Security & Surveillance
PoE Power Budget Reference
IEEE 802.3af/at/bt standards, PSE vs PD watts, cable loss factors, typical device draws and headroom guidance for PoE switch sizing. Use alongside the PoE Power Budget Calculator.
Last updated: July 2026
Terms
- AWGAmerican Wire Gauge
- The American sizing system for wire thickness. Confusingly, a smaller AWG number means a thicker wire — AWG 10 is thicker (and carries more current) than AWG 14.
- PoEPower over Ethernet
- A way to send electrical power and network data through the same network cable, so devices like cameras and Wi-Fi access points don't need a separate power cable or nearby outlet. It comes in power tiers — af (lowest), at, and bt (highest) — matched to how much power a device needs.
The Budget Check
Source W per device = class W × (1 + cable loss) × quantity
Usable budget = switch PoE budget × (1 − headroom%) [× 0.8 thermal derate]
Utilization = total source W ÷ usable budget — plan for ≤ 80%; above 100% the switch cannot power everything.
The switch must deliver more watts at the source (PSE) than the device consumes (PD) — the difference is lost as heat in the cable. Total budget being sufficient does not guarantee each device works: every port must also support the device's class.
IEEE PoE Standards
| Standard | Common name | PSE output (source) | PD input (device) | Pairs used | Typical devices |
|---|---|---|---|---|---|
| 802.3af (Type 1) | PoE | 15.4 W | 12.95 W | 2 | Fixed cameras, VoIP phones, door readers |
| 802.3at (Type 2) | PoE+ | 30 W | 25.5 W | 2 | PTZ / IR cameras, Wi-Fi 5/6 APs, intercoms |
| 802.3bt (Type 3) | PoE++ / 4PPoE | 60 W | 51 W | 4 | Multi-sensor cameras, Wi-Fi 6E/7 APs, small NVRs |
| 802.3bt (Type 4) | PoE++ / 4PPoE | 90 W | 71.3 W | 4 | PTZ with heater/wiper, thin clients, displays |
PSE = Power Sourcing Equipment (the switch/injector). PD = Powered Device. Budget maths always uses PSE (source) watts.
Cable Loss Planning Factors
Approximate planning-level share of power lost in the run, scaled linearly with length up to the 100 m channel limit.
| Cable | Loss at 100 m | Loss at 50 m | Loss at 25 m | Note |
|---|---|---|---|---|
| Cat5e | ~15% | ~7.5% | ~3.8% | Highest conductor resistance — worst case for PoE |
| Cat6 | ~10% | ~5% | ~2.5% | Good default for camera and AP runs |
| Cat6a | ~8% | ~4% | ~2% | 23 AWG conductors — lowest loss, best for 802.3bt |
Power is lost as I²R heat in the conductors. Higher-power classes (bt) and bundled cables in warm ceilings increase the effect — prefer Cat6/Cat6a for high-power PoE.
Typical Device Draws (planning values)
| Device | Typical draw | Plan with class | Watch out for |
|---|---|---|---|
| Fixed dome / bullet camera | 4 – 10 W | 802.3af | IR illuminators at night (+2 – 6 W) |
| PTZ camera | 12 – 25 W | 802.3at | Motor peaks, heater in outdoor housings |
| Multi-sensor / PTZ with heater | 25 – 50 W | 802.3bt T3 | Cold-weather heater start-up |
| Wi-Fi 5/6 access point | 10 – 20 W | 802.3at | Full radio load; USB add-ons |
| Wi-Fi 6E/7 access point | 20 – 35 W | 802.3bt T3 | May degrade features on an at port |
| Door controller / reader | 3 – 13 W | 802.3af | Lock strike inrush |
| Video intercom | 8 – 15 W | 802.3at | Speaker peaks |
Always verify the actual maximum draw in the device datasheet — planning with the full class ceiling (as the calculator does) is conservative and safe.
Headroom & Derating Guidance
| Situation | Recommended reserve | Why |
|---|---|---|
| Standard indoor install | 20% | Inrush at cold start, measurement uncertainty, small adds |
| Outdoor cameras with IR / heaters | 25 – 30% | Night-time IR and heater load well above daytime draw |
| Growing site (planned expansion) | 30%+ | Every later device otherwise forces a switch swap |
| Warm / unventilated cabinet | + thermal derate (×0.8) | PSU output falls with temperature; vendors derate budgets in hot enclosures |
Frequently Asked Questions
Why is the switch budget smaller than ports × class watts?
Vendors size the shared PoE power pool for realistic mixes, not for every port at full class draw. A 24-port PoE+ switch would need 720 W for all ports at 30 W — most ship with 195 – 400 W pools. That is exactly why a budget check matters before rollout.
Can I mix af, at and bt devices on one switch?
Yes — classes negotiate per port. The two constraints are the shared pool (total budget) and per-port class support. A bt device on an at-only port will either not power up or fall back to a reduced-power mode.
What happens when the budget is exceeded?
Depends on the switch: most deny power to the next device that requests it (usually by port priority), some shed low-priority ports. Either way, devices drop unexpectedly — plan to stay at or below 80% of the usable budget.
Do PoE injectors count against the switch budget?
No — a midspan injector powers its device from its own supply, bypassing the switch pool. Injectors are a practical fix for one or two high-power devices on a switch with a tight budget.