PoE budgets: why the eighth camera fails

This one has a specific signature. A set of identical devices is installed, most work, and the last one or two behave oddly: they boot, run for a while, then reboot, or they never come up at all. Swapping the device changes nothing. Swapping the port sometimes helps, which sends everyone down the wrong path.

It is almost always the power budget.

Two numbers, not one

A PoE switch has a per-port maximum and a total budget, and they are not related in the way people assume. A 24-port switch advertising PoE+ on every port does not have 24 × 30 W available. It has a supply — often 180 W, 370 W or similar — and that supply is shared.

The per-port figure says what one device may draw. The total says how many of them can draw it at once. A switch supporting 30 W per port with a 185 W budget runs six devices at full draw, not twenty-four.

Work it out before ordering

The arithmetic is simple and almost nobody does it:

1. Take each device's maximum draw from its datasheet, not its typical draw. A camera's typical figure is measured in daylight; the maximum is with infra-red illuminators on, and the illuminators come on at exactly the time all the cameras need to be working.
2. Add a margin for cable loss. Power dissipates along the run, and the switch supplies more than the device receives — the longer the run, the bigger the gap.
3. Compare the total against the switch's budget, and leave headroom for the device somebody adds next year.

A pan-tilt-zoom camera is the one that catches people out: it draws modestly at rest and far more while the motor is running, which is a brief peak that happens at the worst moment.

What the switch does when it runs out

Behaviour varies and none of it is obvious from the front panel. Some switches refuse power to the next device that asks, which is at least clear. Others shed load by priority, which means an unrelated device goes dark. Others try to supply everything and brown out, which produces the reboot loop.

The last of these is the one that wastes days, because the device looks faulty and behaves faultily and is not.

How to confirm it in a minute

Every managed PoE switch will report power consumed against power available, per port and in total. If consumption is sitting near the budget, that is the answer — no further diagnosis is needed.

The fix is either a switch with a larger supply, splitting the load across two switches, or an injector for the devices that need the most. All three are cheaper than the week spent replacing working cameras.