Choosing a PoE switch: budget and standards

An eight-port PoE switch sounds like eight cameras, until the data sheet shows a power supply that cannot feed them all. The port count is the cheap number on the box. The PoE budget is the one you pay for.

The short answer

Choose a PoE switch by its total PoE budget in watts first, and by the standard each port supports second. PoE (Power over Ethernet) sends power over the same cable as the data: the switch is the "PSE" (power sourcing equipment), the camera or access point is the "PD" (powered device). IEEE 802.3af (Type 1) delivers up to 15.4 W per port, 802.3at (Type 2) up to 30 W, and 802.3bt (Types 3 and 4) up to 60 W and 90 W [3]. The device always receives less than the port sends, because the cable eats part of it [3][9]. So add up what your devices can reserve at the port, add some margin, and compare that sum with the budget the maker publishes for the whole switch. Never compare it with "ports × 30 W".

How it works

The standard describes every PoE link with two words: Type (what the switch can do) and Class (how much power one device is allowed to ask for). The two tables show both. Port watts are the PSE side, device watts are the PD side.

1. Where the numbers come from. The first PoE standard, 802.3af, appeared in 2003 and 802.3at in 2009. 802.3at introduced the idea of a Type, and 802.3bt added Types 3 and 4 and the classes 5 to 8 [3]. IEEE marks the 802.3bt-2018 amendment as superseded on its own site, and the current edition of the base standard is IEEE 802.3-2022, whose scope includes the provision of power over twisted-pair cabling [1][2]. The PD figure for class 3 is 12.95 W in the standard text. Some overviews, including one by the Ethernet Alliance, round it to 13 W [3][9]. The standard speaks of Types and Classes. Labels such as "PoE+" and "PoE++" are shop shorthand, and "PoE++" in particular is used loosely. Our assessment: read the Type or the watts per port on the data sheet, not the label.
2. Port watts versus device watts. The switch is rated at the port. The device gets what is left after the cable. Our own arithmetic on the table above: the gap is 2.45 W for class 3 (15.9 percent of the port figure), 4.5 W for class 4 (15 percent), 9 W for class 6 (15 percent) and 18.7 W for class 8 (20.8 percent). Juniper's documentation says the line loss is typically below 16 percent of the maximum power [9]. For the switch budget this means one thing: count in port watts. A camera that needs 12.95 W at the device occupies 15.4 W of your budget when it announces class 3.
3. How a device asks for power. A switch first checks that a real PD is attached and only then applies power. Then it classifies the device: the PD signals a class, and the switch answers with the class it grants. The PD may refine its demand with the LLDP protocol [3]. For Type 3 and Type 4 switches, this hardware classification is mandatory. Class 0 is an alias for class 3 and is deprecated in the newer clause [3]. Two optional bt features affect budgets. "Autoclass" lets a switch measure the real loss in the cable and allocate accordingly, so it can power more ports from a limited supply. "Power demotion" lets a switch give a device a lower class when it cannot meet the demand [3].
4. Why the total budget is not ports × watts. The power supply inside the switch sets the budget. How the switch spends it depends on its allocation mode, and vendors differ. Cisco's Catalyst guide describes an "auto" mode, where power is granted as long as the budget lasts, and a "static" mode that reserves a fixed wattage even when nothing is plugged in [7][8]. Cisco also states that the initial allocation for class 0, 3 and 4 devices is 15.4 W until the device asks for more through CDP or LLDP [8]. Juniper defaults to "class" mode, where the class of the device sets the allocation. In its static mode, ports are served in port-number order, and higher ports lose PoE when the budget runs out [9]. HPE Aruba's AOS-CX allocates by actual usage by default and can be switched to allocation by class, which then reduces the budget by the full class power [10]. Our assessment: when the data sheet does not say how the switch allocates, plan with the class values. That is the cautious case.
5. 2-pair, 4-pair and what "bt" on the box means. Classes 1 to 4 may run over two or four pairs. Classes 5 and above always need all four pairs [3]. A Type 3 switch can be built as 2-pair only, and then it stops at class 4, which is 30 W at the port [3]. So a box that says "802.3bt" does not yet tell you that every port delivers 60 W, or that all ports do so at once. Look for the class per port and the total budget. The bt standard also defines a channel for 2.5, 5 and 10 Gbit/s Ethernet with PoE [4]. If a Wi-Fi access point needs more than gigabit, a switch with multigig ports is a separate decision from the PoE type.
6. Cable. IEEE 802.3 requires Category 5 or better cabling and a maximum link length of 100 m, and it limits the DC resistance of the link [11]. The Ethernet Alliance gives 12.5 Ω as the worst-case loop resistance per pair set [3]. Four-pair powering also halves the cable loss for classes that could run on two pairs [3]. Watch out for copper-clad aluminium (CCA) cable. Its resistance is about 55 percent higher than solid copper of the same diameter, it heats up more, and it delivers less voltage at the device [12]. Fluke Networks also points out that IEEE already asks for a 10 °C lower maximum ambient operating temperature of the cable for Type 2 operation when all cable pairs are energised, and that nothing allows for the stronger heating of CCA [12]. Buy solid copper.
7. Passive PoE is not PoE. Standard PoE checks first and powers afterwards. "Passive PoE" has voltage on the port all the time, and its voltage, current and wire pairs can be anything. A standard PD on a passive port can be damaged, and the Ethernet Alliance says this in plain words [6]. Teltonika, for example, states for one of its routers that the passive-PoE input is not compliant with IEEE 802.3af, and that powering it from an IEEE 802.3af supply will damage it [13]. Check the PoE mode of every port that offers more than one, and never plug a standard device into a port set to a passive mode.
8. Measure instead of trusting the data sheet. Data sheets give maxima. Real draw is lower and changes with the state of the device, for example with night-time infrared lighting on a camera. A measured number is better, but it needs a method. Our measurement protocol separates standby, stable idle, typical load and a controlled load, and writes down the duration of each state. Apply it to the cable: read the draw per port if your switch shows it, otherwise measure with an inline meter that supports PoE, and log the class next to the value. As of 25 Oct 2026 we have no PoE port measurements on record at Bytearium, so this guide contains no measured values of our own. The Ethernet Alliance runs a certification program for PoE products in two generations (Type 1 and 2, and Type 3 and 4) [5]. A certification mark is evidence that a product passed an interoperability test. It is not a statement about your budget.

