landing for example. We have a Genny rated at 800A sized per 220.87 using the last years max demand as per POCO. Poco has a 150 kva tranny out side capable of delivering 400A continuous.
> 
> Most of 220 is way off base IMO.



I think this is more of a problem for residential houses with AC where they think they only need a 12kw generator but actually need a 20,...as an example.


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