220.87 Peak Demand - hourly vs 15 minutes

[gadfly56]

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.

I wonder if the wording is an attempt to enforce the use of a rolling window of measurement, rather than cutting the hour into 4 segments defined as "0-15", "15-30", "30-45", and "45-60".

 

[Hv&Lv]

And PD is the maximum observed average power over what time interval?

Cheers, Wayne

15 minutes
Sorry, should have stayed on course with the thread and stated the interval.

 

[Ricko1980]

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.

 

[Hv&Lv]

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.

No. The meters have to be programmed. Sorry

 

[Macbeth]

I get this all the time. the 15min interval is not four quadrants of an hour (00:00 to 00:15, 00:15 to 00:30, 00:30 to 00:45, 00:45 to 01:00) but a maintained 15min Sliding window. The window must be continuously Maintained for a period of 15min. As each sample is taken the sliding window moves forward, it must calculate the average of the samples taken in the pervious 15min. It is not the Peak of any one sample.

So then what is the sample interval requirement. As a rule I will not be able to find any thing on this but a real reading sample of 1min or less is acceptable, but over 1min the Peak Reading found during the sample period is stored and used.
If you store samples every 5min then every sample stored must be the highest recorded during that 5min sample period and then averaged with the previous 2 stored readings.
If you store sample every 15min then the Peak reading during that sample period is used. Oddly enough you can see this would yield an 15min average reading that is equal to the Peak reading.
If you use something like 10min samples then you use the Peak sample taken and do not average it. This also yields the Peak reading.

So Peak readings can be used as they will always be higher that the average.

 

[wwhitney]

I get this all the time. the 15min interval is not four quadrants of an hour (00:00 to 00:15, 00:15 to 00:30, 00:30 to 00:45, 00:45 to 01:00) but a maintained 15min Sliding window.

That's a reasonable way to do things, but what NEC language can you point to indicating that is the requirement?

Also, it's only when the sliding window peak is quite rare that the answer would differ significantly between the two methods. If during the total observation period the sliding window peak (or close to it) would be hit multiple times, then it is likely at least one of those times would be captured within an hour-quadrant (barring some reason the peaks are correlated to the wall clock minute hand).

Cheers, Wayne

 

[Hv&Lv]

I tell guys to look at the old relays, meters, etc. to get an understanding of how new electronic devices emulate old equipment.

google merz-price demand

 

[Ricko1980]

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.

 

[jaggedben]

That's great information. Also nice to have my intuition corroborated. It's more or less exactly what I expected.

You can look up proposed changes to this section at nfpa.org.

 

[wwhitney]

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.

Can you clarify that? If the multiplier was normally distributed (or log normally distributed), then you could determine the 97.5% percentile from the median and the standard deviation. But you have the full sample distribution, so you can just find the 95% percentile. Was it 1.70x?

Cheers, Wayne

 

[Ricko1980]

Can you clarify that? If the multiplier was normally distributed (or log normally distributed), then you could determine the 97.5% percentile from the median and the standard deviation. But you have the full sample distribution, so you can just find the 95% percentile. Was it 1.70x?

Cheers, Wayne

Yes, 95% of the residences had meter data such that multiplying the hourly average by factor of 1.70 was sufficient to meet or exceed the 15-minute average.

I suggested x2 only because it seems like the NEC might want better than 19/20, and there were a few dwellings with relatively high demand that managed to be up in that range for the multiplication factor. You’re completely right about the statistical conclusions.

 

[MyCleveland]

Can you clarify that? If the multiplier was normally distributed (or log normally distributed), then you could determine the 97.5% percentile from the median and the standard deviation. But you have the full sample distribution, so you can just find the 95% percentile. Was it 1.70x?

Cheers, Wayne

WW
Please help me understand this discussion.
220.87(1) exception is only considered when utility cannot provide a 1 year demand history.
You are then left to install a recording meter to capture MAX amps or kW over a 2800+ 15 minute intervals in a 30 day period.

Is Ricko1980 attempting to use utility kWhr data is some way to achieve this? Even if you could use the data you still need to confirm you have captured all loads as the exception goes on to explain. I believe this is why you need the recording equipment (meter) to satisfy this section and complete it by verifying all other loads.
If it is residential, just do a load calc.

