Power over Ethernet (PoE) Budget Calculator
Check a switch's PoE budget against connected devices using 802.3af, at and bt class limits, with cable loss included.
Inputs
Power Required at the Switch
483.8W
Worst-Case Power if All Ports Draw Full Class
806.4W
PoE Budget Used
65.4%
Ports Supported at Full Class
22
Budget Status
Adequate for average draw, but oversubscribed at full class
Step by step
Values used
Switch PoE budget = 740 W; Powered devices = 24 devices; PoE standard per port = 802.3at Type 2 (PoE+) — 30 W; Average draw as a share of the class limit = 60 %; Cable dissipation loss = 12 %
Power over Ethernet (PoE) Budget
required = devices × class limit × average draw share × (1 + cable loss); ports at full class = budget ÷ (class limit × loss factor).
Power Required at the Switch
= 483.8 W
Worst-Case Power if All Ports Draw Full Class
= 806.4 W
PoE Budget Used
= 65.4
Ports Supported at Full Class
= 22
Budget Status
= Adequate for average draw, but oversubscribed at full class
How it works
PoE standards define the power delivered by the sourcing equipment, and part of it is lost as heat in the cable before it reaches the device. Because most devices draw well under their class limit, the average figure sizes normal operation while the worst-case figure is what a full-load event demands. PoE budget exhaustion does not fail gracefully: the switch sheds power from the lowest-priority ports, and the cameras or access points that drop are rarely the ones you expected.
Formula
Power over Ethernet (PoE) Budget
required = devices × class limit × average draw share × (1 + cable loss); ports at full class = budget ÷ (class limit × loss factor).
- class limit
- 802.3af 15.4 W, at 30 W, bt Type 3 60 W, Type 4 90 W at the PSE
- cable loss
- Power dissipated in the twisted pair, typically 10–15%
Frequently Asked Questions
How is Power over Ethernet (PoE) Budget calculated?
required = devices × class limit × average draw share × (1 + cable loss); ports at full class = budget ÷ (class limit × loss factor). PoE standards define the power delivered by the sourcing equipment, and part of it is lost as heat in the cable before it reaches the device. Because most devices draw well under their class limit, the average figure sizes normal operation while the worst-case figure is what a full-load event demands.
Why does Power over Ethernet (PoE) Budget matter?
PoE budget exhaustion does not fail gracefully: the switch sheds power from the lowest-priority ports, and the cameras or access points that drop are rarely the ones you expected.
What values do I need to enter?
This calculator takes 5 inputs: Switch PoE budget, Powered devices, PoE standard per port, Average draw as a share of the class limit, Cable dissipation loss. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
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