Estimating KWh electicity billing using clamp-on amp meter

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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:
http://www.tequipment.net/ProductImages/Fluke/33_ap_w.jpg
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.
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**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
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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?
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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
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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%.
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"hr(bob) snipped-for-privacy@att.net" 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?
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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. ;-)
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" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote:

A volt-amp is a watt, when the load is resistive.
If I perform high-speed sampling of both the voltage and current, and if I multiply each reading together to get VA for each sample, and if integrate those VA samples over time, I will get the actual watts or KWh that I should be billed for. That would correctly take into account reative / inductive loads (like motors, light ballasts, computer power supplies, etc).
If I simply calculate watts as equal to VA based on the current measurement from a clamp-on meter, then I'm over-estimating what the billing meter is "seeing" because I'd be assuming that all my loads are resistive. In other words, my calculation of watts = VA can't help but assume that current and voltage are in phase with each other.
The billing meter knows how to calculate wattage correctly when the current and voltage is out of phase.
I guess a clamp-on amp meter that also had a couple of voltage probes so that it could simultaneously measure the voltage could measure true wattage would be needed.

As long as the watt-hour meter is working correctly.
Which seems highly suspect given all the info I've been posting here.
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On 5/28/2011 8:40 AM, Home Guy wrote:

...
Far too little real data to infer that (at least yet)... :)
I'd figure it's likely a combination of estimated or leveled billing w/ a catchup period annually and some tenant or group of tenants w/ loads you're unaware of.
Not that there aren't errors both in manual reading and that the meter can fault but generally those get caught in routine maintenance checks and if the meter were, as you say, replaced recently it's not a high probability.
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That's a *BIG* assumption.

Yes, or you could just use the kWH meter the power company gave you.

That's what everyone has been telling you, yes. Did you actually read any of this thread?

...and it ignores harmonics. It's a smart little thing.

Nope. You're still reading VA, not watts.

Have them calibrate it (they're replace it with one that is calibrated).

Maybe *you* suspect it. They're really pretty good.
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BOTH
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"hr(bob) snipped-for-privacy@att.net" wrote:

That error should be no higher than 10%. The billing meter is measuring power (watts) that is phase-corrected. My method (of just measuring current) is not phase corrected. So my calculation of watts will always be higher by 5 or 10%. If all the loads in the building were purely resistive, then my method should exactly match the billing meter.

The variability or error in actual device or appliance usage is (I would think) quite low, given that this is an office environment where the duty cycle (ie the on/off power cycle pattern) of all of the 15 or so computers is very regular and known, as are the lights.
The lights and computers probably account for 50 to 75% of the electricity usage, the rest being fixed electronic infrastructure (networking routers / switches / DSL modem, multi-function printer-fax, multi-line phone system) a few low-power exit lights, a couple out-door 75 watt out-door mercury vapor lights (on a mechanical timer), an 18 CF refrigerator, small microwave, a small capacity water distiller / chiller (instead of bottled water), a toaster, a couple of small bathroom ventillation fans wired to the bathroom light switches, the furnace fan (granted it's a large motor, 220 VAC and probably 1 hp) but it's not running all the time, a couple of radios.
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wrote:

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?
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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.
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Do you know what a demand meter is? Typically commercial meters register peak demand and that peak demand figure is used as a multiplier
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RBM wrote:

I have no idea. I would think that most utility companies try to communicate as infrequently and as minimally as possible with their customers.
We bought this building 6 years ago. What-ever meter it had is what-ever meter it had. When we wanted the lights to come on, we placed a call to the local utility and at some point soon after the lights came on.
They don't seem to be in the habbit of sending a welcome basket along with a nice, comprehensive information package telling us what choices of meters and electrical service we have, nor a copy of the NIST-traceable certification for the meter.

Never heard of it.

What would be the criteria for determining when a "demand meter" is installed in a given premises, vs a non "demand-meter" ?
Does a "demand meter" give a more accurate measure of energy consumption?
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On 5/28/2011 9:26 PM, Home Guy wrote: ...

