Yes I accept that equation if only 125v circuits (L1 or L2 to neutral) are being used, but I’m almost sure 250V circuits current flow from L1 at the top of the tranformer, through CT1 (getting counted there) through the load, then back to the transformer on L2, getting counted on the way again, as a separate power reading by CT2. Seems like with the given equation I’d be counting the same current in 250v circuits twice. No?
OK, so disregard the 120 V loads, and write out the equation for the power in terms of the voltage you measure (V) and the current you measure (I3) (assuming as I did that the two legs see exactly the same voltage each side of neutral). Then manipulate it into the same form as the last term in the line you’re complaining about.
