# 3-phase question

#### talwart

Joined Jun 21, 2007
2
I need to measure the energy consumption of a motor hooked into 3-phase, 208V AC. I know the current on each of the three wires connected to the motor (from a clamp meter). My question is what is the total amp draw of the motor.

For example, I have:
Blue wire: 4.2 amps
Red wire: 3.7 amps
Black wire: 3.9 amps.

Is the total amp draw the simple average of these, the square root of 3 times the average, or something else?

Thank you!

#### beenthere

Joined Apr 20, 2004
15,819
I believe the total draw is the sum of the three currents. I would also think that a motor with currents that far out of balance has some damage in the windings.

#### recca02

Joined Apr 2, 2007
1,212
in three phase current enters from one wire to to another so three currents are not un related but same current divided in different branches at different phases.
the current from phase to phase is vector sum of current in each phase.
for power calculation phase current into phase voltage gives power consumed by that phase.summing wud give the total power.

also vector sum of current for balanced 3 phases shud be zero.

#### beenthere

Joined Apr 20, 2004
15,819
Well, I learned something.

#### subtech

Joined Nov 21, 2006
123
Talwart
I don't know just how accurate you want to be, but here goes.
There are a couple ways to calculate VA or watts (in your case watts, we'll ignore power factor for now)

1. If you know the phase to phase voltage:
Power= I*sqrt3*line volts (208V and avg. of the three currents 3.93 A)

2. A more accurate method would have been to measure each voltage phase to neutral at the same time you were taking the amp readings and calculated power in each phase.

Assuming a balanced phase to neutral voltage of 120 volts then:

A phase power= 120*4.2
B phase power= 120*3.7
C phase power= 120*3.9
Adding the three powers will yeild total energy consumed by the motor.