Is there a reason to use 48v power over 12v on the rb562a with dual 400mw cards?
Well… P = I * U and U = R * I is probably the best way to explain it 
To elaborate…
If you need say 10 Watt at the consuming point you would need either 12Volts * 0.83Amps OR 48Volts * 0.20Amps at that point
So what does that mean… and why is that important?
Normally it wouldn’t matter much BUT if you are using POE and having some meters cabling this could be troublesome… all cabling has a DC resistance, normally the thinner the cable the more resistance and network cables do not have thick strands of copper!
Since U = R * I (voltage loss over the cable equals the resistance times the current) a 12V POE feed would loose 4 times the voltage compared to a 48V feed.
So how high is the resistance (DC, not high frequency) of a Cat5 cable? The specification says that it should not be higher then 9.38 Ohms per 100meters… actual measurements shows that it’s somewhere between 5 and 7 Ohms.
So lets do a quick calc… say 20 meters feed = 1 Ohm, since you are going both ways you would need to count it twice so the resistance is 2 Ohms.
48 Volt feed at 0.20 Amps
- cable loss 0.20 * 2 = 0.4Volts
- volt at consuming point is 48 - 0.40 = 47.60 Volts
12 Volt feed at 0.83 Amps
- cable loss 0.83 * 2 = 1.66Volts
- volt at consuming point is 12 - 1.66 = 10.34 Volts!
As you see, the higher the current the more voltage you will loose in your transmission lines… that’s why the power company uses VERY high voltage on long power / long distance lines!
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/Jörgen
I used regular 12 or 18V for POE 50m.
On the other hand I have serious trouble using 112 and 48V. After power failure routerboard is not working. The only way is to unplug routerborard, start power supply, plug in routerboard.
That’s why I don’t use any 48V.