The power delievered to a 3-phase load can be measured by the use of two wattmeters only when

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3-phase load is connected to the source of 3-wires

Explanation: The two-wattmeter method is a universal technique for measuring power in a 3-phase system provided that the system is a 3-wire system. This is based on Blondel's Theorem, which states that for an N-conductor system, N-1 wattmeters are required to measure the total power.

Working Principle: According to Blondel's Theorem, the total power in a system of N conductors can be measured by connecting the current coils of N-1 wattmeters in series with N-1 lines and connecting the potential coils between the respective line and the remaining conductor (the common return path). In a 3-phase 3-wire system, $N=3$, so $N-1=2$ wattmeters are sufficient, regardless of whether the load is balanced or unbalanced or connected in star or delta.

Key Formulas: $P_{total} = W_1 + W_2$ — Total power in 3-phase 3-wire system; $P = \sqrt{3} V_L I_L \cos(\phi)$ — Power for balanced load (Reference)

Key Points: Blondel's theorem is valid for any waveform and any load condition (balanced or unbalanced).; If a neutral wire is present (4-wire system), 3 wattmeters are required to measure total power.; The two-wattmeter method is independent of the star or delta configuration of the load.; Reading of one wattmeter may become negative if the power factor is less than 0.5 (lagging).

Advantages: Works for both balanced and unbalanced loads.; Applicable to both star and delta connected systems.

Disadvantages: Requires three wires only; cannot be used for 4-wire systems without modification.; Calculation of power factor requires both readings.

Applications: Industrial 3-phase motor power monitoring.; Utility metering in 3-phase 3-wire distribution.

Additional Info: In a 4-wire system, if the load is balanced, the neutral current is zero, and 2 wattmeters would technically suffice; however, for general unbalanced cases, 3 wattmeters are mandatory.. Option A and B are incorrect because the number of wattmeters depends on the number of wires, not the load balance.

Answer: C is correct — The two-wattmeter method is a direct application of Blondel's Theorem for measuring power in any 3-phase 3-wire system.

Exam Tip: Always remember that in electrical instrumentation, the number of measuring elements is determined by the number of lines (conductors) in the circuit, not the nature of the load impedance.

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