Estimating KWh electicity billing using clamp-on amp meter

[Home Guy]

Just a note: I haven't read the last dozen or so posts to this thread, so I'm not responding to anything there because (naturally) I don't know what direction this thread took in those posts.

To summarize: This thread started by me speculating how or if I could use a clamp-on amp meter to measure and then estimate the monthly kwh electricity usage for a small commercial building with a three-phase,

120/208 volt, 400 amp service.

The motivation for making my own such measurement was to get a reality check on a recent electricity bill that had me using 4960 kwh worth of power during the month of March, when my previous two bills were well under 2000 kwh each.

Today I spoke with the head meter technician of the municipal utility that provides my electricity, and this is what he told me:

My meter was replaced on Feb 28 (I knew this). The new meter started with a reading of 0 (zero) on that date. The meter was read on March 4, and it read 2 (two). It was read again on April 4. It read 25. It was read again on May 4, and it read 47. Finally, it was read again just yesterday - and it read 68. Those are numbers that he saw on my account on his computer.

Now, what do those numbers mean?

I don't know if this is universal or not, but from what I gathered, if your electric service is 200 amps or less (one, two or three phase) then all the current you're using is flowing through the meter. The meter will report the total kWh used on it's display.

If your service is more than 200 amps, then the meter is measuring the current using a transformer or coil and the number displayed by the meter needs to be multiplied by a factor (I think it's labelled as "PAF") to convert to kWh. In my case, the factor is 80. So when you look at the above numbers and multiply them by 80, you get the kWh that I'm being billed for.

Now, when you look at the above numbers, and specifically the May 4 number of 47, well, that was not the number I have on my bill. The number I have is 87. A difference of 40. Which when multiplied by 80 gives 3200 kwh that I was incorrectly billed for.

So this looks like some sort of transcription or data-entry error.

The new meter is capable of time-of-use measuring (which makes me think now that I should call the guy back and ask how well it can really do that given the crude resolution it has). Our city it not (yet) on a time-of-use billing scheme (at least not for home and small commercial customers). Both the old and new meters are capable of measuring peak or demand usage, but I'm not and will not be billed based on demand because my use is less than 50 kw.

I then called the billing department and described the situation and (last I heard) they are going to send a new bill for that month.

This 80 factor also explains something else - why I was seeing several months come in with exactly the same kWh usage, down to the exact number. The meter only shows whole integer numbers, so the effective resolution is 80 kWh, which in my case represents 5% of typical monthly usage.

[Malcom "Mal" Reynolds]

so the question becomes: given your previous description of electricity usage, why is your service more than 200 Amps?

[Home Guy]

Before I answer that question, I want to make it clear that this billing error really didn't have anything to do with what sort of meter I have or what numbers it reports. This was a human transcription problem at some level somewhere in the system.

Now, to answer your question - I am not the original owner of the building.

The building was obviously spec'd for a different style of usage. More people, different equipment. During the design phase, the projected electricity usage was calculated, a fudge factor was applied (which was either appropriate or way over-estimated) and the corresponding service was installed.

[dpb]

On 6/8/2011 7:50 PM, Home Guy wrote: ...

...

Very similar problem to what I and several others propositioned--that it wasn't a "real" usage problem but either a human or computer-generated problem... :)

--

[Home Guy]

The meter was read correctly - to the extent that the reader entered the correct numbers into a large handheld device of some sort.

You wouldn't think that there would be any other human hands touching the data after that point, at any other point in the chain that leads to the preparation of my monthly bill. Because I can't imagine the upwards of 50 to 100 thousand customers having their bills similarly "touched" by human hands, on a monthly basis.

As for this being a computer-generated problem, you would think that any code that transforms "47" into "87" would have been caught long ago.

[Home Guy]

The numbers I showed was after the utility adds a "loss factor" to the meter's reading. Something like 4 or 5 percent.

[Home Guy]

Yes, I do recall another similar high reading a year ago that I'm going to look into in the next few days.

I expect that it would have been corrected for the next billing cycle, as this one would have.

[Malcom "Mal" Reynolds]

But the fact remains that you are in an inappropriate service tier and should do something about that

[Home Guy]

You should define what you mean by "innappropriate service tier".

