to calculate the expected heat dissipation from a switch, add the total amount of power drawn from power supply by the system's configuration, then divide the total amount of power by the efficiency of the power supply. multiply the result by 3.415 to get the system heat dissipation in btus/hr.

    first example (system without any powered devices):

   

    all catalyst 4000/4500 power supplies have different efficiencies, an average efficiency figure of 75% was chosen.

    second example (same system but this time with one ieee class 3 device):

   

   

    although a class 3 device needs 15.4 w to power up, 17.3 w need to be generated from the backplane in order to have 15.4 w at the switch port. 17.3 w comes from the ws-x4248-rj45v dc-dc converter’s efficiency (89%).