Power over Ethernet (PoE)

Power over Ethernet (PoE) sends electrical power and data down the same network cable, so access points, cameras, phones and sensors need no separate power outlet. It simplifies installation and lets you back up many devices from one UPS in the network room. The catch is planning: you need switches with enough power budget, cabling suited to the load, and a clear idea of what each device will draw.

How PoE works

A PoE system has two sides. The power sourcing equipment (PSE) is usually a PoE switch, or a midspan injector placed between a regular switch and the device. The powered device (PD) is the access point, camera, phone or other load at the far end.

Standard PoE is negotiated. Before applying full power, the switch checks that a compatible device is connected and what class of power it needs. Many devices then fine-tune their request over the network using LLDP. This handshake is why standards-based PoE does not damage ordinary computers plugged into a PoE port.

The PoE standards

PoE is defined by IEEE 802.3. The main types are summarized below. Power at the device is lower than power at the switch because some is lost in the cable.

Types 1 and 2 use two of the four pairs in the cable for power. 802.3bt uses all four, which spreads current and allows more power. Check the data sheet of each device: newer WiFi access points often need more than PoE+ to run every radio and feature at full capability, and may switch features off if they receive less.

Sizing the switch power budget

A PoE switch has a total power budget that is often less than every port at full power. A 48-port switch might not be able to deliver PoE+ to all 48 ports at once. To size it:

1. List every device the switch will power and its maximum draw from the manufacturer data sheet.
2. Add them up and compare with the switch's PoE budget, not its port count.
3. Leave headroom for devices added later and for higher-power replacements, such as a WiFi upgrade.
4. Check how the switch behaves when it runs out of budget. Most let you set port priorities so critical devices stay up.
5. If switches are stacked or have redundant power supplies, check whether the budget changes when a supply fails.

Basic switch concepts such as stacking and uplinks are covered in network switch basics.

Cabling, distance and heat

PoE runs over the same structured cabling as data, with the same 100 metre channel limit for twisted-pair Ethernet. Higher power brings two cabling concerns.

• Heat in cable bundles. Current through copper generates heat. In large, tightly packed bundles, especially in enclosed spaces or insulation, the cables in the middle warm up. That raises insertion loss and can shorten cable life. Industry guidance from TIA and cable manufacturers addresses bundle size and installation for high-power PoE; follow the manufacturer's instructions and the applicable electrical code.
• Conductor size. Larger-gauge cable, typical of Cat 6A, runs cooler under PoE load. This is one reason Cat 6A is often chosen for access point drops. See Cat 6 vs Cat 6A vs fibre.
• Connector quality. Unplugging a connector under load can cause small arcs at the contacts. Use quality jacks and patch cords rated for PoE.
• Termination and testing. A bad termination that still passes data may fail under power. Certified testing of every link catches most of these problems.

A worked power budget

Suppose one telecom room will feed a floor with WiFi access points, a set of cameras and desk phones. The arithmetic is simple, but each step has a trap.

1. Use the maximum draw, not the class. If the access point data sheet lists its maximum draw at full features, use that figure. If it lists only a class, use the class maximum at the switch port. For example, 20 access points that each need a PoE+ port at roughly 30 W at the switch account for about 600 W before anything else is connected.
2. Add the rest of the load. Cameras with heaters or motors draw much more in winter than on a mild day. Desk phones are small individually but there are many of them. Add every device on the same switch or stack.
3. Compare with the budget per switch, not per stack. In many stacks each switch has its own budget. A floor whose access points all land on one switch can overload it while its neighbour sits idle. Spread high-draw devices across switches.
4. Plan headroom. The next generation of access points may draw more. Leaving room in the budget now is cheaper than replacing switches with the WiFi upgrade.
5. Size the UPS from the same total. The figure you just built, plus the switches themselves, is the load the UPS has to carry for your target runtime.

When a PoE device will not come up

Keeping port-to-device records up to date makes every one of these quicker to trace. The cable labelling and documentation guide describes what to record.

Backup power and uptime

Because PoE devices draw power from the switch, keeping them running in an outage means keeping the switch running. Size the UPS in each telecom room for the switch plus the full PoE load, not just the switch's own draw. Decide which devices matter during an outage: emergency phones, security cameras and access control usually do; desk displays may not. Think about cooling as well, since a closet full of switches under full PoE load gives off heat. The telecom rooms guide covers room power and cooling.

Common mistakes

• Buying switches by port count without checking the PoE budget
• Assuming every access point runs fully on PoE+
• Large, tight bundles of high-power cables through insulated or enclosed spaces
• Using unmanaged injectors or passive PoE without documentation
• Sizing the UPS for the switch alone
• No record of which port powers which device

How ehZee can help

PoE is part of every wireless design we do, from the installation side of our survey checklist (cabling, ceiling type and IDF/MDF space) to switch selection and power budgets. Through our WiFi design and build service we plan the cabling, switching and power together, then verify that every access point comes up with the power it needs.

Common questions