You guys have discussed the words "reached" and "maintained"...I am struggling with the use of the word AVERAGE when we are trying to capture a peak or max value.
Maybe it is just my ignorance of how or what data these meters are capturing.

 

[jaggedben]

WW
Please help me understand this discussion.
220.87(1) exception is only considered when utility cannot provide a 1 year demand history.
You are then left to install a recording meter to capture MAX amps or kW over a 2800+ 15 minute intervals in a 30 day period.

Is Ricko1980 attempting to use utility kWhr data is some way to achieve this? Even if you could use the data you still need to confirm you have captured all loads as the exception goes on to explain. I believe this is why you need the recording equipment (meter) to satisfy this section and complete it by verifying all other loads.
If it is residential, just do a load calc.

You guys have discussed the words "reached" and "maintained"...I am struggling with the use of the word AVERAGE when we are trying to capture a peak or max value.
Maybe it is just my ignorance of how or what data these meters are capturing.

I think there's two discussions...

First discussion is that the code is basically silent about what legitimately qualifies as 'demand' data. So the discussion is about what kind of data could be legitimately used under 220.87. It's kind of odd that the exception has more concrete language than the requirement above it, but since that's the clearest language in the code, people are looking to the exception.

It seems like if the utility is recording 'max demand' data for billing purposes then that data can be used. What's less clear is whether/how max demand can be derived from kWh data. There's no physical or mathematical reason that one isn't as good as the other if the kWh data is granular enough. But maybe 'reached and maintained for 15min' means something precise enough to some people to argue that it can't be used.

Second discussion is about what's actually realistic variation between 15 minute and hourly demand, given that hourly kWh data is more widely available (at least for some utilities). Rick has given us some great number crunching on that.

 

[electrofelon]

WW
Please help me understand this discussion.
220.87(1) exception is only considered when utility cannot provide a 1 year demand history.
You are then left to install a recording meter to capture MAX amps or kW over a 2800+ 15 minute intervals in a 30 day period.

Is Ricko1980 attempting to use utility kWhr data is some way to achieve this? Even if you could use the data you still need to confirm you have captured all loads as the exception goes on to explain. I believe this is why you need the recording equipment (meter) to satisfy this section and complete it by verifying all other loads.
If it is residential, just do a load calc.

You guys have discussed the words "reached" and "maintained"...I am struggling with the use of the word AVERAGE when we are trying to capture a peak or max value.
Maybe it is just my ignorance of how or what data these meters are capturing.

That is not correct. You are recording max demand not max amps/KW. Demand is a 15 minute average. This should also explain your question in your 3rd paragraph.

 

[Ricko1980]

If it is residential, just do a load calc.

The reason I'm interested in figuring out how to improve 220.87 code language so we can use hourly data is that oftentimes the actual demand is WAY WAY WAY lower than when you do the calcs another way. I've seen houses where people want to add car charging or an induction stove, and the 220.83 calcs come in at suddenly needing a service upgrade from 100A to 200A service, but the actual demand data shows 15-minute demand of only 40A (thus leaving enough space for a charger or stove right away)!

Since there's going to be a lot of converting appliances from gas to electric coming up, I think it'll be good to be able to more accurately judge current usage patterns. Hopefully 220.87 will be somehow amended to allow for hourly data to be used with a conversion factor, thanks jaggedben for the suggestion to look up the proposals on nfpa.

 

[MyCleveland]

I think there's two discussions...

First discussion is that the code is basically silent about what legitimately qualifies as 'demand' data. So the discussion is about what kind of data could be legitimately used under 220.87. It's kind of odd that the exception has more concrete language than the requirement above it, but since that's the clearest language in the code, people are looking to the exception.

It seems like if the utility is recording 'max demand' data for billing purposes then that data can be used. What's less clear is whether/how max demand can be derived from kWh data. There's no physical or mathematical reason that one isn't as good as the other if the kWh data is granular enough. But maybe 'reached and maintained for 15min' means something precise enough to some people to argue that it can't be used.

Second discussion is about what's actually realistic variation between 15 minute and hourly demand, given that hourly kWh data is more widely available (at least for some utilities). Rick has given us some great number crunching on that.

Thanks…but
FP-does not the exception tell us what qualifies….if we are trying to use the exception.

SP - I do not see any means to convert kWhr data to kW.

TP - to what end if you cannot convert to kW. We would still need to validate all loads were captured.

