Estimating KWh electicity billing using clamp-on amp meter

[Home Guy]

I have a small commercial building with a 3-phase, 120/212 400 amp service.

The billing meter is in a locked cabinet (for which the local utility has a key) and out of this cabinet run 4 large cables (each about 1 or

1.25 inches in diameter) that run to their own insulator or terminal blocks where smaller cables (3/8" diameter) connect to them and run to separate panels and switch boxes.

All of these large cables are black, and one of them is (I believe) a neutral or ground (it has a white stripe running down it's length).

A voltage reading from this neutral wire to each of the other 3 terminal blocks is 120 vac, and voltage readings between the 3 terminal blocks is about 212 vac.

I have a hand-held amp meter (Fluke 31 true RMS clamp meter) which looks exactly like this:

I don't know exactly how old this meter is, but I believe it's at least

10 years old and quite possibly 15 years old.

The electrical devices in the building are typical for an office with some very light manufacturing. About 15 desktop computers, many with small UPS backup, telephone system, some networking switches and routers, a few printers, flourescent lighting, a few soldering irons, microwave, coffee maker, fridge, water cooler / distiller. At this time or year neither the building's furnace (forced air natural gas) or AC unit is running (the breaker powering the outdoor AC unit is off).

When I put the meter clamp around each of the 4 large cables, I read anywhere from 10 to 20 amps on them during normal day-time electrical usage inside the building. During a test when all computers, monitors, printers and lights are turned off (but all UPS's are still turned on) I read a total sum of about 4 or 5 amps across all 4 power cables.

So my questions are:

1) when coming up with a total current measurement, do I include the current flowing on the neutral line? Should I indeed measure any current on that line at all - or should the current on the neutral be equal to the sum of the currents on the other 3 lines?

2) I am not computing the instantaneous power as a product of the instantaneous voltage and current because I don't know the phase relationship between the current I'm measuring with the clamp-on meter and the AC line voltage. But if I take the meter's RMS amp reading (or the sum of the 3 or 4 readings for each cable) and multiply that by 120, will I get a power or wattage measurement that is AT WORST the highest possible energy consumption number I can have (ie - equivalent to if all the loads were resistive and not inductive) ?

3) There are (on average) 30.4 days in a month, and therefore 730 hours in a month. If I take the above wattage calculation (120 x total_current) and multiply it by 730, then divide by 1000, I should get a quantity energy measurement (KWh) that should match (or approximate) my bill from the local utility - assuming that the load in use at the time of the readings are representative of daily or continuous use. If this method of obtaining a representative monthly KWh measurement is not correct (or needs more refinement) then please state what, why or how.

[RBM]

**You're not going to be the least bit accurate trying to calculate that way. There is no reason that you shouldn't be able to look at the electric meter. If you really need to self meter the service get an "emon demon" for 3 phase 4 wire 208 volts

[Home Guy]

RBM used improper usenet message composition style by full-quoting:

I want to establish several use-case situations, primarily a "worst-case" KWh monthly usage by assuming that all the devices that are normally on during a week-day 9-am to 5-pm work day and turned off at all other times are instead left on continuously 24/7.

I can also get current readings for other use-case situations (evenings and week-ends) that should give me a more closer-to-reality current reading and factor in their time-of use over the course of a month.

This is a small office - not a home. There are fewer variable involved.

The meter is in a locked cabinet. The only time I get to see it is when the meter-reader guy comes around once a month to read it.

I suspect the meter is in a locked cabinet to prevent tampering / bypass (the meter is inside the utility / furnace room of the building and is not accessible from the outside).

And besides, having the ability to lay my eyes on the meter won't tell me anything about the accuracy of the meter, or the real-time current consumption.

All I want to know is - should the current readings from a clamp-on meter (when extrapolated across the 730 hours of a typical month) jive with the accumulated KWh reading as measured by a typical billing meter? Do I have to do any "special" math to the wattage I calculate with the meter to arrive at what the billing meter is measuring?

[Mark]

yes you are correct you can get a rough reading this way.. you need to measure and sum only the 3 black cables, do not include the white striped cable in the sum.

Add up the 3 currents and multiply by 120 and this is your VAs and the actual Watts will be equal to or less then the VAs.

Multiply by hours and you have Watt Hours.

Divide by 1000 and you have kWh which is how you are billed.

