Test Instruments & Measuring DevicesWATTMETERSEquation (14-16) is the mathematical representation for calculating power in an AC circuit.(14-16)PV_{Rms}I_{Rms}cosq or PI^{2}RThree-PhaseWattmeterTotal power in a 3f circuit is the sum of the powers of the separate phases. The total powercould be measured by placing a wattmeter in each phase (Figure 12); however, this method isnot feasible since it is often impossible to break into the phases of a delta load. It also may notbe feasible for the Y load, since the neutral point to which the wattmeters must be connected isnot always accessible.Normally, only two wattmeters are used in making 3f power measurements (Figure 13).Figure 12 Wattmeters in Each PhaseIn balanced 3f systems, with any power factor, total power is calculated by adding the A andB phase powers. Equation (14-17) is the mathematical representation for calculating total power(P_{T}).(14-17)P_{T}W_{A}W_{B}whereW_{A} and W_{B} are the power readings in Phase A and Phase BRev. 0 Page 21 ES-14