PoE Budget Calculation

What the PoE standards actually deliver

Power over Ethernet sends DC power and data over the same twisted-pair cable. The power-sourcing equipment (PSE), usually a switch, feeds the powered device (PD), like a camera. Three IEEE amendments matter for security work.

The two numbers per row are not a mistake. The switch sources more than the device receives because some power is lost in the cable.

PSE power versus PD power

The gap between PSE output and PD input is cable loss. The standards assume up to 100 m of Category cable and a worst-case resistance. For 802.3af, that loss budget is about 2.45 W. For 802.3at, about 4.5 W. The standard reserves this margin so a device at the far end of a long run still gets its rated power. Shorter runs and better cable lose less, but design to the standard, not to luck.

What this means in practice: a PD that needs 25.5 W requires an 802.3at (30 W) port. You cannot run it on an 802.3af port even though the device draws less than 30 W, because af tops out at 15.4 W sourced.

Endspan versus midspan

An endspan is a PoE switch. Power and data come from the same box. This is the normal choice for new installs. A midspan, sometimes called a power injector, sits between a non-PoE switch and the device and adds power to the cable. Use midspans to add PoE to existing data switches or to reach higher wattage than the switch supports. Either way, the per-port and total-budget math is the same.

The total power budget

A switch has a total PoE budget that is usually less than the sum of all ports at maximum. A 24-port switch rated 30 W per port does not always provide 720 W. The shared power supply might deliver 370 W or 740 W depending on the model. Read the spec sheet for the total PoE budget, not just the per-port rating.

The budget formula:

Total required = sum of (each PD power draw)
Headroom = Total budget - Total required
Plan so Headroom stays positive after derating.

Derating

Do not load a PoE switch to 100 percent. Heat, supply tolerance, and future additions all argue for margin. A common practice is to keep total PoE draw at or below 80 percent of the rated budget. Hot environments and cabinets without airflow push that number lower. Also account for the camera’s peak draw, not its idle draw. Pan-tilt-zoom motors, infrared illuminators, and heaters spike well above the nameplate average.

Worked example: 16 cameras

Assume 16 fixed dome cameras that each draw 12 W at the device, with infrared on. Account for cable loss by budgeting at the PSE class.

• Each camera fits 802.3at (PoE+) for headroom on IR spikes: budget 13.5 W sourced per camera as a planning figure.
• 16 cameras x 13.5 W = 216 W required.
• Apply the 80 percent rule: 216 W / 0.8 = 270 W minimum switch budget.

A 24-port PoE+ switch with a 370 W total budget covers this with room for four to six more cameras. A switch rated only 250 W total would be over budget once IR and motors spike, even though the per-port rating looks fine.

Quick rules

• Match the PD class to a PSE class that sources enough, not just enough at the device.
• Size to the switch total budget, not the per-port number times port count.
• Plan for peak draw with IR, heaters, and PTZ.
• Keep total load at or below 80 percent.

Last updated 2026-06-14.