Peak Demand NEC 220.87

[Daggerdale]

I am not that familiar with recording ammeters and their functionality but had a question as it pertains to NEC 220.87. When adding load to a service, I have always gotten the past 12 month demand from the utility company to comply with 220.87. If I can?t get that data and want to use the exception for a 30 day recording ammeter reading, which value do I use, the highest ?average? or the highest peak.

NEC 220.87 says??the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15 minute period)??. The meter readings I have from a testing company is a graph that shows the maximum, minimum and average values. The statement in parenthesis is what is confusing me.

 

[charlie b]

In any given 15 minute interval, the power drawn (in units of kw) varies up an down. There should be a single number assigned to that 15 minute period, and the number should represent the average power of that interval. In the language of calculus, it would be the area under the curve divided by the duration of the period. The testing company should have given you that number for each and every 15 minute interval over the entire 30 days of the test. That would give you a total of 2,880 numbers, representing the 2,880 values of "average power in a 15 minute interval" that take place in a 30 day period. The highest single number on this list is the one you use for calculating the available capacity for new load.

 

[Daggerdale]

Thank you Charlie. that helps some. However, the graph indicates Maximum, Minimum and average values every 15 minutes for the 30 days and where I am still confused is the terminology in NEC that says "average ..." Am i to use the 'average' value which is less than the maximum from the graph or the 'maximum' number?

 

[charlie b]

Without seeing the actual data, I cannot tell for certain. But if you were given an average value for the first fifteen minutes, and an average value for the next fifteen minutes, and an average value for each of the next 2878 sets of 15 minute intervals, then that is the set of numbers I would use. Look for the single largest number from that set, and that will be your "present load." Add 25% to that number, them subtract the results from the system capacity, and that tells you how much spare capacity is available for the addition of loads.