It's the tariff schedule. Call the business office or look at the bill; it'll tell you what your rate(s) are.
It's not unusual as others have said for commercial to be on such a tariff but it's more generally so for manufacturing than simply office but it'll all depend on where you are.
As somebody else has noted, there will be very specific rules in the state in which you're located regarding what the utility is required to do regarding a question regarding billing. Look at that information that is bound to be available from your state rate commission or whatever the equivalent is called where you are.
Messing around as you're doing isn't going to get you anywhere at all useful.
Have you evn verified that the meter is actually physically read on a monthly basis rather than estimated and the balloon "reading" isn't simply the annual catchup when they finally do read it? As mentioned before that would certainly be one possible explanation.
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@Home Guy:
A "demand meter" comes into play as a factor when:
-- The size of the electrical service is exceeds a certain predetermined size...
-- The property is zoned commercial/industrial usually will have a demand meter unless you are a small tenant with a separate meter and your electrical service capacity is smaller than the predetermined size mentioned above...
A "demand meter" is no more or less accurate than the meter you have installed on your house -- it is simply keeping track of an additional aspect to your power use, rather than only being an odometer counting how many kWh of energy you are using in a month, it is also keeping track of what your greatest simultaneous draw of energy (a.k.a. your "peak demand") was during the billing period which like others have said acts as a multiplier or determines which billing rate you will be charged for your electric bill for that month...
With a "demand meter" service, a business which uses a consistent 20KW of electrical energy during a billing period would pay a different rate than a customer who uses 100KW of electrical energy in short bursts even if the overall consumption of kWh during said billing period was identical...
~~ Evan
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Here's a question no one has asked. Why is the meter in a box that is locked? I've never seen this before. Do you know if similar meters are all locked? Why not ask the electric company to leave the box unlocked? That way at least you could see what the actual meter is doing.
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" snipped-for-privacy@optonline.net" wrote:

That question was posed earlier in this thread, and I speculated as to the reason for a locked cabinet in a post I made on Friday night (8:47 pm est):
===========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). ========= Based on the following (and others which I'm not including) I don't think it's uncommon for billing meters to be located in locked cabinets, or "cupboards" (a term used in the UK):
===================http://www.horizonpower.com.au/downloads/comm_partners/metering/metering_requirement.pdf
Multiple occupancy premises must have their meters readily accessible at all times and comply with section 6 of the WAER, unless remote reading facilities are installed. Where meters are located within a locked meter-box/ cabinet or enclosed area, an approved Horizon Power master lock must be fitted, allowing Horizon Power access to the meters at all times. The disengaging of electronic security systems to obtain access to the meters is not acceptable. Such cases will require an automated reading system (AMR) to be installed. =================== See also:
http://forums.moneysavingexpert.com/showthread.php?t "07277
And this:
=================http://thestar.com.my/news/story.asp?file=/2011/5/21/sarawak/8727246&sec=sarawak
See explained that Lees electricity meter was locked inside a centralised cabinet outside his premises and three of the eight meters inside the cabinet were found with signs of attempted tampering. ================== And this:
================http://forums.whirlpool.net.au/archive/1414092
I live in a small townhouse block with 3 dwellings. We are in an Energy Australia distribution area.
Our electricity meters are all together in a locked metal cabinet near the street. These connect with underground wiring to a breaker box in each house's garage. ================= And this:
==============http://www.theanswerbank.co.uk/Law/Question512492.html
When the houses were built, it was decided that the electricity meters of each property would be placed in a 5ft x 3ft metal wall cabinet on the outside gable end wall of an end property of each terrace of houses. Recently, a new tenant has moved into the end property of my terrace of houses and he has proved to be quite an objectionable and difficult character. He is very unsympathetic towards my need to check my meter and to make matters worse, the council has locked the cabinet and provided him with a key just like they've done with other terrace blocks on the estate. ================
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