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Max length of a USB power cord

Last response: in Components
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February 20, 2013 7:37:07 PM

Hello,

I am looking to determine the maximum length of a USB cable that will be solely used as a 5V bus. The current draw is 500mA, and voltage drop of no more than 0.25V
a c 128 ) Power supply
February 20, 2013 8:18:37 PM

Welcome to Tom's Hardware Forums,
Although the formula depends upon wire gauge and I have not checked what size the wires are in a USB cable, assuming 22ga stranded copper, you should be able to go about 15', thicker wire will get you further, thinner not as far. Here's a handy calculator I found for that kind of stuff... http://www.bdbatteries.com/wirerules.php
Although 5v is skipped in their calculator, checking 6 and 4 volts can give you an interpolating point. Here's what they say if you want to do the math yourself "Calculate Voltage Drop in Wire
To calculate voltage drop, plug in the values: V = DIR/1000
Where I is the amperage,
R is from the ohms/10' X 100 column left,
and D is the total distance the current travels
(don't forget to add the length of the neutral and hot together - ie: usually double cable length). Design rules in the CEC call for a maximum voltage drop of 6% (7V on 120V circuit)."
Hope it helps
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