220.87 Peak Demand - hourly vs 15 minutes

Ricko1980

Member
Location
San Francisco
Occupation
Electrical Contractor & Architectural Designer
I heard from someone that they use either 15-minute OR HOURLY demand from smart meters to do optional calc 220.87 to see actual demand. I know the 15-minute demand data for 1 year is good per 220.87, but, looking at the NEC, I can't find this 15-minute increment mentioned anywhere. Is it acceptable to use the 1-hour interval, which will obviously result in a lower peak demand? Can someone point me to a definition of how long the interval should be? I've got a multifamily building that's got some meters on 15-minute and some on 1-hour, and I'd love to just do the calc with the data I have.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Did you read the exception to 220.87(1)

Exception:
If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
A 15 minute interval is mentioned in the exception to 220.87 (1). I find it odd that the exception specifies the interval but the main section doesn't.

For a conservative approach, take the max from the 1 hour demand data and multiply by 4 to get a fifteen minute maximum equivalent.
 

Ricko1980

Member
Location
San Francisco
Occupation
Electrical Contractor & Architectural Designer
I saw the 220.87 exception, but since I'd heard from another local contractor that he uses 1 hour when necessary, I thought I'd post here... Is that "definition" in the exception what people are going on for the 15-minute interval being the required time length? The 4x multiplier would be a very, very conservative approach, I'd be willing to bet the actual multiplier for typical residential use would somewhere between 1.3 and 2... (Although, obviously, unless this is in the code somewhere, this can't just be made up.)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Be careful with the information you get from the smart meters. Many utilities such as mine provide 15 minute usage data, but do not provided 15 minute demand data.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Im not sure about that Don. Demand IS the average power in an interval, not peak power in that interval.
Not sure, my utility says they are not interchangeable numbers and that the usage number they provide is not permitted to be used 220.87 calculations.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Exception:
If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.

Not sure, my utility says they are not interchangeable numbers and that the usage number they provide is not permitted to be used 220.87 calculations.
The bolded part above is new as of the 2017 NEC. Perhaps earlier it was possible to read "maximum demand" as the instantaneous peak, but now it is clearly the 15 minute average power for the exception. Your utility's interpretation seems to be out of date, NEC-wise. They may have their own rules for matters under their jurisdiction.

Cheers, Wayne
 
The bolded part above is new as of the 2017 NEC. Perhaps earlier it was possible to read "maximum demand" as the instantaneous peak, but now it is clearly the 15 minute average power for the exception. Your utility's interpretation seems to be out of date, NEC-wise. They may have their own rules for matters under their jurisdiction.

Cheers, Wayne
It did occur to me that that new definition is poorly worded. The "reached and maintained" part does not seem to have any purpose, and in fact makes the whole thing nonsensical.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
It did occur to me that that new definition is poorly worded. The "reached and maintained" part does not seem to have any purpose, and in fact makes the whole thing nonsensical.
While I agree that "reached and maintained" is superfluous, I don't see it as affecting the meaning. The highest observed average is perforce "reached and maintained" as an average.

Cheers, Wayne
 
While I agree that "reached and maintained" is superfluous, I don't see it as affecting the meaning. The highest observed average is perforce "reached and maintained" as an average.

Cheers, Wayne
I would say the "reached" word doesn't affect things, but "maintained" certainly does. It just seems to me that opens the door for other ways to calculate it besides the average.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I would say the "reached" word doesn't affect things, but "maintained" certainly does. It just seems to me that opens the door for other ways to calculate it besides the average.
While I would say that an average is automatically "maintained" over the period over which the average was taken.

But I agree now that "reached and maintained" is at best unnecessary verbiage and potentially confusing.

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
While I would say that an average is automatically "maintained" over the period over which the average was taken.

But I agree now that "reached and maintained" is at best unnecessary verbiage and potentially confusing.

Cheers, Wayne
If the mathematician doesn't know what it means for an average to be 'reached and maintained' then I don't either. :sneaky:

One thing that occurs to me is that you could get a different result between the following:
a) the highest consumption among any intervals starting at 12:00am, 12:15am, 12:30am and so on...
b) The highest average consumption for a 15 minute interval that is calculated from smaller interval measurements that may start at any time on the clock. e.g. a period composed of 15 one minute intervals, and, say, you find the highest average consumption among any of the data was for the 15 one-minute intervals starting 12:21am to 12:35am.

I don't know if utility meters actually measure and report something like (b). The only meter data (utility or otherwise) that I've ever gotten in spreadsheet form more resembles (a).
 
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