Power Over Ethernet Calculator

Stephen Foskett’s Power Over Ethernet (PoE) Calculator

Version 2.0

This calculator allows you to calculate the voltage drop and required power supply for homebrew Power over Ethernet hardware. What’s Power over Ethernet? Check out this page where it’s explained (for a DiY audience) better than I could.

Just enter your information in the five “Device Information” boxes and click on “Compute”. I’ve filled in the input boxes with generic information to save a second or two for testing. It does some basic sanity checking, but anyone playing with homebrew hardware should test their own sanity, too!

This app is written in JavaScript (1.0!) so you can save the page and use it without being connected to any server.

Important Note! The two most important inputs for power over anything are wire gauge and amps used. Wire gauge is easy to figure out (it’s either 22 or 24 for common Ethernet cable) but actual input amps is much trickier. You see, most devices list their maximum current draw, not their “normal” draw. So if your access point says “6V at 1.5A” and is 100 feet away on 22 gauge wire, you would be tempted to use a 7.2V supply. But the AP probably only really pulls in half an amp or less under normal conditions, so you should be feeding it just 6.4V. You can fry your equipment by putting too much voltage into it! So what to do? Simple, just measure the normal current draw when the device is in use. Don’t have the tools? Well, then, just guess. Most devices that don’t have a hard disk, Pentium 4, or other power sucker actually draw very little current, making this whole calculator less useful than you might think. Guess the actual draw by reducing the listed amps by 50% to 75%. And hope for the best!

Tested in IE 6. Different browsers may offer limited or non-functional support for JavaScript, so this calculator may not work for you. Be sure to verify the results by hand until you are comfortable with the results.

Links

Wire gauge resistance information and more information.
Power over Ethernet overview
DiY Power over Ethernet information and more information
Information on Ohm’s Law

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