Why does 802.3bt deliver more power than 802.3af?

Q: How far can I run Cat 6 for PoE?

PoE distance is bounded by the same 100 m Ethernet channel, but power delivery also depends on resistive loss over that run. This calculator estimates I²R loss from the conductor gauge and length, then adds it to the device draw to find the PSE port power. Longer runs and thinner conductors raise loss, so 4-pair 802.3bt at 23 or 22 AWG carries high-wattage loads farther than 2-pair 802.3af on 24 AWG.

Q: How much power does a PoE switch need per port?

The PSE port must source the device draw plus the resistive cable loss, so per-port PSE power equals PD watts plus I²R loss in the channel. IEEE 802.3af allows up to 15.4 W per port, 802.3at up to 30 W, and 802.3bt Type 3 and Type 4 up to 60 W and 90 W. The tool also confirms the device draw stays within each standard's PD class maximum.

Question

Why does 802.3bt deliver more power than 802.3af?

Accepted Answer

IEEE 802.3af and 802.3at carry power on 2 pairs, while 802.3bt Type 3 and Type 4 parallel all 4 pairs, halving the loop resistance and cutting cable loss. This calculator models that by computing loop resistance as ρ×L for 2-pair types and ρ×L/2 for 4-pair types. Lower loss plus higher PSE limits is why Type 4 reaches 90 W per port.

Why does 802.3bt deliver more power than 802.3af?

Answers