Why is PoE power output 15.4W?

When IEEE 802.3af was written, the committee had CAT3 cable in their mind. They assume it’s the worst case. And 350 mA is the maximum continuous current considered to be safe for CAT3 cabling. The typical PoE voltage is 48Vdc but may drop to 44V, which is the minimum.

Therefore, the minimum guaranteed power entering the cable is 44V × 0.35A = 15.4W.

The IEEE standard sets the overall PoE loop resistance of a 100-meter CAT3 cable at a fixed 20Ω. So cable losses are 0.35 × 0.35 × 20 = 2.45 W. Then the minimum guaranteed power entering the powered devices (PD) is 15.4W - 2.45W = 12.95W.

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