IT-01

PoE Budget

IEEE 802.3af / at / bt · NEC 725 · NEC 800 · ANSI/BICSI 007-2024 · ASHRAE thermal guidelines

01 · Powered devices

Device

Nothing added yet

02 · Cabling

Cable categoryⓘ

03 · Switch

Total PoE budgetⓘ

PoE ports available

Power supplies fitted

Simultaneity factorⓘ

Growth headroom

Awaiting input7 fields left: Device · Cable category · Total PoE budget · PoE ports available · Power supplies fitted · Simultaneity factor · Growth headroom

Capacity and layout

The drawing appears when the inputs are complete

01
Budget Power over Ethernet across a switch: per-class draw for 802.3af/at/bt Types 1–4, cable-length loss, PSU redundancy, simultaneity and headroom.
02
Tip. Budget against the class the switch reserves, not the wattage the device actually draws. A switch allocates the full class to every powered port at negotiation, so a rack of devices sitting at half their class can still exhaust the supply on paper and refuse to power the next one.
03
15.4 W — Type 1 at the PSE
04
30 W — Type 2 (802.3at)
05
60 W — Type 3 (802.3bt)
06
90 W — Type 4 (802.3bt)
07
100 m — Maximum channel

IEEE 802.3 Power over Ethernet

PoE budgeting has two figures that are easy to confuse. The PSE figure is what the switch must supply and reserve for a port; the PD figure is what the device can actually draw at the far end. The difference between them is the loss allowed in the channel, and it is why cable length and category belong in a power calculation at all.

Types and classes

Each type sets a maximum at the switch and a guaranteed minimum at the device. Types 1 and 2 power two pairs; Types 3 and 4 use all four, roughly halving the channel resistance.

Use cases

Type 1 (802.3af) — 15.4 W PSE, 12.95 W PD
Type 2 (802.3at) — 30 W PSE, 25.5 W PD
Type 3 (802.3bt) — 60 W PSE, 51 W PD
Type 4 (802.3bt) — 90 W PSE, 71.3 W PD

Formula

Channel loss = I²R, where I = PD watts ÷ bus voltage

IEEE 802.3af / at / bt

Budget and headroom

A switch reserves the full class for every powered port at negotiation, whatever the device actually draws. Budget on class, then add headroom and check the supply redundancy.

Use cases

Sum class allocations, not measured draw
Add the switch’s own base consumption
Simultaneity — will everything draw at once
Headroom for growth, typically 20 %

Formula

Supply = Σ(class W) × simultaneity × (1 + headroom) + switch base

100 m maximum channel

Cable resistance is the reason a 90 m run behaves differently from a 5 m one. Thinner conductors and longer channels dissipate more of the supplied power as heat before it reaches the device, which is why Cat 5e at full length can fail to deliver a load that Cat 6A carries comfortably.