220.87 Peak Demand - hourly vs 15 minutes

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Ricko1980

Ricko1980

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San Francisco
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Electrical Contractor
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.
 
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.
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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.
 
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.)
 
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.
 
electrofelon said:
Im not sure about that Don. Demand IS the average power in an interval, not peak power in that interval.
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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.
 
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.
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don_resqcapt19 said:
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.
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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
 
wwhitney said:
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
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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.
 
electrofelon said:
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.
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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
 
wwhitney said:
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
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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.
 
electrofelon said:
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.
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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
 
wwhitney said:
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
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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).
 
Just thought I'd follow up on this, as I've gotten more info. Some AHJ apparently allow the 1-hour data in California (although this seems crazy to me, since it would probably be way lower!). PG&E data in San Francisco is sometimes reported as 1-hour and sometimes as 15-minute data, depending on the customer, but ALL of the 15-minute data sets that I've looked at from PG&E just evenly split the 1-hour usage across equal 15-minute intervals, which means it's actually an hourly average demand and not useable per 220.87.

If someone had access to a large data set of 15-minute data, it occurred to me that it would be possible to calculate a conversion factor (probably best to do by use type - SFR, multifamily residential, small commercial, etc). The 15-minute data could be used to make hourly averages, and then you'd just have to compare the two to find out how much lower the hourly averages are by contrast to the 15-minute averages.

The conversion factor would be even more accurate if the data could be filtered into sub-groups (such as with elec. heating/cooling vs with gas heating). I'd love to see this conversion factor, if anyone out there has access to such data! I know it wouldn't be code-useable, but it'd be a great fast way of understanding if you're in the right range and to know if it's worth taking 15-minute readings for a month!
 
I give that information freely almost once a week.
Our meters actually scroll "PD", so anyone can see it. We reset all of them in spring and fall to capture PD (peak demand) in summer and winter for transformer overloading.
 
Hv&Lv said:
I give that information freely almost once a week.
Our meters actually scroll "PD", so anyone can see it. We reset all of them in spring and fall to capture PD (peak demand) in summer and winter for transformer overloading.
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Do you have residential data? How do I sign up to get on your good list to get some of it for personal research use?
 
Ricko1980 said:
Do you have residential data? How do I sign up to get on your good list to get some of it for personal research use?
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I was talking about the residential meters...
Its easy now for the meters to do this. All that needs to be done is to add the peak demand data (program) to the scroll on most models.
 
Hv&Lv said:
Our meters actually scroll "PD", so anyone can see it. We reset all of them in spring and fall to capture PD (peak demand) in summer and winter for transformer overloading.
Click to expand...
And PD is the maximum observed average power over what time interval?

Cheers, Wayne
 
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