PoE Power Calculator

Power over Ethernet saves a lot of cabling, but every switch has a limited PoE budget. Plugging in one demanding device too many leaves ports without power, usually the last ones to come up. The page adds up what a switch must reserve for the devices you plan and tells you how much room is left.

How a switch allocates power

When a device is plugged in, the switch detects it and assigns a class, which says the most power it may draw. The switch then reserves that class’s maximum from its budget, whether the device later draws it or not. This is static allocation, and it is why the sum of class maxima, not of real consumption, decides whether everything fits. Switches that use LLDP power negotiation can reserve exactly what a device asks for, which stretches the budget; the page assumes the conservative class-based reservation.

The class table

The standards define nine classes. At the switch (the PSE) they reserve: class 0 and class 3 15.4 W, class 1 4 W, class 2 7 W (all 802.3af, Type 1); class 4 30 W (802.3at, Type 2, “PoE+”); class 5 45 W, class 6 60 W (802.3bt, Type 3); class 7 75 W and class 8 90 W (802.3bt, Type 4). Cable resistance means the device receives less: 12.95 W of a 15.4 W port, 25.5 W of 30 W, 51 W of 60 W and 71.3 W of 90 W. Both figures are in the table the page shows.

Using the page

Enter the switch’s total PoE budget from its data sheet, then the number of devices per class. Presets load a 24-port switch with a 370 W budget and a mix of cameras and access points. The result shows the number of powered devices, the power to reserve, the budget, what remains in watts and percent, the share used, and how many more devices of the largest class in use would fit. A notice appears when more than 85 percent of the budget is committed, since that leaves little headroom for devices that turn out to need more.

A worked example

The 24-port sample has 17 powered devices that require 364.8 W of the 370 W budget. That leaves 5.2 W (1 % free), the budget used is 99 %, and zero more devices of the largest class in use would fit. The page flags that more than 85 percent of the budget is reserved. The remedy is a switch with a larger budget, a separate PoE injector or midspan for the biggest devices, or switching some ports to a lower class where the device allows it.

Limits

The page checks the budget only, not the number of PoE ports, the cable length or the cable quality. Runs near 100 m, thin or copper-clad aluminium cable and bundles that heat up all lose more power than the table assumes. Devices may also draw less than their class maximum, which is why real switches often run comfortably with a budget that looks tight on paper. Use the switch’s own power report to confirm. Wireless access points and cameras are often the heaviest PoE loads, so plan them with the Link Budget Calculator and CCTV Storage Calculator in mind.

Related tools

• CCTV Storage Calculator sizes disks for the cameras you power.
• Server Rack Planner totals power, heat and weight of a rack.
• Network Cable Chart lists cable categories and length limits.
• Link Budget Calculator plans the wireless links that access points provide.