Question

Why we don't use 3-wattmeter method in 3-phase power measurement?

Answer 1

Answer:

Should the power factor become very low one watt-meter meter will start to read negative which is the case if the load is highly inductive or capacitive but not if pure resistance. see Measurement of Three Phase Power for maths. and Measurement of Three-Phase Power by Two-Wattmeter Method for proof..