Home Guy wrote:
harry wrote:

Yes you can determine the power being consumed but only at the
instant you have taken a reading.

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 three large cables are the phases.
The neutral is smaller because it only carries the "out of balance
current".

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".

Multiply the current by the voltage (120 in your case) for each of
the three cables and then add them together & divde by 1000
This actually gives Kva. If there is only heating/incandescent
lighting in the building, this is the same as Kilowatts.
If there are electric motors or uncorrected fluorescent lights,
the Kva needs to be multiplied by the "power factor" to get Kw
which you don't know. In practice it could be anywhere between
1 (unity) or 0.7. For office premises, you could assume 0.9

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?

You're pushing a very big rock up a hill to nowhere.
Your meter will give you little useful information.
You need to KNOW the phase.
Sticking your finger in your favorite orifice and pulling out a power
factor number is just that...a useless number. You don't
even want to think about the orifices you find here.

Why do you care?
A decision tree is often helpful.
If you think the equipment is faulty, you should enlist the power company.
I've found 'em to be very knowledgeable and helpful. They have the
equipment to determine whether your meter is faulty. A friendly
conversation with customer service should get you a call from
a real engineer. If it turns out to be faulty, make sure you
get calibration numbers off the old system so you can negotiate
a refund.

If you think they're intentionally screwing you, you need to hire
an electrician with the equipment to measure WATTS. That's what you
pay for. Measuring VA is an exercise in futility. Your "finger"
ain't gonna hold up in court anyway.

You might be able to get some advice from the local electrical
inspector.

If you think one of the tenants is charging their electric car
when you're not looking, your amp meter can point you in the right
direction.

I've used current clamps into a computer to log AMPS. Useful
for determining relative consumption from the same load...in
my case it was a water heater.
RMS amps is better than peak or average amps, but still not
a short path to WATTS...well, in the case of a water heater,
it is, but that's a special case of resistive load.

Depending on how the power meter's made, you can get cheap wireless
monitors that you might get the utility to let you clamp
on the meter...but that will have the same systematic
errors that the meter has. But it might help you find
any clandestine loads at odd hours.

Newer digital readout meters have an infrared light that blinks in concert
with the load. Mine is one blink per watt-hour. I programmed
a pda to read out and log consumption in real time. Again, for
my relative use. It has same systematic errors as the meter.
But you still gotta be able to "see" the meter...or put a fiber
optic cable to the outside.

You can buy clamp-on current transformers that also hook to the volts
and measure REAL power accurately. But it's much easier to pick up
the phone and have the power company help you.

The question about the 5% adder is one for customer service.

Did I mention...rock..hill...futility?