Mark

[hrhofmann]

You may be lucky and only have an error of 10 - 20%, but I wouldn't be surprised if your error was as much as 25%.

[RBM]

yes you are correct you can get a rough reading this way.. you need to measure and sum only the 3 black cables, do not include the white striped cable in the sum.

Add up the 3 currents and multiply by 120 and this is your VAs and the actual Watts will be equal to or less then the VAs.

Multiply by hours and you have Watt Hours.

Divide by 1000 and you have kWh which is how you are billed.

Mark

This is a commercial service and metering equipment. How is he going to guestimate demand?

[RBM]

**You need to put 3 clamp on meters on the wires and monitor them continuously over the desired period of time, which still won't be accurate because they won't give you the peak demand. If you have some reason to believe that the utility company equipment is faulty, you can request that the utility company hang a testing meter to verify the accuracy

[Home Guy]

"hr(bob) [e-mail]" used improper usenet message composition style by full-quoting:

When you speak of this error, do you mean

a) an error between (what the billing meter is reading in terms of KWh for some arbitrarily short time span) AND (what my amp-meter derived watt-measurement would give for the same arbitrarily short time span)

or

b) an error between (the monthly power use of the office derived from several different measurements with various devices and appliances turned on or off as per time-of day and day-of-week) AND (the actual or real pattern of device usage over the course of a real month).

What is the error you speak of?

[Ed Pawlowski]

"Home Guy" wrote

.>

There is no special math, but I'd wonder just how accurate your number is going to be. Don't for get about seasonal changes too, more lighting in winter, AC in summer, etc. At work, I take readings on some of our utilities daily, others, monthly, and can spot a trend when correlated with material used on a given day, etc. This will often tip you off as to problem areas and waste when you see aberrations from the norm.

Have you talked to the utility company? Some will do the work for you and put a recording meter on the line for a week or two to get you want you want. Your approach, of course, is in the interest of energy conservation. They are big on selling you less these days.

Reading the meter on a daily basis would be a help to determine trends also. Perhaps they will allow a window to the meter so you can take readings.

[Home Guy]

RBM used improper if not deplorable usenet message composition style by full-quoting:

Demand (or load) doesn't have to be guestimated.

There are a fixed set of lights, appliances and devices in this office, as well as a relatively fixed schedule of use for them. We have a basic week-day and week-end pattern, and for the week-days we have diurnal (day - night) pattern of usage. Since the average month has 730 hours, comprised of 4.34 weeks, we know that there will be 209 "week-end" hours and 521 "week-day" hours. The week-day hours can be further decomposed as 217 "day-time, week-day" hours and 304 "night-time, week-day" hours.

So we have 3 different loading conditions where the energy consumption during each condition is assumed to be constant: Week-day day-time, week-day night-time, and week-ends. If a current measurment is performed once for each of the 3 conditions, and then extrapolated over their projected duration over the course of a month (217, 304 and 209 hours respectively) then if the three total are summed the result should approximate what the billing meter should measure if that same exact device and appliance usage pattern is replicated during a typical month.

The exact hours of each loading condition can be exactly specified to match a given utility bill if the meter-reading dates are known for the bill in question (that will tell us how many week-end and week-days actually occurred during the billing month of interest).

Based on 5 years of previous bills, about 75% of the monthly meter readings are between 1750 and 2250 kwh. Very few go higher than 3000, and only 2 have ever gone above 4000.

[Metspitzer]

They make clamp on amp meters that graph usage over time. I have no idea how much they cost to buy or rent.

We took amp measurements on a parallel run of 3-500. I was amazed that supposedly equal lengths of copper varied as much as 25 amps on the same phase.

[Home Guy]

Yes, I have contacted the "front-line" customer service people and expect to hear more from someone more technical or in more authority (or at least someone more "male") next week.

Our last bill was for 5,160 kwh, with a billing period from april 5 to May 4. The previous month (March 5 - April 4) was 1915 kwh.

Infact, looking at the last 12 bills I see some very suspicious things:

5162.41 April 1915.26 March 1582.17 Feb (meter changed end of Feb) 1998.53 Jan 2011 1915.26 Dec 2081.8 Nov 2664.7 Oct 1498.9 Sept 3414.15 Aug 3913.78 July 1998.53 June 2248.34 May 1915.26 April 1915.26 March 5162.86 Feb 3247.61 Jan 2010 2331.62 Dec 2009

Our meter is always read on the first business day of each month. The meter was changed at the end of February. The old meter was electronic (it had a digital LCD readout). The new meter *I believe* is different in that it can do time-of-use measuring and it can do RF (wireless) data transmission. Time-of-use billing is not yet being performed in our area, and at least for our new meter a person is still coming around to read it.

