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PoE power budgets, without the brownout
A PoE switch has one power budget shared across all its ports — and it is almost always less than the ports could draw at once. This is the reference: the IEEE 802.3 classes, the port-versus-device watts that catch people out, and how to size a closet with room to grow.
One closet, budgeted
- Draw
- sum the PD watts per device
- Budget
- demand vs switch PoE capacity
- Headroom
- 20% power, 25% ports in reserve
- Warning
- flagged before anyone installs
The number that decides whether the closet holds up.
The classes
What IEEE 802.3 delivers
PoE is defined in four generations, each with a power class and two numbers that matter: what the port sources, and what the device receives.
Every powered device negotiates a class with the switch, and the class sets its power ceiling. The switch sources the higher figure at the port (PSE); the device receives the lower figure after the cable takes its cut (PD). Design against both.
| Standard | Type | Class | At the port (PSE) | At the device (PD) |
|---|---|---|---|---|
| 802.3af | Type 1 | Class 0–3 | 15.4 W | 12.95 W |
| 802.3at | Type 2 | Class 4 | 30 W | 25.5 W |
| 802.3bt | Type 3 | Class 5–6 | 60 W | 51 W |
| 802.3bt | Type 4 | Class 7–8 | 90 W | 71.3 W |
The gap between the two columns is resistive loss in up to 100 metres of copper. A Type 4 port sources up to 90 W to guarantee 71.3 W has arrived at the device — which is why a long run to a hungry PTZ or a high-power access point is where PoE quietly under-delivers.
The inputs
What a closet budget is made of
A PoE budget is arithmetic against a shared supply. Four inputs decide whether the switch holds up.
-
01
The device demand
The sum of the PD power of every device that homes to the closet, by class. A fixed dome at 12.95 W and a heater-bearing PTZ at 71.3 W are not the same load, and the budget has to see the difference.
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02
The switch’s real budget
The switch’s advertised PoE power budget — the shared figure, not ports times maximum — is the number that actually has to cover the total device demand.
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03
The headroom
Roughly 20% of the power budget and 25% of the ports held in reserve. Growth, a failed-over load, and the next camera all need somewhere to go that is not a switch swap.
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04
The long runs
On high-class devices near the 100-metre limit, the PD power still has to arrive. A budget can pass in total while a single distant device falls short.
On the platform
The closet budgets itself as you design
SiteOps Command does not wait for a spreadsheet — the budget assembles from the placed devices.
Every device carries its PoE class from the datasheet, and budgets assemble per closet from the cable-path assignments: maximum and typical draw, checked against the switch’s PoE capacity, with the 20%-power and 25%-port headroom rule applied and short budgets flagged before install. The closet details panel shows the budget live as you place devices, and the proposal prints the table with utilization status — so the power conversation happens at design time, not on the day the breaker trips.
PoE budget questions
Why is a switch’s PoE budget less than its ports times the maximum?
Because the PoE budget is a separate, shared power supply — not a guarantee on every port at once. A 48-port switch might advertise a few hundred watts of PoE, far less than 48 devices at 30 W each. The budget is the number that matters: the sum of what your devices draw has to fit inside it, with headroom, or ports stop delivering power when you can least afford it.
What is the difference between the port watts and the device watts?
The higher figure (PSE) is what the switch port sources; the lower figure (PD) is what survives the cable to reach the device. The gap is resistive loss in up to 100 metres of copper. Budget the switch against the PSE figures it sources, but confirm each device gets the PD power its class guarantees — a long run to a hungry device is exactly where PoE quietly under-delivers.
How much headroom should a PoE budget carry?
Enough that the next camera does not force a switch swap. A common rule — the one SiteOps Command applies — is roughly 20% spare power and 25% spare ports on the recommendation, so growth and a failed-over load both fit without a forklift. A budget planned to 100% is a budget that fails the day someone adds a device.
How does SiteOps Command budget PoE?
Per closet, from the design. Every device placed from the product library carries its PoE class — read from the datasheet or inferred from maximum draw — and budgets assemble from the cable-path assignments: maximum and typical draw, checked against the switch’s PoE capacity, with the headroom rule applied and short budgets flagged before install. The closet panel budgets it live as you place devices, and the proposal prints the table.
Budget PoE from the design, not a guess
Place the devices and the closet budget assembles — draw, capacity, and headroom, with the shortfalls flagged before install. Talk to us about what your team powers.