Why does an 802.3bt PSE provide 90W of power while a Class 8 PD is only rated for 71.3W?

Question

Accepted Answer

The discrepancy between the power supplied by the PSE (90W) and the power received by the PD (71.3W) is due to power loss in the Ethernet cable. 
 The IEEE 802.3bt standard accounts for this loss, guaranteeing that even with the maximum allowed cable length of 100 meters, the PD will receive the minimum required power for its class. 
 The PD's internal circuitry is designed to strictly limit its power draw to 71.3W, regardless of the available power on the line or the cable length, ensuring compliance with the standard.

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LTC4292: FAQ