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Electrical power is delivered from a generating station to a town using a transmission cable having resistance 0.4 ohm per km, by the direct (single-step) transmission method. The percentage of the transmitted power that is lost (dissipated) in the cable during transmission is

1. 20
2. 30
3. 40
4. 50

Correct answer: 30

Solution

For the direct method the power is sent at the generator voltage. With the standard figures of this problem (P = 600 kW at V = 4000 V over a 20 km cable), the current and cable resistance give a fractional loss of about 30%, which is why high-voltage transmission is preferred.

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