Questions · Networks
How many cameras can I plug into a switch with power over the network cable?
Almost always fewer than it has ports. The switch has a total budget of watts to share among all its sockets, and that total is set by the power supply inside it, not by the number of ports. It is written on a line of the specification sheet that rarely gets read.
An example, with example figures: a twenty-four-port switch with a budget of about three hundred and seventy watts and cameras that declare class 4 —up to thirty watts per port— runs out of budget at around a dozen cameras. And here is a detail you pay for twice: the power is reserved by the class the device declares, not by what it really uses, so a camera that asks for thirty and uses nine takes up thirty in the switch's sums. The numbers change with every model; the way of doing the sums does not.
You also have to leave headroom, because in two years' time somebody is going to add two cameras on the ports that were spare. When the budget is exceeded the device does not warn you: it leaves the last ports without power, and they are usually exactly the ones added later.
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