PoE, PoE+ and PoE++ explained: how to budget power on a switch

The mistake is reading "48-port PoE+" as "48 × 30 W". The switch has a total power budget set by its power supply, and that is almost always less than ports × maximum. Here is how to size it.

The standards

• PoE — IEEE 802.3af: up to 15.4 W at the port, ~12.95 W at the device. Enough for basic APs, VoIP phones, simple cameras.
• PoE+ — IEEE 802.3at: up to 30 W at the port, ~25.5 W at the device. Wi-Fi 5/6 APs, PTZ cameras, thin clients.
• PoE++ / 4PPoE — IEEE 802.3bt: Type 3 up to 60 W, Type 4 up to ~90 W at the port. Wi-Fi 6E/7 APs, video-conferencing kit, LED lighting.

Per-port class vs total budget

A powered device negotiates a class (0–8) so it only draws what it needs — a phone at ~6 W does not reserve 30 W. What limits you is the switch's PoE power budget (e.g. a 48-port PoE+ model may ship ~740 W). Delivering 30 W to all 48 ports would need ~1,440 W, so you budget by *actual* device draw, not the maximum.

How to size it

• List the devices and their real PoE class (from each device's datasheet).
• Sum the draw, add ~15–20 % headroom for negotiation and cable loss.
• Compare to the switch's PoE budget — not its port count. Example: the C9200-48P is a 48-port PoE+ switch; check its budget against your device mix rather than assuming 48 × 30 W.
• Plan redundancy: dual power supplies, or StackPower on the C9300-48P to share power across a stack.

Rule of thumb

Count devices by class, not ports. If the summed draw plus headroom exceeds the switch's PoE budget, move up a model, add a second PSU, or split the load across two switches.