 

[MyCleveland]

That is not correct. You are recording max demand not max amps/KW. Demand is a 15 minute average. This should also explain your question in your 3rd paragraph.

What is not correct?
The exception states we can capture max amps OR kW, does it state anything about kWhrs?
Where is it written that demand is a 15 minute average?

 

[MyCleveland]

The reason I'm interested in figuring out how to improve 220.87 code language so we can use hourly data is that oftentimes the actual demand is WAY WAY WAY lower than when you do the calcs another way. I've seen houses where people want to add car charging or an induction stove, and the 220.83 calcs come in at suddenly needing a service upgrade from 100A to 200A service, but the actual demand data shows 15-minute demand of only 40A (thus leaving enough space for a charger or stove right away)!

Since there's going to be a lot of converting appliances from gas to electric coming up, I think it'll be good to be able to more accurately judge current usage patterns. Hopefully 220.87 will be somehow amended to allow for hourly data to be used with a conversion factor, thanks jaggedben for the suggestion to look up the proposals on nfpa.

It just seems you are trying to find a way to avoid a service upgrade.
Even if 15 was changed to 60….you still need to satisfy the entire exception.

There have been many threads on EV load additions and the majority states you can add in the EV load at 40%….optional calc method. Have you tried doing a load calc with this…you still might be under 100amps.

 

[jaggedben]

Thanks…but
FP-does not the exception tell us what qualifies….if we are trying to use the exception.

I don't think people are actually trying to use the exception. They are wondering if we can use annual kWh data to comply with the main section above the exception. We only look to the exception because the language is more specific about the data required.

SP - I do not see any means to convert kWhr data to kW.

Let's take an example...
If I have an hourly consumption data point of 7.2kWh between 6pm and 7pm, then the most extreme possibility for demand (as defined by the exception) is that 7.2kWh was consumed within 15 minutes and zero kWh was consumed for the rest of the hour. This is accomplished simply by multiplying 7.2 by 4 (60mins/15mins). This method produces a conservative result of 28.8kW, and applies to literally all the possible 15minute intervals that start and end within the hour (i.e. start between 6pm and 6:45pm, and end between 6:15pm and 7pm)

With respect to intervals that overlap with neighboring hours (e.g. 5:53pm to 6:08pm), the most extreme assumption is that both hours entire consumption occured within that 15 minute interval. So if my kWh for 5pm to 6pm was 4.5kWh, then the maximum possible demand (as defined by the exception) is (7.2+4.5)*4=46.8kW.

Btw, these sorts of numbers are typical of what I see for homes I look at. And the two different calcs come out to 120A and 195A respectively.

What Rick's numbers show us is that such calculations (without the overlapping versions,actually) are outrageously conservative, i.e. they likely overestimate real demand (as defined by the exception) by more than double for 95% of homes. It's to be expected that they are very conservative overestimates because of course most people have a constant low amount of load and high loads are usually for shorter times. But Rick has helped us quantify the likely size of the overestimate.

I think a point to be made about all of this is that since the exception is a legit way to meet 220.87, then these sorts of kW calcs ought to be too.

TP - to what end if you cannot convert to kW. We would still need to validate all loads were captured.

See above. Also demand data is agnostic with respect to the actual loads.

 

[wwhitney]

Is Ricko1980 attempting to use utility kWhr data is some way to achieve this?

The statistics Ricko1980 gathered are about the following data: suppose you have a house (or rather 2251 of them), and you install two logging meters on it. One tells you the average kW used over each 15 minute period, and the other tells you the average kW used over each 60 minute period. The former number is always going to be at least as big as the latter number. So what does the ratio look like, and how does that ratio vary over the population of 2251 houses? (And of course you don't actually need two meters, as with the data from the first meter, you can recreate what the second meter would show, simply by grouping the string of 15 minute numbers into groups of 4, and taking the average of each group).

The interest in that ratio is that if you are presented with a house with only the 60 minute data, you could estimate what sort of result you would get if you did the 15 minute data gathering. For example we learned that for the sample considered, 95% of the time the ratio would be 1.70 or less. So if you consider 95% confidence good enough, you could estimate the 15 minute data from the 60 minute data by multiplying by 1.7.

Of course, as 220.87 (1) Exception is currently written, that estimate would not be sufficient. But a large enough study like the above might be sufficient evidence to support a change in the code for that exception that would allow more flexibility.

Cheers, Wayne