PoE — Power over Ethernet
Power over Ethernet (IEEE 802.3af / 802.3at / 802.3bt)
For switch selection including PoE budget, see networking/network-switch-selection. For HDBaseT PoE, see glossary/hdbaset.
PoE (Power over Ethernet) delivers DC electrical power to network devices over the same Cat cable that carries data, eliminating the need for a separate power supply at the device location. In AV systems, PoE powers IP cameras, PTZ cameras, VoIP phones, wireless access points, HDBaseT receivers, Dante endpoints (select models), touchpanels, and digital signage media players. Specifying adequate PoE budget in the network switch is critical — insufficient PoE capacity causes devices to power on and off randomly, and is one of the most common commissioning surprises in AV installations.
PoE Standards
| Standard | IEEE | Power at Port | Power at Device | Pairs Used | Notes |
|---|---|---|---|---|---|
| PoE | 802.3af | 15.4 W | 12.95 W | 2 pairs | Legacy; cameras, phones |
| PoE+ | 802.3at | 30 W | 25.5 W | 2 pairs | Most common; AP, PTZ cameras |
| PoE++ (Type 3) | 802.3bt | 60 W | 51 W | 4 pairs | High-power devices |
| PoE++ (Type 4) | 802.3bt | 90–100 W | 71.3–90 W | 4 pairs | Thin clients, laptop charging |
The power at port is the injected power; the power at device is after cable resistance loss (approximately 20% loss at 100 m). Cat5e/Cat6 resistance at 100 m reduces delivered power; PoE devices declare their class (0–8) and the switch powers them accordingly.
PoE Budget and Switch Selection
Every PoE-capable switch has a total PoE budget — the maximum simultaneous power it can deliver across all ports. Common switch budgets:
- 24-port access switch: 370W (15W average per port — insufficient if all devices need PoE+)
- 48-port enterprise switch with PoE+: 740W (about 15W average per port)
- 24-port PoE+ dedicated: 480W–740W (20–30W per port — better for AV applications)
Never assume all ports can deliver maximum PoE simultaneously. A 24-port 370W switch cannot power 24 PoE+ devices at 30W each (720W required). Power budget the switch:
Total required = Σ (each device's PoE class power)
Switch budget ≥ Total required × 1.2 (20% safety margin)
AV Devices Powered by PoE
| Device | Typical PoE Class | Power |
|---|---|---|
| IP camera (1080p PTZ) | Class 3 (PoE) | 12–15 W |
| IP camera (4K PTZ, high-power) | Class 4 (PoE+) | 20–25 W |
| VoIP phone | Class 0–2 (PoE) | 3–10 W |
| Wireless access point (Wi-Fi 6) | Class 4 (PoE+) | 20–25 W |
| HDBaseT receiver | Class 3–4 (PoE/PoE+) | 13–25 W |
| Crestron touchpanel (TSW-770) | Class 4 (PoE+) | 25 W |
| Dante AVIO adapters | Class 2–3 (PoE) | 4–12 W |
| NVX decoder (PoE+) | Class 4 (PoE+) | 25 W |
Midspan Injectors and Endspan
PoE can be delivered by the switch port directly (endspan, 802.3at/bt-compliant switch) or by an inline midspan injector inserted between a non-PoE switch and the device. Midspan injectors are useful when:
- The existing switch has no PoE capability
- More power is needed than the switch's budget allows
- Specific PoE++ Type 4 (90W) is required and the switch only supports Type 3
Single-port and multi-port (4-port, 8-port) midspan injectors are available from Cisco, Phihong, and OEM sources.
Common Pitfalls
-
Switch PoE budget exhausted, devices randomly losing power. All switch ports are assigned PoE, but the total draw exceeds the switch's budget. The switch begins throttling power to lower-priority ports. Fix: calculate PoE requirements before specifying the switch; choose a switch with a budget 20% above calculated requirements; use priority settings to ensure critical devices (cameras, touchpanels) get power first.
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Using PoE for HDBaseT receiver but switch only supports 802.3af (15W). Some HDBaseT receivers require PoE+ (25W); an 802.3af switch port only delivers 12.95W at the device, causing the receiver to fail or reboot. Fix: verify the HDBaseT receiver's PoE class requirement; use a PoE+ switch or midspan injector.
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Powering PoE devices through a patch panel without verifying PoE passthrough. Some patch panels include PoE filters or are wired incorrectly for PoE (e.g., only passing 2 pairs). Fix: verify patch panels are PoE-transparent (pass all 4 pairs, no filtering); use PoE-rated patch panels and couplers.
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Running PoE over cable longer than 100 m. Power loss over cable resistance means very long runs may not deliver sufficient power to the device even if the switch budget is adequate. Fix: keep all PoE runs under 100 m; use PoE extenders (active midspan injectors inline) for longer runs.