PoE Engineering Guide

PoE basics

Power over Ethernet lets Power Sourcing Equipment such as a switch deliver DC power over twisted-pair Ethernet to a Powered Device such as an AP, phone or camera.

PoE standards

Power available at the PD is lower because the cable consumes power. Exact values depend on the negotiated class and standard.

Power budgets

A switch may have a port capable of delivering the required power but still be unable to power every port simultaneously. Calculate worst-case simultaneous demand and compare it with the switch-wide PoE budget.

Classes and negotiation

PoE negotiation allows the PSE and PD to establish the required power class. Check device documentation for the required standard and class rather than assuming every PoE device has the same demand.

Design considerations

• Use worst-case device power for capacity planning where appropriate.
• Consider startup and recovery loads.
• Check cable length and environmental conditions.
• Keep spare PoE capacity for growth.
• Consider port prioritisation for critical devices.
• Check whether 2-pair or 4-pair power is required.

Troubleshooting

1. Confirm PoE is enabled.
2. Check remaining switch-wide budget.
3. Check negotiated class and requested power.
4. Inspect cable length and pair continuity.
5. Test with a known-good PD or cable.
6. Check logs for overload or protection events.