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Old 21st September 2003, 09:33 PM   #1
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chris ma's Avatar
Join Date: Sep 2001
Location: Pickering, Canada
Default Long and Short Power Cables

It seems to be the norm to accept long AC cables for all electronics, but why it is not so with DC cables especially with high current?

Can someone please kindly enlighten me.

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Old 22nd September 2003, 03:10 AM   #2
haldor is offline haldor  United States
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Join Date: Aug 2001
Location: Columbus, Ohio
It's more about high voltage verses low voltage than it is AC verses DC. The reason why this is normally more of an issue with DC is that DC is typically used at lower voltages than AC (ignoring tube circuits).

To explain why, remember that Power = Volts x Amps. So when you cut the voltage in half you must double the amps to deliver the same power. For losses in a cable however, it is worse than this simple linear relationship. Restate the power formula using Ohms law (V = I x R) and you get Power = Amps^2 x Resistance. Double the amps flowing in a cable and you increase the resistive losses in that cable by a factor of 4. More amps requires either a fatter or shorter cable to deliver the same power at the same level of losses.

That's why long distance power lines use such high voltages (500,000V or higher). AC makes it a lot easier to convert voltage levels efficiently, so that's the main reason why power companies use it (plus you can make brushless motors with AC, a brushless DC motor is simply an AC motor with a DC to AC inverter).

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