How do I plan a PoE power budget for access points, phones and cameras?

Plan a PoE power budget by adding up the maximum power each device can draw, comparing that total to the switch’s overall PoE budget, checking each device against its port’s limit, and leaving about 20 percent spare. The switch’s total budget is often the limit that catches people out, not the port count.

Two limits: the switch budget and the port limit

A PoE switch has two separate limits, and you have to stay inside both.

The total PoE budget is the most power the switch can supply across all ports combined. A 24-port switch does not necessarily have enough power for 24 devices. Many switches have a budget well below what every port could deliver at full power.

The per-port limit is set by the PoE standard the port supports. IEEE 802.3af (PoE, Type 1) delivers up to 15.4 W at the switch port, and a device is guaranteed at least 12.95 W. 802.3at (PoE+, Type 2) delivers up to 30 W at the switch port, with at least 25.5 W at the device. 802.3bt (sometimes called PoE++ or 4PPoE) goes higher, with two tiers: Type 3 up to 60 W at the switch port and Type 4 up to 100 W. Check the switch’s data sheet for the exact figures and for which ports support which standard.

A device that needs more than its port can give will not run properly, however much budget is left overall.

Adding up device draw

Use the maximum draw, not the typical draw. Manufacturers usually list a power class or a maximum consumption figure on the data sheet. Use that.

Budget at the switch, not at the device. Some power is lost in the cable, so the switch has to supply more than the device receives. A power class lists the switch-side maximum, which covers this. If the data sheet gives only the device’s own consumption, add a margin for cable loss.

Count everything that takes power from the switch. That includes access points, desk phones, cameras, and anything else plugged into a PoE port, such as door controllers or small sensors.

Then add them up:

1. List every device by type and quantity.
2. Write down the maximum draw for each.
3. Multiply by quantity and total it.
4. Add about 20 percent headroom.
5. Compare the result to the switch’s total PoE budget.

Worked example

These numbers are round samples for illustration only. Use the real figures from your own devices’ data sheets.

Add 20 percent headroom: 242 W × 1.2 = 290.4 W, so call it about 290 W.

Now compare. A switch with a 370 W total PoE budget has room to spare. A switch with a 240 W budget does not, since it falls short of even the 242 W raw total. A switch with a 300 W budget fits, with a little margin.

Then check the ports. An access point drawing 25 W at the switch is within the 30 W that PoE+ allows, but too much for basic PoE (15.4 W), so it needs a PoE+ port. The phones and cameras in this example fit within basic PoE ports. If a device is listed at a higher draw than its port can supply, the budget total does not matter, because that port is the problem.

Why 20 percent headroom

Headroom covers draw you did not plan for. Devices can draw more when they start up, when a camera turns on its infrared lighting or heater, or when a phone has a display or accessory attached. It also leaves room to add a device later without replacing the switch.

About 20 percent is a common rule of thumb, not a standard. A switch running right at its limit may drop or refuse to power ports, depending on how it handles an overload.

Common mistakes

• Using typical or idle power instead of the maximum.
• Assuming that a switch with many PoE ports can power all of them at full load.
• Checking the total budget but not the per-port limit, or the reverse.
• Forgetting small devices such as door controllers, sensors or extra phones.
• Ignoring that cameras can draw more at night, or when heaters or lighting are active.
• Not allowing for growth, such as extra access points or cameras next year.
• Assuming a long cable run is free. Ethernet cabling is limited to 100 m, and poor-quality cable can reduce delivered power, so check the cable type and the maximum run length in the device and standard documentation.
• Ignoring how the switch behaves when the budget runs out. Some models cut power to lower-priority or higher-numbered ports, and the behavior varies by model, so read the data sheet. If a decision is expensive or safety-related, confirm it with a qualified network installer or electrician.