220.87 Peak Demand - hourly vs 15 minutes

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Hv&Lv,
When you say it's easy for the meters to do this, you mean it's easy for the utilities to set this up, but not electrical contractors, right? I assume there's no way for me to set this up for residential customers on my end? Forgive this rather uninformed question, I haven't had to deal with meters before other than to have the utility come and install them after I've provided the meter boxes. I would be very, very happy to learn there's some way for me to set meters to give 15-minute demand data that doesn't involve my contacting the utility and going down a 1-year rabbithole.
My meter has a demand register, but that is not one that my utility has set up. The do have the 15 minute usage data set up and I can log into my account and download that information.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
It seems like the peak reading method will be more conservative than code, and I'm really hoping for something that doesn't add in extra demand beyond the code requirement. I do work on whole-house electrification, and the best case for speed, cost, etc, is if if the calcs can show that no service upgrade is required.

Meanwhile, I got my hands on a large chunk of PG&E data. Out of 2251 non-solar homes with more than 6 months of 15-minute interval data (00:00-00:15, 00:15-00:30...), this 15-minute average demand was larger than the hourly average demand by a factor of 1.30x, with a median of 1.26x and a standard deviation of 0.2x. This means that 95% of homes had 15-minute interval average demand that was less than 1.7 times the hourly average.

15 of the 2251 entries had 15-minute averages that were more than 2.0x the hourly average. The highest was 2.75x the hourly average, and *almost but not all* the ones with the highest factors were all very low-demand readings, so I suspect that we were looking at situations where adding a small load (AC or heat turning on) was able to bump the factor up above 2x quickly if the heat came on for only the last 15 minutes of the hour, for example.

This may not be a way to calc to code, but I'm going to find it super useful for understanding if service upgrades are actually necessary, since all PG&E clients have hourly data at the very least. Using 2.75x the hourly average seems like it would be the most conservative way to interpret this data, but 2x is probably good for most typical SFR houses.

Anyone know anyone who's working on the next NEC code cycle? With the big push for electrification, this is going to be an important section of code to take a close look at.
The only PIs submitted fir 220.87 were related to the current exception. Two PIs wanted it deleted, and a 3rd wanted to be able to turn off the solar and load shedding for 30 days to use that data. The CMP rejected those proposed changes. The only action they tool was to delete the exception and put the same language in as the last sentence of the section. There were no Pubic Comments, so what you see in the first draft report is very likely what will be in the 2023 code.

You can see the First Draft Report by going to nfpa.org/70 and clicking on "next edition". The first draft report shows the action on all of the proposed changes that were submitted and shows the language that would be in the 2020 code assuming there were not public comments that would make changes.
You can also read all of the public comments, but the actions on those comments will not be available until March.
 

NewtonLaw

Senior Member
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.
It has always been my electric utility experience that all of our electrical equipment is rated on a 15 minute thermal demand. This is how both equipment is rated from the manufacturers and used as a measure of system losses. Thus a collection of peak load data should not be considered thermal demand for a piece of equipment like a transformer or load panel. Peak load level measurements should be used to ensure you do not exceed the maximum allowed load above which instantaneous damage is going to occur. For instance, pulse loading of an oil filled transformer that could cause a hot spot to develop within the windings of a transformer exceeding 190 degrees C will permanently damage the paper insulation. Below this level, pulse loading will cause an average thermal loading and that is the rating you would use in this example. This type of rating is true for most electrical equipment.

Using the peak load will cause you to over specify the needed equipment which may be necessary based on other considerations. As an example, the graph below shows peak load readings in blue, the 15 minute average demand is shown in red. As you can see there would be a big difference in specifying a load center good for 150 kW versus one for 100 kW.

Also you may want to review: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6889045


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