The collective average power read by the two wattmeters is the total delivered power distributed across the three connected phases.
An intuitive way to understand why this is possible is to recall Kirchhoff’s laws as applied to the delta system.
Because the phase-phase voltages around the delta must sum to zero, knowing two voltages is sufficient to reproduce the third.
Similarly, the currents must also sum to zero.
Thus, three voltage and three current readings gives an overdetermined system due to the delta arrangement, and only two pairs are required for a total power reading.
This White Paper Includes: