PoE Budget Calculator

Size a PoE switch budget properly: per-device class draw, cable loss over distance, total switch load and the headroom left for growth.

Calculator Cabling & Power Runs in your browser
From the datasheet, not the class maximum.
Try:

Result

Power per port (at PSE)
12.65 W
Cable loss per run
0.65 W
11.26 Ω loop at 60 m
Total switch load
303.6 W
Budget utilisation
82%
Headroom
66.4 W
≈ 5 more devices
Standard limit per port
30 W PSE / 25.5 W PD
PoE budget used304 of 370 W

Working

ItemValue
Devices24
Device draw12 W
Current per port (approx, 50 V)240 mA
Loop resistance11.26 Ω
Loss as % of device power5.4%
Limited headroom
Above 75% utilisation there is little room for growth or a failed power supply in a redundant pair.

About PoE Budget Calculator

PoE budgets fail in two ways: adding up class maximums instead of real draw and over-provisioning, or forgetting cable loss and under-provisioning. This calculates the power the switch actually delivers, including the resistive loss in the run - which on a long 24 AWG cable is not a rounding error.

Negotiation, classes and what the switch reserves

A powered device signals its class during detection, and the switch reserves that class maximum from its budget whether or not the device draws it. LLDP-MED and 802.3bt allow a finer negotiation that can release the difference back, but only when both ends support it. That gap is why a switch reports its budget as nearly full while the measured load is half of it — and why the fix is often enabling LLDP power negotiation rather than buying a bigger power supply.

Class maximum versus real draw

A device negotiates a class that reserves power at the switch, but draws only what it needs. A camera in Class 4 reserves 30 W and may consume 9 W. Budget on measured or datasheet draw, and remember the switch reserves on class - so a rack full of over-classified devices exhausts the budget on paper before the load exists.

Common use cases

  • Sizing a switch before ordering for a camera or AP deployment.
  • Checking whether a 90 m run can still power a Type 3 device.
  • Explaining why a switch shut ports down after two more APs were added.

Edge cases and gotchas

  • 802.3bt Type 3 and 4 use all four pairs; two-pair-only cable or a splitter will not carry them.
  • PoE budgets derate at high ambient temperature - check the datasheet curve for hot comms rooms.
  • Cable loss scales with the square of current, so high-power devices on long runs are hit hardest.

Frequently asked questions

Why does a 25.5 W device need 30 W at the switch?
The standard guarantees the PD figure at the far end of a worst-case 100 m run. The difference is the power dissipated in the copper.
Can I run PoE over 100 m?
Not within the standard. Ethernet is limited to 100 m and PoE assumes that maximum. Beyond it, use fiber with a local injector or a PoE extender that re-clocks and re-powers.