# How to measure the total current driven in a building by observing the electricity meter?

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?

• P = U*I, Watts = Volts * Ampères -> I=P/U, A = W/V, where U = your main's nominal line voltage. – JimmyB Apr 12 '16 at 10:58
• I read around 95000kWh. Possible to estimate the current drawn? – user16307 Apr 12 '16 at 11:24
• @user16307 Divide it by the used voltage times three. If your phases are not used symmetrically you have no means to calculate the individual load from this one figure. – Ariser Apr 12 '16 at 11:29
• U mean 95000/(230x3) ??? – user16307 Apr 12 '16 at 11:32
• "Electricity meters show power.' No, they show energy used (power x time) so just dividing by a voltage doesn't work, that will produce an amp-hr value. – JIm Dearden Apr 12 '16 at 11:36

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.

• So relating to OP's comment he probably saw an energy meter not a power meter. – Ariser Apr 12 '16 at 11:41
• Well some of them show power. We have some fancy modern thingies in Italy, they can trip the main switch if you go over max power of some x% for more than some t time. And they show you the power ofc. I am with you anyway, I highly doubt OP has a way to access a power meter that shows 95MW (figure given in comments). That's probably MWh. – Vladimir Cravero Apr 12 '16 at 11:41
• Yes but if I wait 5 hours and take two kWhr readings and the diffetence in kWhr and 5 hr time; can I estimate the current driven? – user16307 Apr 12 '16 at 11:44
• However an average current can be calculated by noting two readings after a short interval(maybe 1hour for example) and dividing the difference of those two readings by that interval and dividing again by voltage. Reading difference = energy consumed. Energy / time = Power. Power/rms voltage = rms Current. – Whiskeyjack Apr 12 '16 at 11:45
• assıming load is constant – user16307 Apr 12 '16 at 11:45

Typical Power factors:

Fluorescent lamp 0.95

Incandescent lamp 1

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.