A balanced three-phase load on 400 V, 50 Hz, three-wire RYB mains is measured by two wattmeters. W₁ is in line R, W₂ in line B. Line current is $1/\sqrt3$ A. If W₁ reads zero, find W₂ in watts.
For a balanced load, the two wattmeter readings are $V_LI_L\cos(30^\circ\pm\phi)$. One reading is zero when the relevant angle is 90°, so the power-factor angle magnitude is 60°. The other reading has magnitude $V_LI_L\cos30^\circ=400(1/\sqrt3)(\sqrt3/2)=200$ W. With the stated consuming-load convention, W₂ reads +200 W.
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