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When do I need power over ether net (POE)?

POE has become a sort of watch word in deploying wireless access points out of doors.  In many cases it is necessary and should be considered; however, in many other cases it is an unnecessary expense.

In my work it has become necessary for me to be able to make this decision and to that end I have prepared a table which I find useful.  I put it here in the hope that it would also be useful to others.

Wire is sold in various diameters and configurations.  The costs of the wire in an installation can surpass the cost of the hardware employed in doing POE.  I do not consider those costs here.  This table simply tells me how much of one wire I may use to replace one foot of the wire which came attached to my "wall wart" power supply.  Your access point ships with a "wall wart" power supply.  Attached to that power supply is a "zip" wire that leads to the power plug which provides the connection to the access point.  Typically that "zip" wire is #20 AWG braided copper.  That wire will be marked and you should use the markings on that wire to look into the table below to decide how much of some other wire can be used to replace *ONE FOOT* of the existing wire.

Reading down the left hand side of the table, find the wire currently attached to your power supply.  Suppose mine is #18 AWG (next to last row).  You can now read across to the type of wire you wish to use to replace the #18 AWG, I am going to replace the #18 AWG wire with #12 AWG wire.  Under the column marked #12, I see that I can replace one foot of #18 AWG with four feet of #12 AWG.  I will be replacing six feet of #18 AWG therefore I can cut six feed of #18 from the power supply and splice in 24 feet of #12 AWG.  This will extend my cable by 18 feet (24'-6') and the resistance properties of the power cable will not change.
 

#10 #12 #14 #16 #18 #20
#10 1.0' 0.63' 0.40' 0.25' 0.16' 0.10'
#12 1.6' 1.0' 0.63' 0.40' 0.25' 0.16'
#14 2.5' 1.6' 1.0' 0.63' 0.40' 0.25'
#16 4.0' 2.5' 1.6' 1.0' 0.63' 0.40'
#18 6.4' 4.0' 2.5' 1.6' 1.0' 0.63'
#20 10.1' 6.4' 4.0' 2.5' 1.6' 1.0'

Another example:  I have a Linksys WAP-11.  It has attached to the power supply six feet of #20 AWG "zip" wire.  I need to extend the power cable to forty feet.  I can replace one foot of #20 AWG with 10.1 feet of #10 AWG so I could remove four feet of the existing #20 AWG and replace it with 44 feet of #10 AWG.

You must remember that you are both removing and adding cable therefore the total length of your extension will be the length added minus the length removed plus the length not modified.

It's not all that complex really.  You will discover that *MOST* home installations will not require POE.

Hope this is useful.

Michael
osiris@deltaville.net