Davidbeach,
Yes you're right I guess I got hung up on the question of measuring 3rd phase without a transducer for a few reasons.
1/ My control system is Opto22 with two inputs per module.
2/ I have often seen the 2 Wattmeter circuit for measuring 3 phase Watts so it seemed reasonable that 2 CTs would be able to measure 3 phase current.
3/ At some point finding the formula became the challenge
As I said several times math is not my strong point so I posted the question thinking it would be a simple task for someone with an engineers skills.
I got to thinking about your "Kirchhoff's current law" and although it seems to be applied to DC if you think about a 3 phase circuit at any instant in time with the phases at 0, 120 & 240 degrees it should still apply.
Jghrist,
"Maybe there is a formula, but I haven't had time to work it out" I searched high and low but I couldn't find one, several posters hinted at one but someone else would find a flaw.
If I think about a circuit with CTs on A & B, if both currents are equal you don't know if the load is between A-B or A-C & C-B or some combination therof
but if the A & B aren't equal then you know there's current in C but how much, not A-B obviously because of the phase angle.
It seemed like a simple question at the time, I'm sorry I wasted everyones time.
As I said I will use 3 transducers but I won't stop wondering!
Kind regards
Roy