When it comes to the rate I pay for power, my electricity provider does not distinguish me (with a 3-phase, 400 amp service) from any other small commercial operation under 50 kw that might have a 1 or 2 phase,

100 or 200 amp service.

If you mean that I should change my service from 3 phase 400 amp to 2 phase 200 amp, and by doing so change my switch gear, fusing, distribution bus and re-wire my panels, do you honestly believe such a change would result in anything different in the short or long term other than my wallet being several thousand dollars lighter?

[dpb]

Home Guy wrote:

Home Guy wrote:

Well, I wouldn't presume anything -- if it were fully automated there'd be no need for the manual reader to come look to begin with.

Your previous posting said (at least that's that I thought it said) you had confirmed it to have been a transcription error--you're nowing saying that wasn't found to be so but is another supposition you're making not what the utility rep confirmed?

Other possibility is that whatever the transfer mechanism is from the reader's collection to the central billing computer is error-prone. Do you know how/what technology they're using?

What if the reader;s data entry device prints out a record and that is sent thru an OCR device for entry to the accounting software? I can imagine that could create such a repetitive mistake. Or it's a wireless connection and is flaky. If the utility is still stuck in an early technology introduction mechanism I can imagine all sorts of possible error-inducing things _could_ be doing. (In another life, I remember having nuclear generating power plant data returned to central office by punch paper tape--more effort was spent in making software to correct the errors inherent in the process than the time required to do the actual engineering calculations. Eventually, stuff was upgraded but again it was a first-generation system and reasonable technology for the first deployment but had been passed by during the time from initial specification to deployment by other developments. As noted elsewhere, am involved heavily in our local rural electric co-op; we're going thru a similar issue of moving from manual readers to automated and have had several trial pieces of gear during the last ten years since began that have proven unequal to the task in the specific application. If we had instead taken one of these at face value from the vendor and made a deployment we could easily have had a case of generating many billing errors such as the one you've described because they had bad communications, primarily altho some were related to loss of onboard memory or other glitches that caused bad readings. In your case, since apparently the meter is mechanical and the reading manual, the problem is a little different probably in inception but conceptually the same.

Would be interesting to find out what final resolution turns out to be...as another poster noted and I had mentioned before, the earlier real outlier value seems as though should have similar explanation and perhaps a billing credit if hasn't somehow been otherwise corrected.

Whatever the actual mechanics, it turns out that it isn't an actual locad problem which was the point I had been trying to make all along that the issue really couldn't be a "real" physical usage problem with the kind of characteristics of the posted data.

[Home Guy]

dpb double-quoted the following for some reason:

I talked to the guy that runs the metering department. I don't know exactly what his title is, something like Senior meter technician or some sort.

He looked at my account on his computer and called up the last few meter readings. These are the numbers displayed on the meter, which are NOT the actual kWh. To get the kWh, you take those numbers and multiply by

2. The number that he had for my May reading was 47. But on the invoice that I get in the mail, the number showing was 87.

This guy running the meter department does have access to what I would have been seeing on my bill. I think he said it was a different computer system that generates the bill I get in the mail. He claimed that just by what he can see, he can't tell me if there is a transcription or billing error because he doesn't have access to my bill. He told me what my bill *should* say, and that if it doesn't say that, then there's a problem somewhere.

I then talked to someone in the mail customer-support dept, and told them that my bill said (87) and your main guy in the meter dept (I mentioned him by name) said that the bill should say (47). After a few minutes on hold, I was told that I'd either get a call back (I guess telling me that their 87 was right) or I wouldn't get a call-back but that I'd receive a new invoice in the mail.

I haven't yet got a call back, so I assume I'll be getting a new invoice next week.

They basically don't want to admit to any error, at least not over the phone.

Utility operators can be cagey buggers in this regard.

I'd have to take a really close look at the hand-held device the meter reader is using the next time he comed to do a read. I have a snow-balls chance in hell to pose your question to the customer service dept and actually get an answer.

I'm going to look into that earlier reading and see how those numbers jive with the expected numbers. I'll probably also call the senior meter tech and see if he has the read values for that month as well.

[dpb]

...

...

