The short definition

PoE injects DC power onto two or four pairs of a Cat5e, Cat6, or Cat6a cable alongside the Ethernet data signal. A PoE switch (power-sourcing equipment) and a PoE-capable device negotiate the wattage at link-up. If the device asks for more than the port can deliver, the link doesn't come up. That negotiation keeps a 90-watt camera from melting a 15-watt switch port.

For IP camera installs, PoE replaces a separate 12V or 24V power run from a wall transformer to each camera. One Cat6 cable carries data and power, terminating at a PoE switch in the IDF closet. That single change made the modern IP-camera install economically practical at scale.

The three IEEE standards

  • IEEE 802.3af (PoE), 2003. Up to 15.4W at the source, 12.95W usable at the device. Two-pair power delivery. Enough for fixed-dome cameras, basic access points, VoIP phones. Most enterprise switches still support af for backward compatibility.
  • IEEE 802.3at (PoE+), 2009. Up to 30W at the source, 25.5W at the device. Two-pair. The current default for modern fixed and varifocal cameras, including most outdoor units with heaters.
  • IEEE 802.3bt (PoE++), 2018. Two power classes. Type 3 delivers up to 60W at the source, 51W at the device, four-pair. Type 4 delivers up to 90W at the source, 71W at the device, four-pair. Required for PTZ cameras, multi-sensor cameras, integrated lighting, and some Wi-Fi 6E access points.

Source: IEEE 802.3 working-group specifications. The newer 802.3bt also formalized two-event physical-layer classification, which lets the device and switch agree on power class without firmware-level negotiation.

Camera implications

Power class drives the camera-to-switch port match. Quick reference for typical IP-camera power draws (averaged across Axis, Hanwha, Avigilon, and Verkada datasheets):

  • Fixed indoor dome. 4 to 8W. PoE (af) port is fine.
  • Outdoor varifocal bullet. 8 to 13W in cold weather with heaters off, 18 to 25W with heaters on. PoE+ (at) port required for most exterior installs.
  • PTZ camera. 25 to 60W depending on motor load and IR. PoE+ (at) at the low end, PoE++ (bt) Type 3 for typical PTZs, Type 4 for the largest models.
  • Multi-sensor (4x or 8x lens) camera. 25 to 50W. PoE++ (bt) Type 3 typical.
  • Outdoor camera with integrated lighting. 60 to 90W with the lights on. PoE++ (bt) Type 4 required.

Specifying a 30W port for a PTZ that needs 60W is the most common install mistake. The camera will appear to work and then power-cycle the moment the heater kicks on or the PTZ motor moves under load. Datasheets list the worst-case draw; that's the number to match against the switch port budget, not the typical-draw figure.

Switch PoE budget

A switch's per-port maximum is one number; its total PoE budget across all ports is another, and it's where buyers get caught. A 24-port PoE+ switch advertises 30W per port but might have a total budget of 370W. Twenty-four cameras at 18W average is fine (432W exceeds budget; some ports throttle). Twenty-four at 25W average overcommits by 230W and the switch refuses to power some.

Before scoping the switch, sum the worst-case PoE draw across every port carrying a camera, access point, or other powered device. Add 20% headroom for cold-weather heater draw and future expansion. That's the budget to specify, not the per-port number.

For larger installs, mid-span PoE injectors or dedicated PoE switches at each IDF rebalance the budget without an oversized core switch. Tec-Tel installs typically use 48-port PoE+ or PoE++ switches at each IDF, sized for 80% load plus headroom.

Distance, surge, and other constraints

PoE inherits the Ethernet 100-meter (328-foot) maximum cable run. Beyond 100m, the link drops or the power voltage sags below the device's tolerance. Three workarounds: a PoE extender (a small repeater that re-injects power, gets you to 200m), a fiber run with a PoE injector at the remote end, or a dedicated mid-span PoE switch closer to the camera.

Outdoor cameras need surge protection on the PoE line. A direct lightning strike anywhere on the cable run will fry the camera and the switch port if there's no surge suppressor inline. The standard practice is a PoE surge protector at the camera side and another at the switch side, with both grounded to the same building ground. We've seen six-figure switch failures from a single uncomforted lightning event.

Cable spec also matters at the upper PoE classes. PoE++ Type 4 (71W at the device) drives meaningful current through the cable and benefits from Cat6a or better, especially on runs near the 100m limit. Cat5e works for PoE and PoE+ but starts to lose efficiency at the higher power classes.

Where PoE shows up in real installs

Three patterns drive most of our PoE conversations:

  • Adding a PTZ to a fixed-camera install. The existing PoE+ switch budget is fine for the fixed cameras and short on the new PTZ. Solution is a single PoE++ port at the IDF, not a full switch swap.
  • Outdoor parking-deck cameras with heaters. Cold-weather heater draw doubles the worst-case wattage. Specify PoE+ ports and budget for 80% load with 20% headroom, not the steady-state draw.
  • Multi-site retail rollout. Standardized switch spec across 200 stores. PoE budget per store sized for the worst-case camera count plus a buffer for AP additions. We've found rebuilds where the original integrator sized the switches for the camera count and forgot the access points.

For installs where the IDF and PoE budget question shows up early, see the AI video analytics service page (analytics camera draws can run higher than expected) and the vendor comparison matrix for camera power-class data.