A worked budget (example, not a measurement)

The devices below are invented for the calculation. Replace them with your own list. The class values come from the table above.

3 cameras,        class 3:  3 × 15.4 W = 46.2 W
2 access points,  class 4:  2 × 30.0 W = 60.0 W
1 desk phone,     class 2:  1 ×  7.0 W =  7.0 W
Reserved at the ports:                  113.2 W

Planning margin of about 20 % (our rule of thumb, not a standard):
113.2 W ÷ 0.8 = 141.5 W  →  look for a total PoE budget of at least 142 W

If the switch allocates by actual usage and the devices really draw less, for example 6 W per camera, 15 W per access point and 4 W for the phone (assumed, not measured), the draw would be 3 × 6 + 2 × 15 + 4 = 52 W. You cannot count on that, because the allocation mode decides what the switch reserves. Plan with 113.2 W and be pleasantly surprised.

Now change one assumption: the two access points negotiate class 6. That gives 3 × 15.4 + 2 × 60 + 7 = 173.2 W reserved, and 173.2 ÷ 0.8 = 216.5 W as the target budget. Two ports must then supply class 6 over all four pairs, and the switch must be able to do so at the same time as the other four. An eight-port switch is plenty for six devices. A switch with a 65 W budget is not, even if all eight ports say "PoE". Also remember the UPS: if the switch sits behind one, the switch's own power draw plus the PoE output must fit what the UPS can deliver. Related pages cover power units and UPS sizing.

Checklist

• List every PoE device and write down its class, or "unknown" if the data sheet does not name one.
• Add up the class watts at the port (PSE side), not the device watts.
• Add a planning margin of about 20 percent and write down that this is your own rule.
• Compare the sum with the total PoE budget of the switch, and ignore "ports × 30 W".
• Check that every port feeding a class 5 to 8 device is 4-pair capable, and that the switch can supply all such ports at once.
• Read how the switch allocates power (by class, by LLDP or by usage) and which mode is the default.
• Check that the cable is solid copper, Category 5 or better, and not longer than 100 m.
• Check the PoE mode of every port, and keep passive 24 V devices away from standard ports.
• Measure the real draw per port in your own states, and log it with class and date.
• Recheck the budget whenever you add a device or update the switch firmware.