I say at the moment they're still in the resolution process so it's early in the game for that quite yet. If you get the revised bill consonant w/ the lower usage, that says "we goofed" ipso facto... :)

...

Perhaps; worst they can do is not answer the question. My couch it as "how did or could the recorded 47 get transmuted into 87?" instead asking directly for details of the system.

Altho I forget that as member of the distribution co-op that supplies us and board member as well that have inside information and our interaction w/ our "customers" isn't the same as the typical large utility... :)

Here, if a member asks a question, we strive to answer it.

[davidstevens]

The current in the neutral will NEVER be more than the highest current on o ne of the 3 lines. It will ALWAYS be less than the highest current on one o f the three lines unless there is only current on one line. If the current in all 3 lines are balanced the current in the neutral should be ZERO. This would occur if all of the loads were three phase motors.

If this were a 2 phase panel, such as almost all residential panels are, it would have 2 hot lines and one neutral. In this case if the loads are bala nced the current in the neutral will be zero. With unbalanced loads the cur rent in the neutral is the difference between the 2 hot lines. Because the current in one is 180 degrees out of phase with the other the current from one cancels out the current from the other. Best practice is to try to keep the loads balanced as much as possible.

With a 3-phase/4-wire panel the current in each line is 120 degrees out of phase with each other so the math for calculating the current in the neutra l when the loads are unbalanced becomes more complex.

The above is based on simple resistive and inductive loads back 50 years ag o when I worked with this stuff every day. Modern day loads with lots of el ectronics and switching power supplies`make things even more complex.

[Annie H]

[snip]

Neutral current is actually the sum of the lines, but considering phase.

(for 3 phase):

neutral_current = line1_current + (line2_current at 120_degrees) + (line3_current at 240_degrees).

for example (10A on each phase):

10A + (10A at 120deg) + (10A at 240deg) = 0

[Walt's Electric]

I've never seen a two-phase residential panel at lowes or home depot.  Since they're so common, would you provide a link to one at homedepot.com or lowes.com?

[trader_4]

, it would have 2 hot lines and one neutral. In this case if the loads are balanced the current in the neutral will be zero. With unbalanced loads the current in the neutral is the difference between the 2 hot lines. Because the current in one is 180 degrees out of phase with the other the current f rom one cancels out the current from the other. Best practice is to try to keep the loads balanced as much as possible.

? Since they're so common, would you provide a link to one at homedepot.c om or lowes.com?

Oh no, here we go again.

[Ralph Mowery]

Yes, another pissing contest about split phases and 2 phases.

[davidstevens]

? Since they're so common, would you provide a link to one at homedepot.c om or lowes.com?

OK, It has been 45 years since I have worked as an electrician. Perhaps the proper term should be dual phase or split phase. Does that work for you? I really didn't expect I'd be waking anyone up by responding to a 7 year old post.

[gfretwell]

Or just single phase with a center tapped secondary. (split phase is a type of motor winding)

[trader_4]

On Monday, July 23, 2018 at 8:24:43 PM UTC-4, [e-mail] w rote:

  Since they're so common, would you provide a link to one at homedepo t.com or lowes.com?

he proper term should be dual phase or split phase. Does that work for you? I really didn't expect I'd be waking anyone up by responding to a 7 year o ld post.

We went through this round and round a few years ago. I was somewhere clos e to your position. I said something along the lines that there are in fact two phases present, 180 degrees apart. But that they are derived from one of the three phases coming down the street and commonly referred to as split-phase. But when you spit-something, what do you get? Two things. I wouldn't call it a two phase panel though, that's not the common term for it. But it is very common to refer to each of the hots as a phase.

I even found a presentation a power industry engineer made at a conference where he made the case that to correctly analyze a split-phase system you have to treat it like what it is, a system with two phases.

The other argument is that if I start with a 3 phase system going into a building, everyone agrees that there are 3 phases there, right? You can see them on an oscilloscope. Now let;s remove one. How many phases now? Two. Now instead of the phases being 120 apart, let's move one so that they are 180 apart. How many phases now? Two. And if you look at it on a a scope, it looks exactly like a split-phase gsystem going into the same building and it behaves exactly like it. Electrically you can't distinguish one from the other, they are the same thing.

That's my case, others will probably draw different conclusions.