Understanding PoE Standards: 802.3af, 802.3at and 802.3bt

Each standard is backward compatible: an 802.3at switch will power an 802.3af device, negotiating down to what the device requests. The negotiation happens at connection time, which is why a device that draws too much simply will not power up rather than browning out the switch.

PSE (Power Sourcing Equipment) is the switch or injector; PD (Powered Device) is the camera or access point. The gap between the two figures is the worst-case loss over 100 m of cable.

Always specify equipment against the PD number. A device needing 25 W requires an 802.3at port, not an 802.3af port, even though 15.4 W looks close on paper.

A switch's PoE budget is the total wattage available across all ports at once. A 24-port switch with a 370 W budget cannot run 24 devices at 30 W each — that would need 720 W.

Size the budget by adding the actual draw of every powered device and leaving margin. In the Lysora range the LCS-24GS-P carries a 370 W budget, while compact models such as the LCS-4GS-P and LCS-8GS-P suit smaller edge counts.

As a planning rule of thumb, most fixed cameras sit in the 802.3af band, while heaters, motors, and higher-power radios push devices into 802.3at or 802.3bt. Confirm each device against its own datasheet before finalising a budget.

Standards define electrical behaviour, not environment. A rooftop or roadside cabinet needs an industrially rated switch regardless of PoE type — the LS2T-8GT2SFP-HP carries a 240 W budget in a DIN-rail body rated well beyond commercial limits.