[Asterisk-Users] # IP601's with POE per Catalyst 3560G-48PS

[Rich Adamson]

Jay Wilton wrote:
> Hello people,
> 
> I am having difficulties figuring out the POE power draw in
> watts from a Polycom IP601.  I want to know how many
> IP601's can be powered from the Cisco Catalyst 3560G-48PS.
> 
> The IP601 wallwart has: Input 120VAC 60hz 19W, Output 24VDC
> 500mA.  I assume the output is appropriate value to figure
> out how many phones can be powered.

Not sure where the above numbers came from, but the IP600's wall wart is 
rated at 12 vdc at 400 ma (or 4.8 watts max rated power).

I don't think you can assume the phone will draw the same amount of 
power as the maximum rated (500ma) shown above. That value is the rated 
current draw as stated by the manufacturer (essentially, the 
not-to-exceed current draw). Its very common to use an over-rated power 
supply to keep from burning them up.

If I touch the wall wart on an IP600, its warm but not hot. That would 
suggest it's running at something less then max rated draw. (I know, not 
very scientific, but its a relative indicator.)

I'd run the calculations again using 300 ma as a reasonable guess, and 
see where that takes you. If you want something more accurate, cut one 
of the wires on a wall wart and measure it with a $20 radio shack 
voltmeter. Cheap investment considering the cost of the 3560.

> The Cisco 3560 datasheet says "the 48-port PoE
> configurations can deliver the necessary power to support
> 24
> ports at 15.4W, 48 ports at 7.7W, or any combination in
> between"  ...370 Watts POE max output.
> 
>>From a UPS planning site:
> Volts * Amperes = VA
> VA * .8 = Watts
> 
> To convert small units into large units you divide.
> Amperes = 1000/500mA = .5A
> 
> VA = 24V * .5A = 12VA
> Watts = 12VA * .8 = 9.6 Watts
> 
> 370 Watts maximum output / 9.6 Watts/phone = 38 phones
> Does this logic hold water or change with line loss?

You don't have to go through the above gyrations to calculate watts.
Watts = volts * amps
Watts = 24 * .5 (which is 500 milliamps, using your numbers provided)
Watts = 12 watts
Again, that is the max rated power of the wall wart, not what the phone 
actually consumes.