Power Adapter Barrel Size for PoE

I would note I have had great success powering many different devices from POE / High POE using an appropriate injector at the head end and splitter at the device end. In the case of the ReadyNAS Ultra/2 the power requirements as listed here -> http://www.readynas.com/?page_id=3962#TechnicalSpecifications note 35W typical with two 2TB drives. If you use a 4 pair high POE injector -> http://www2.microsemi.com/PowerDsine/Documentation/DataSheets/PD-9501G.pdf and a high power 4 pair splitter providing 12V -> http://www2.microsemi.com/PowerDsine/Documentation/Datasheets/PoE_951_Splitters.pdf you still have 19W (allowing for heat dissipation in the cable) of headroom (in case drive and fan spinup or boot draw more). By the numbers that setup should work- the supplied PS is rated at 2.5A@12V and the 4 pair splitter is rated at 2.5A@12V. The eBay item listed is a passive device that I could not recommend as low DC voltages do not survive long cable runs as well as higher voltages, hence the active splitter taking 50V-57V down to 12V.

Of course from a cost perspective the active POE devices I've linked are much higher and probably not worth the expense - unless you already had a multiport injector and really needed the closet to be POE powered only.