PoE — Power over Ethernet

Power over Ethernet (PoE) delivers DC electrical power through standard Ethernet cables alongside data, eliminating the need for separate power supplies at each networked device. In AV installations, PoE simplifies cabling for cameras, IP intercoms, speakers, wireless access points, and control panels — often reducing installation cost and time significantly.

How PoE Works

PoE injectors or PoE-capable switch ports supply voltage (typically 44–57 VDC) over the Ethernet cable. The powered device (PD) includes circuitry to detect PoE presence, negotiate the power class, and convert the voltage to whatever the device requires internally.

The power sourcing equipment (PSE) — the switch port or midspan injector — first performs a detection handshake to confirm the connected device is a legitimate PoE device before applying power. This detection prevents damage to non-PoE equipment accidentally connected to PoE ports.

Power Negotiation (LLDP and CDP)

After the initial IEEE detection handshake, many devices use LLDP-MED (Link Layer Discovery Protocol — Media Endpoint Discovery) or Cisco's proprietary CDP (Cisco Discovery Protocol) to negotiate precise power requirements. This allows a device to request exactly the wattage it needs rather than negotiating to the maximum class wattage.

For example, a camera rated as PoE+ Class 4 (maximum 30W) might use LLDP-MED to negotiate for only 18W, freeing up PoE budget on the switch for other devices. This power negotiation happens automatically when both the switch and device support LLDP-MED.

Power travels over the cable's conductor pairs. Older PoE standards (802.3af/at) used two pairs; 802.3bt (PoE++) uses all four pairs to deliver higher wattages.

PoE Standards

Note the difference between PSE (switch port) wattage and PD (device) wattage — cable resistance causes power loss along the run. A device requiring 25 W at its input needs a PoE+ port, not a standard PoE port.

For new AV installations, specify CAT6 or better for all PoE runs. Higher-gauge conductors reduce resistance losses, delivering more available power at the device end of long runs.

Common AV Devices and Their PoE Requirements

Always consult the manufacturer's datasheet for exact PoE class and wattage requirements. Marketing materials sometimes understate power draw; the datasheet's "maximum power consumption" figure is what you should use for planning.

Planning PoE Switch Budget

Every PoE switch has two power figures:

1. Per-port maximum — the highest wattage the switch will supply to a single device
2. Total PoE budget — the aggregate wattage the switch can supply across all ports simultaneously

A 24-port switch rated for PoE+ (30W per port) might have a total PoE budget of only 185W. That's enough to run about 7 ports at full PoE+ load — not 24. If you connect 24 PoE+ devices drawing 20W each, you need 480W total, which exceeds the switch's budget.

Planning process:

1. List every PoE device with its maximum draw from the datasheet
2. Sum the total PoE load
3. Add 20% headroom for future devices and power fluctuations
4. Specify a switch whose total PoE budget meets or exceeds this figure

For large installations, stackable switches or distributed IDF closets with separate PoE switches may be more economical than a single high-budget core switch.

Example calculation for a medium conference room:

This room needs a PoE switch with at least 182W total budget and at least one 802.3bt Type 3 or Type 4 port for the video bar.

Midspan Injectors

When you have an existing switch that isn't PoE-capable, a midspan injector can add PoE to individual ports or entire cable runs. Single-port injectors are common for adding PoE to one device; rack-mounted midspan units provide PoE to 8, 16, or 24 ports from a central location.

Midspan injectors are also useful for delivering 802.3bt PoE++ to devices on switches that support only 802.3af/at. Verify the injector is compatible with the switch's data path and that the cable run meets length requirements.

PoE and Cable Distance

Standard Ethernet maximum run is 100 meters (328 ft). For PoE devices at or near that distance, cable resistance increases power loss. The voltage drop across a 100m run of 24 AWG CAT5e can reduce available power at the device by 5–15% compared to the switch port output.

In practice: for high-power PoE++ devices more than 60 meters from the switch, verify the device's operating range on the cable you've specified. Consider running a shorter cable home-run to an IDF switch closer to the device rather than stretching a long run.

CAT6 vs. CAT5e for PoE:

CAT6 uses thicker conductors (23 AWG vs. 24 AWG in CAT5e), which reduces resistance. For 802.3bt Type 4 devices at long runs, CAT6 or CAT6A provides noticeably more available wattage at the device end compared to CAT5e.

Proprietary PoE Variants

Some manufacturers implement non-standard PoE (sometimes called "passive PoE") that applies voltage without the IEEE negotiation handshake:

• Ubiquiti 24V passive PoE — used on older Ubiquiti equipment; applies 24V without negotiation
• Ubiquiti 48V passive PoE — used on some Ubiquiti airFiber equipment
• Some IP intercoms — use passive 12V or 24V PoE

These are not compatible with standard IEEE PoE switches. Connecting a passive PoE injector to a device expecting IEEE 802.3af/at can damage the device, and connecting an IEEE PoE switch to a passive PoE device can also cause damage if the device lacks detection circuitry. Always verify compatibility before mixing PoE types.

Common Pitfalls

• Exceeding total switch PoE budget — This is the most common PoE planning mistake. Devices fail to power on, or some devices power on and others don't, seemingly at random (the switch depowers lower-priority ports when budget is exceeded). Always calculate total load against total budget.
• Using CAT5e for high-power PoE++ runs — While 802.3bt technically works on CAT5e, long runs with high-current devices see significant voltage drop. CAT6 or CAT6A is strongly recommended for 802.3bt installations.
• Forgetting to derate for long runs — Standard PoE planning assumes a 100m run. Longer runs (with extenders or fiber/copper converters) may reduce available wattage below what the device requires.
• Assuming all PoE switches support 802.3bt — Many switches sold as "PoE+" support only 802.3at (30W). High-power devices like video bars and 90W cameras require true 802.3bt. Read the datasheet, not just the product name.
• Mixing IEEE and passive PoE — Connecting a passive 24V PoE device to an IEEE 802.3af/at switch (or vice versa) can damage equipment. Always match PoE type to device requirements.
• Not accounting for LLDP-MED negotiation — Some switches only supply the maximum class wattage if LLDP-MED is not negotiated. Without LLDP-MED, a device requesting 18W from a PoE+ port may receive only the default class allocation. Verify LLDP-MED is enabled on switches where precise power negotiation is important.