To calculate 3 phase kWh from measuring amps on each phase

Use this formula; KW=(volts(avg) x amps(avg) x power factor x 1.732) divided by 1,000.
If you don'thave a power factor use 0.9.
Also make sure you use the larger voltage of the system voltages, ex) if it's 480/277 use 480 in the formula...208/120 use 208...240/120 use 240.
If possible use a volt meterto read the actual voltage between each phase.
Use the average of the three voltage readings(V1+V2+V3 / 3).
Take the amperage reading from each phase and average them (A1+A2+A3 / 3).
Tofind KWh, just multiply KW by the amount of hours your system will be serving the load.

Example:
 An operating three phase motor has voltages measured with a voltmeter on each phase of 453, 458,and 461 volts, amperage measured on each phase with an ammeter are 14.1, 13.9, and 13.8 amps,power factor was measured as 0.82.
(453 + 458 + 461) / 3 = 457V(14.1 + 13.9 + 13.8) / 3 = 13.9A(457V x 13.9A x .82pf x 1.732) / 1000 = 9.02 Kwatts
  To calculate the kWh for one days use:
9.02 Kw x 24hrs = 216.48 kWh

 If you are serving unbalanced loads and the amps vary between phases, you can calculate eachphase separately. In this case, you are calculating three separate single-phase values, so the 1.73multiplier is not used, and you use the phase to neutral voltage.

Example:
Phases measured with a voltmeter at 272, 264.4, and 266.2 volts (A-N, B-N, C-N), amperagemeasured on each phase with an ammeter are 10, 25, and 16 amps, power factor is assumed as 0.9:
(272V x 10A x .9pf) / 1000 = 2.45kw
 
(264.4V x 25A x .9pf) / 1000 = 5.95kw
 
(266.2V x 16A x .9pf) / 1000 = 3.83kw
 
2.45 + 5.95 + 3.83 = 12.23kw total

To calculate the kWh for one days use:
12.23 Kw x 24hrs = 293.52kWh (IF the load remains constant!)

This method also works for balanced loads, it just takes longer and is more work, but you get thesame answer (actually slightly more accurate, since you are not averaging) as the first method!
Modification of the example above for an accurate total kw:
 If you are serving unbalanced loads and the amps vary between phases, you can calculate eachphase separately, however you must still average the phases, and multiply by the square root of 3.

Example:
Phases measured with a voltmeter at 272, 264.4, and 266.2 volts (A-N, B-N, C-N).
We will assume480v phase to phase
. Phase to neutral voltages will not be used in the calculation. Amperagemeasured on each phase with an ammeter are 10, 25, and 16 amps, power factor is assumed as 0.9:
(10 + 25 + 16) / 3 x 480 x 1.732 x 0.9 / 1000 = 12.71 kw total

To calculate the kWh for one days use:
12.71 Kw x 24hrs = 305.26 kWh (IF the load remains constant!)