Q3Electronic Measurement & Instrumentation

Blondel's Theorem states that the total power in an $N$-phase, $N$-wire system can be measured accurately by using $N$ wattmeters.

However, if the common connection point for all the wattmeter potential (voltage) coils is connected directly to one of the $N$ wires, the power measured by that specific wattmeter becomes zero. Therefore, only $N-1$ wattmeters are required to measure the total power.