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

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

(a) load is unbalanced (b) load is balanced (c) three phase load is connected to the source through three wires (d) three phase load is connected to the source through four wires
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The power delivered to a 3-phase load can be measured by the use of two wattmeters only when the three phase load is connected to the source through three wires 

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