`Time for some ohm law here guys. The radio you are powering is a load in`

`series with a transmission line that has resistance. The ohms of the`

`line and the ohms of the load add together to determine the effective`

`load for determining the amount of current flow that will be drawn. If`

`you already know the number of amps that the radio draws then this gets`

`easier to calculate. 24 AWG wire has a resistance of roughly .0302 ohms`

`per foot. If you are going 300 foot to the load then you actually have`

`300 foot on the positive side and 300 foot on the negative side of the`

`load. This yields 600 X .0302 = 18.12 ohms. Now here is where those with`

`no electronics background may start to glaze over a bit. We actually`

`have two lines for positive and two lines for negative when using two`

`pairs of a 24 AWG cat 5 cable. When you have two identical loads in`

`parallel the resistance gets cut in half. So the effective resistance of`

`the transmission line for 300 feet of Cat 5 24 AWG cable is 9.06 ohms.`

`Let's assume the radio being powered has a current draw of 1.2 amps. The`

`current flow in a series circuit is the same throughout the circuit so`

`the voltage drop across the transmission line would be determined by`

`multiplying the amps of current times the ohms of resistance which means`

`1.2 amps X 9.06 ohms = 10.872 volts. This means that if a radio needs`

`12 volts with 1.2 amps current flow then you would need to supply a`

`minimum of about 24 volts at the POE injector. This is because if we`

`start with 24 volts and lose 10.872 across the transmission line this`

`leaves about 13.128 volts to the radio. If you do not know ohms law then`

`you need to do some reading on it. It is easier than I have described`

`here once you play with it a bit and you really do use it in your life`

`if you do anything at all with electricity. Here is a page I just`

`Googled and seems to cover most of the basics:`

http://www.awrr.com/descorn.html Cheers, Scriv

Brian Rohrbacher wrote:

I'm a little confused here. I'm working on a 300 ft run of cat5 andhave a question. The radio is acting sparatic. The power supply hasalready been upped from a 18v to a 24v. Both 1 amp. Will it hurt toput a 24v 2 amp power supply in? If I "over do" on amps or volts,what blows a radio. One or both? I seem to remember being told thata radio only takes what amps it needs, so putting a higher amp powersupply in won't hurt, but if you put too many volts in, that will frythem. Please clarify me on this.

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