Electricity meters show power. How can I measure roughly how many total current in amperes is driven at an instant in a building including a 3 phase AC motor?
Electricity meters show power
No, they show power integrated over a time period (energy) i.e. kWhours and not kW actually being consumed in a particular moment. On this basis, you cannot know what the load current is in a particular "instant".
You could set a stop watch and record the change in kWhours. Then convert this back to kW (knowing the time period elapsed on the stop watch). Then estimate average RMS current over that time period but you'll have to take a stab at power factor which by no means will be either unity or constant over various load conditions that the 3ph motor might encounter over that time period.
Typical Power factors:
Resistive load 1
Fluorescent lamp 0.95
Incandescent lamp 1
Induction motor full load 0.85
Induction motor no load 0.35
Resistive oven 1
Synchronous motor 0.9
120V =208V 3ph.
Current = 95000 X1000 / sqrt(3) X V X PF
current= 95000000 / 1.732 X 208V X 1
current = 95000000/360.26
current = 263,698.44 Amps
or 263.7 kA Resistive load worst case scenario
Electric motor PF .85/.35
= 224.15 kA, 92.29 kA respectively......
You can buy a meter that clamps onto the wire and will tell you precisely how much current is being drawn.