I note that I see 1915.26 show up suspiciously 4 times during the past year (what are the odds that we'd use the exact same amount of electricity for any 2 months, let alone 4?).

It was the large jump from 1915 to 5162 that tweaked me on this and I had an assistant enter all previous numbers from other bills in a spreadsheet. The large spike back in Feb 2010 to almost exactly the same number of 5162 is also suspicious.

I would have expected our electricity use to decline from March to April given a reduced furnace on-time in April vs March.

I have no idea what remedy or proceedure my utility company follows in disputes of this nature. I have the vague idea that they circle the wagons and defend their meters to the last drop of their blood, and that the laws or service contracts may favor them and not the customer. We shall see.

[Ed Pawlowski]

"Home Guy" wrote

You are correct in being suspicious. My findings from tracking electric, July and August are the highest, January is second highest. There will be a curve on the months between. Winter is high because of running heat and more lighting, then it comes down as you approach spring, then goes up again as the AC use kicks in. Then in September, it comes down then back up for winter use.

I can understand with the old meter that you'd possibly get 4 identical readings if the meter was not read and the bill was estimated. Actual readings, I'd say "no way" you'd have that situation at that use. We do have one tenant that uses very little electric and the meter was originally installed for some machines. It has a multiplier Most bills are the same, in increments of $18. A few a year are $36. Another has bills in the $400 to $800 range and follows the curves a I outlined.

A couple of possibilities I can think of. The meter is incorrect. (yes, it really does happen) A tenant is carelessly leaving on a space heater. a tenant is running both heat and AC out of stupidity, not a maintenance problem. You have some other pump or device that is running all the time.

Good luck and please keep us posted on what happens.

[krw]

An amp meter measures amps, not watts. Even if you assume 120V, you're still calculating volt-amps, not watts.

c) The error between Volt-Ampere-hours and Watt-hours.

The errors you mention above will likely be even bigger, though. Surprisingly, a watt-hour meter (it's already there) is the real way to measure watt-hours. ;-)

[Tony Hwang]

I am wondering why you are trying to do this? First what is the power factor in this circuit? If your goal is to increase efficiency to result in savings, look into that. If you suspect meter is inaccurate you can request for a replacement. Newer digital meters are more dependable and accurate. Even my cabin located in the boondogs have digital power, natural gas meters. You already know typical power consumption pattern. So what is the purpose?

[RBM]

** That's the sort of meter that the utility company hangs on the service to test the accuracy of their equipment

[trader4]

My first guess would be that the logical explanation for 1915.26 showing up 4 times would be that it is some form of an ESTMATED reading used when they didn't take an actual reading. The odds of that number showing up 4 times from a true reading would be very low. And if it has, I'd suggest using it to play the lottery.

[Home Guy]

It's a start.

The main disconnect switch before the meter is name-plate rated at 400 amps. It doesn't mean we're going to ever draw that much.

I know exactly what equipment is in the building - I work there.

I talk to them every day.

What-ever.

Another example of a usenet post that starts with a question, and devolves into "why are you asking?".

I have to waste more time explaining why I want to do something or why I want certain information about measurement techniques, and instead I get a bunch of arm-chair blow-hardts that think they know better.

[Home Guy]

So I shouldn't assume that, say, a bank or three of florescent lights won't necessarily draw a constant amount of current?

Or a dozen PC's?

The neutral is not smaller (physically) than the other 3 cables. It's the same size.

Most charts I see only go as large as AWG guage size OOOO (almost 1/2 inch diameter). In my case, the cables running from the meter to the distribution blocks (a run of about 7 or 8 feet) are at least 1 inch diameter (OD). The conductor diameter is at least 7/8".

So if I don't multiply my VA number by the power factor, then I'm OVER-ESTIMATING my KWh calculation by 5 or 10%.

Tangent:

Why does my utility apply (add) a 5% "correction factor" to the KWh measurement that comes from the meter?

[George]

Poor you and that incredible amount of time you wasted reading three posts after you clearly described you already know and presented everything anyone needs to know and everyone didn't catch on that no questions are allowed and no opinions are wanted..