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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Starting with Resistance
One thing is sure: The medium we have available is copper within a cable. The only other thing certain is we have to share it with data (phantom power techniques).
Let us write the equations for predicting the resistance of a certain length of wire of a certain wire gauge (AWG). The equation which relates diameter in mils to AWG is
image
The following equation looks like the correct equation, but is actually an approximation (metric is alien to AWG and mils):
image
To go from diameter in mils to mm we use
To calculate resistance ...

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