I tried really really hard to find the answer to this (on this forum and elsewhere) but could not.
Looking at https://learn.openenergymonitor.org/electricity-monitoring/ac-power-theory/use-in-north-america it seems that the usual approach is to attach a current transformer (CT) to each leg of the incoming service ensuring that the two CTs are ‘facing’ in the opposite direction. That page then goes on to say that CT1 will see I1 + I3 and CT2 will see I2 + I3. Shouldn’t that be I2 - I3? This seems to be the only way to make the final formula
Total Power = ½ × V × ICT1 + ½ × V × ICT2
I think I can test this if I can shut down all the other circuits in the house and turn on our electric stove which is a (except for the electronic controls) pure 240V appliance. Then I should see the raw input as I3 on (in my case) emontx:power1 and -I3 on emontx:power2.
Can someone please confirm/comment/correct as appropriate?