PoE power budgeting for cameras, displays and access devices

Power over Ethernet is the reason a modern AV and security job runs on so few power outlets. One cable carries data and power to a camera, an access point, a door controller, a small display. It is also the reason installs stall on power-up day, when the last four cameras on a fully cabled switch simply will not come online and nobody can see why.

Two budgets, not one

Every PoE switch enforces two separate limits, and confusing them is where the trouble starts. The first is per-port: a port negotiates a class, 802.3af up to about 15 watts, 802.3at up to about 30, 802.3bt higher still, and it will not deliver more than the connected device can pull. The second, and the one that actually bites, is the total power budget across the whole switch. A 24-port switch does not supply full PoE on all 24 ports at once. Its shared budget is finite, and once you cross it the switch starts refusing power to ports even though every one of them is cabled and configured correctly.

The trap: nameplate versus real draw

Budgeting from the maximum class rating overstates the load and pushes you to buy a bigger switch than you need. Budgeting from the typical draw understates the peaks, and PoE devices peak hard at exactly the wrong moment: a camera pulls its highest wattage when the IR illuminators fire at dusk, a PTZ when the motor drives, a display at full brightness. Budget for the peak, not the idle. The gap between the two is where a switch that looked fine at commissioning falls over on the first dark evening.

Add it up before you hang anything

The reconciliation is simple arithmetic, but it has to happen before the devices are on the wall, not after. List every PoE load on the switch, take each one's peak draw, sum them, and leave headroom under the switch's total budget for the inrush when everything powers up together. Our PoE budget calculator does the sum and flags when you are over, so you catch it in design instead of on a ladder.

Cable length quietly taxes the budget

PoE is delivered over copper, and copper has resistance, so a long run loses some of the power to heat before it reaches the device, the same physics behind voltage drop on any low-voltage run. The device still needs its rated wattage at the far end, which means the switch has to source a little more at the near end. On runs approaching the 100 metre limit this is not negligible. Budget with a margin and long runs stay inside it.

Keep the budget tied to the parts list

The reason PoE budgets drift is that the device count changes after the switch is chosen. Two more cameras get added in a design revision and nobody re-checks the total. When the PoE load lives in the same bill of materials as the rest of the job, adding a camera updates the power sum in the same move, and the switch that was specified is still the switch that works. Get it right on paper and power-up day is boring, which is exactly what you want it to be.