MR57 require more than af or at at full operation if you are not using the USB ports you should budget 30 W of power for each MR57 at a minimum. 802.3at at 30wx21=630W. Cable lengths per AP will vary drastically, best practice is to assume the maximum draw. as your APs power will fluctuate depending on the Tx power and number of users connected, channels in use, etc. etc. etc. etc. A 355-48X should be using a 1025W PS not a 740...
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