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/ProductIma...ke/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 a

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.