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Name: Martin
Status: other
Grade: other
Location: N/A 
Country: N/A
Date: 9/25/2005

High voltage lines are used to reduce the losses in transmission, and to reduce the size of the lines since lower current is required. Why then do the utility companies lower the voltage transmitted during peak power periods when they may not have enough power?

Dear Martin,

You are absolutely right in saying that high voltage lines can transmit an equivalent amount of power (P = IV) with a lower current I when using a higher voltage V. The power loss in the line is just I^2R = IIR (I squared R). So if you want to deliver a certain amount of power, if the line voltage is doubled, the current is halved and the power wasted in the lines is reduced by a factor of 4.

On the other hand if the line is delivering the power to a system with resistance R, the power delivered is P = V^2/R so if the voltage is reduced, the power delivered is reduced, which is exactly what a utility company wants to do when they are unable to generate sufficient power to meet the demand.

This analysis is not exact, since R may change when V is changed. For example, a lower voltage on an incandescent light bulb means the filament is not so hot, which means the resistance is higher. This effect thus reduces the power consumed even more than when the resistance is constant. The argument is otherwise correct for DC (Direct Current), but is a simplification for AC. AC (alternating current) is, of course, used in almost all power distribution systems.

Best, Dick Plano,
Professor of Physics emeritus, Rutgers University

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