Measuring multifilar current...

Thread Starter


Joined Nov 29, 2005
Never thought about it before, and I may need to implement it. Checking how it should work with your opinions...

A U.S.A. plain household electrical utility, supplies 2 branches of 120 VAC. 120X and 120Y and center tap neutral. Some loads L are on the X branch, some in the Y branch . Some 240V loads L between X and Y branches :

120X----------------CT---------------------------L------------neutral-----------L-------------------------------------------120Y (lights)

120X----------------CT---------------------------L------------neutral------------L-------------------------------------------120Y (outlets)

120X----------------------------------------------L------------neutral------------L-----------------------CT-------------------120Y (garage, exterior)

120X----------------------------------------------------------------L---------------------------CT--------------------------------120Y (water heater)

120X---------------------------CT------------------------------------L------------------------------------------------------------120Y (airconditioning)

If current transformers CT are inserted AFTER the circuit breakers, (load side as shown) to read the amperage consumed, Can several load wires of the same branch be passed trough the same CT to read the addition of currents ?

I will need to read the total current consumed by the dwelling and cannot do it at the breaker panel feed input.

If a load wire from branch X and a load wire from branch Y pass trough the same current transformer, to sum the currents, the Y branch should pass in the opposite direction. Is that correct ?

Can one single current transformer be used to read the sum of all load currents observing the direction their wires pass trough the current transformer ?

In other words... the reading of a current transformer is the sum of the currents of several conductors trough it, and the direction matters ?


Joined Aug 7, 2020
Yes. The output current from the CT will be the vector sum of the currents through it.
As the two 120V branches are 180° apart, if you put wires from each through the same CT, currents will cancel out.
If you put the wires from the other “phase” through in the opposite direction, they should add (need to think about that one a little more - it will count current in 240V circuits twice, but as the power is double, that might be what you want)