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.