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?