PoE Power Budget Calculator
Also known as PoE Power-Budget Planner
Add up the PoE load on each switch at the power its ports reserve, and see the headroom and ports left.
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Worked example: 22 devices on a 370 W switch
Headroom: −56.4 W
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How it works
PoE power budget calculation
A PoE switch has one power budget shared by all its PoE ports, and it often runs out of watts before it runs out of ports. Each device counts at the power its port reserves for its IEEE 802.3 class, the PSE power: 15.4 W for an 802.3af Class 3 device, 30 W for 802.3at Class 4 and up to 90 W for 802.3bt Class 8.
The port reserves more than the device receives, the PD power, because the cable turns some of it into heat, so sizing the switch on the port figure counts that loss. Where a device's rated draw is higher than its class power the larger figure is used, and a device with no class counts at its rated draw.
Pdev = max(Pclass, Prated)
Pused = Σ n × Pdev
Headroom = Pbudget − Pused
Utilisation = 100 × Pused / Pbudget
Ports used = Σ n
- Pclass
- power the port reserves for the device's IEEE 802.3 class (PSE), in watts
- Prated
- the device's rated draw from its datasheet, in watts, where it gives one
- Pdev
- load one device puts on the switch, in watts
- n
- number of identical devices on the switch
- Pused, Pbudget
- PoE power used, and the switch's PoE budget, in watts
Standards
- IEEE 802.3af (PoE), Classes 0 to 3
- IEEE 802.3at (PoE+), Class 4
- IEEE 802.3bt (PoE++), Classes 5 to 8
Worked example
Worked example: 22 devices on a 370 W switch
A 24-port PoE+ switch with a 370 W PoE budget and 30 W per port, feeding 16 fixed cameras at 802.3af Class 3 and 6 access points at 802.3at Class 4.
| Step | Working | Result |
|---|---|---|
| Cameras, Class 3 | 16 × 15.4 W | 246.4 W |
| Access points, Class 4 | 6 × 30 W | 180.0 W |
| Power used | 246.4 + 180.0 | 426.4 W |
| Utilisation | 426.4 / 370 | 115.2 % |
| Headroom | 370 − 426.4 | −56.4 W |
The switch is 56.4 W over its budget with 2 of its 24 ports still free. Sized on the power the devices receive instead, 12.95 W and 25.5 W, the same load comes to 360.2 W and looks as if it fits, but the switch also has to supply what the cables lose.
What are the PoE class power levels for 802.3af, 802.3at and 802.3bt?
IEEE 802.3 gives each class a power at the switch port (the PSE): 4 W for Class 1, 7 W for Class 2 and 15.4 W for Class 3 under 802.3af, 30 W for Class 4 under 802.3at, and 45, 60, 75 and 90 W for Classes 5 to 8 under 802.3bt. A device that gives no class, Class 0, gets the Class 3 power. The device itself receives less, because the cable takes its share: 12.95 W at the top of 802.3af, 25.5 W at 802.3at, and 51 W and 71.3 W at the top of 802.3bt Types 3 and 4.
Why size a switch on the PSE figure rather than the device draw?
A switch reserves power at the port (the PSE) above what the powered device (the PD) receives, to allow for cable loss. An 802.3af Class 3 port reserves 15.4 W to deliver 12.95 W, so sizing on the device figure under-counts the load by about 16 per cent. The worst case view uses the port reservation so the plan matches what the switch actually allocates.
How much power does a PoE cable lose?
The cable loses the current squared times its loop resistance, I²R, as heat. IEEE 802.3 sets each class's power at the port above the power at the device to cover a worst case channel: an 802.3at port supplies 30 W at 600 mA, and over the 12.5 Ω worst case loop the cable takes 0.6² × 12.5 = 4.5 W, leaving 25.5 W at the device. 802.3af allows 15.4 W at the port for 12.95 W at the device, and 802.3bt Class 8 allows 90 W for 71.3 W. This planner sizes each switch on the port figure, so the allowance is already in the budget. For the loss on an actual run, the Structured Cabling Channel Planner works out the loop resistance, voltage drop and power at the far end.
What is the difference between worst case and device draw?
Worst case sizes each device at the larger of its PoE class power at the port and its rated draw, which is the figure a switch power supply must plan for. Device draw uses the device's own figure where it is known; that is measured at the device, so the port also supplies the cable loss and the switch carries more. Size the switch on worst case.
Does the planner check cable length or voltage drop?
No. The planner sizes PoE against the switch budget and port limits only. Cable run voltage drop, injector selection and midspan design are outside its scope.
Does my design leave my browser?
The budget is calculated in your browser. Catalogue switches and devices are loaded from the noIM₃ catalogue on our server, but the design you build is not sent anywhere.
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