A PoE switch sends power and data down the same Ethernet cable. That lets you mount a Wi-Fi access point, a camera or a desk phone wherever you can run a cable, without putting a power outlet beside it. In an office or store with ceilings full of devices, it saves electrical work and keeps the install tidy.
The part people get wrong is the power budget. A switch can have 24 PoE ports and still be unable to power 24 devices. Below: how PoE works, which standard you need, and how to add up the watts before you buy.
How PoE works
Power over Ethernet (PoE) is defined in the IEEE 802.3 standards. The switch, or a single-port injector, is the power source. The device on the other end is the powered device. A standards-based switch checks that the device can accept PoE before it sends any power, which is why a laptop or printer plugged into a PoE port comes to no harm.
Cheaper passive PoE gear skips that check and pushes power down the cable regardless. Keep passive equipment and standards-based equipment apart.
PoE standards and power levels
| Standard | Common name | Maximum power at the switch port | Power available at the device |
|---|---|---|---|
| 802.3af | PoE (Type 1) | 15.4 W | 12.95 W |
| 802.3at | PoE+ (Type 2) | 30 W | 25.5 W |
| 802.3bt | Type 3 | 60 W | 51 W |
| 802.3bt | Type 4 | 90 W | 71.3 W |
The gap between the last two columns is power lost as heat in the cable. Plan around the figure at the device, and read the datasheet for the real draw of each model.
What usually runs on PoE
- Wi-Fi access points (many want PoE+ or better, so check the model)
- IP cameras
- VoIP desk phones (see our business phone and VoIP service)
- Door readers and intercom panels
Port count is not the same as power budget
Every PoE switch lists a total PoE budget in watts. That is the most power it can supply across all ports at once. Add up the maximum draw of your devices from their datasheets, not the typical figure, and check the budget covers it.
Here is an example with round numbers. Four access points at up to 25 W each come to 100 W. Six cameras at up to 12 W come to 72 W. Eight desk phones at up to 6 W come to 48 W. The total is 220 W, so a switch with a 150 W budget would run out of power even though it has enough ports.
Leave room to grow. A margin of 20 to 30 percent over your maximum is a common rule of thumb, and it covers the camera or access point you add next year.
Managed or unmanaged
An unmanaged PoE switch is plug and play and fine for a few devices on one flat network. A managed switch adds the controls most businesses end up wanting: VLANs to keep point-of-sale, staff, guest and camera traffic apart (see network segmentation), per-port PoE control so you can restart a frozen access point or camera from your desk, and monitoring that tells you when a port goes down.
For a store with POS terminals and guest Wi-Fi, we would choose managed. The retail Wi-Fi network design guide shows how the segments fit together.
PoE switch or PoE injector?
An injector powers one device and costs less than a switch. It makes sense for a single camera at the far end of a building. Once three or more PoE devices sit in the same area, a switch is simpler to manage and puts one power cable in the rack instead of several.
Cable and distance
An Ethernet run is limited to 100 meters including patch cords, and PoE does not extend that. Cat5e is fine for lower-power devices. For the higher-power Type 3 and Type 4 devices, Cat6 or better gives more margin, since more current means more heat in the cable. We compare the options in Cat6 vs Cat6a.
Buying checklist
- The PoE standard each device needs (af, at or bt)
- Total PoE budget in watts against your maximum, plus margin
- Port count today plus spare ports
- Uplink speed, and multi-gigabit ports if your access points have them
- Managed features, if you need VLANs or remote port control
- Fan noise, if the switch will sit in an occupied room
Frequently asked questions
What does PoE stand for?
Power over Ethernet. It delivers electrical power to a device over the same twisted-pair cable that carries its network data.
Do I need a PoE switch for Wi-Fi access points?
No. Most access points can also run from a power adapter or a PoE injector. A PoE switch is the neater option when the access point is mounted on a ceiling and there is no outlet nearby.
Will a PoE switch damage a normal device?
Not if the switch follows the IEEE standards. It checks for a PoE device before sending power, so a laptop or printer is safe. Passive PoE equipment does not check, so do not mix it with standards-based gear.
How many watts should a PoE switch have?
Enough to cover the maximum draw of every device you plan to connect, plus a margin of around 20 to 30 percent.
Planning a network for an office or store? Scandifix designs and installs business networks across Edmonton and area, including switching, Wi-Fi and cabling. See our network setup service, call +1 (587) 400-9991 or contact us.