PoE classes and wattage: 802.3af, at and bt in one table

A PoE class tells the switch how much power to set aside for a device. The table below gives classes 0 to 8 with two numbers each: the watts the switch port (the PSE, power sourcing equipment) has to be able to supply, and the watts the device (the PD, powered device) can count on at the far end of up to 100 meters of cable. Budget with the first number. It is the one your switch counts against its PoE budget, and it is the one the PoE budget calculator adds up when you list your access points and cameras.

PoE classes and wattage, 0 to 8

A few notes on reading it:

• Class 0 and Class 3 reserve the same 15.4 W, so the calculator lists them as one entry, "Class 0 / 3". Class 0 is what a switch assumes when a device does not announce a class at all, so a sensor that sips 3 W but never says so still costs you 15.4 W of budget.
• Class 4 was set aside as reserved in the original 802.3af and only became a power level with 802.3at, which is why "PoE+" and "Class 4" mean the same thing on most datasheets.
• The Type belongs to the switch port. A Type 2 port powers anything from Class 0 to 4. A Type 4 port powers every class in the table. A device asking for more than the port can give either starts with features switched off or does not start.
• The cable allowance column is simply PSE minus PD: the power the standard expects the cable to turn into heat on a worst-case run.

Types, standards and the pairs that carry power

The three standards came in steps. 802.3af (2003) defined Type 1 and classes 0 to 3. 802.3at (2009) added Type 2 and Class 4. 802.3bt (2018) added Types 3 and 4, classes 5 to 8, and power on all four pairs.

Before any of that power flows, the port checks the cable for a valid device. It looks for a 25 kilohm signature resistance across the pairs, and only then runs a classification step in which the device tells the switch its class. PoE+ and later can also refine the allocation over LLDP, with the device asking for a more exact figure once the link is up. None of this happens with passive PoE: a fixed 24 V or 48 V on the spare pairs, no detection and no classes. A standard switch will not power a passive device, and a passive injector can damage gear that expects 802.3af. Keep them apart and label the injectors.

Classes 0 to 4 use two of the four pairs. A switch may put power on the pairs that carry 10/100 data (pins 1 and 2, 3 and 6, called Alternative A) or on the other two (pins 4 and 5, 7 and 8, Alternative B), and a compliant device accepts either. Gigabit Ethernet uses all four pairs for data anyway, and power rides alongside it through the center taps of the magnetics. Classes 5 to 8 need power on all four pairs, which is the main thing 802.3bt changed on the wire.

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Why the two columns differ

This is the part I explain most often, because it is plain electrics. PoE runs at roughly 50 volts DC, so a 30 W port pushes about 0.6 A, and a 90 W port about 1.8 A. The cable is a resistor. The power it wastes grows with the square of the current, so tripling the current multiplies the loss by nine at the same resistance. That is why 802.3bt spreads the current over all four pairs: more copper in parallel, lower resistance, less heat.

You can see it in the table. Class 4 loses up to 15 % of the port's power on a worst-case run; Class 8 loses up to 20.8 %. Those figures assume a full 100 m channel. A 5 m patch to a ceiling access point loses far less, but the switch still reserves the full class figure unless it allocates by LLDP request or by measured draw.

Two practical consequences. First, do not add up device-side watts in a spreadsheet and call it a budget: you will come out short by the whole allowance column. Second, the cable matters. Copper-clad aluminum cable is not compliant with the cabling standards, has higher resistance than solid copper, and runs warmer under load. Tight bundles of cable carrying four-pair power warm up too, so leave long runs loosely dressed rather than zip-tied into a rope.

How many devices fit a switch budget

The same arithmetic the calculator uses, done for four common budget sizes. Each cell is the number of devices of that class that fit with 20 % of the budget kept free; the number in parentheses is the count with no headroom at all.

This table is budget arithmetic only. It does not know how many PoE ports the switch has, or which class each port can deliver. Thirteen Class 1 devices fit in 65 W on paper, but a four-port switch powers four. A Class 6 access point fits in a 120 W budget on paper, but not on a port that stops at PoE+.

A home closet, worked through

Here is the kind of setup I see most: a 120 W switch with four PoE+ ports, two Wi-Fi access points at Class 4, and two cameras whose datasheets do not give a class, so they count as Class 0. In the calculator that is two groups and 20 % headroom:

access points   2 x 30.0 W  = 60.0 W
cameras         2 x 15.4 W  = 30.8 W
total reserved              = 90.8 W on 4 ports
usable          120 x (1 - 20 / 100) = 96.0 W
verdict         fits, 5.2 W to spare

At the device end the same four loads can draw at most 76.9 W (2 x 25.5 + 2 x 12.95). The other 13.9 W is cable allowance the switch holds back, whether your runs are 8 m or 80 m.

Now the two changes people make a year later. Add a Class 2 desk phone (on a switch with a fifth PoE port and the same 120 W) and the total is 97.8 W: the calculator says over the headroom by 1.8 W. Everything still powers up, there is just no margin left.

Or swap one access point for a newer model that asks for Class 6. On paper the total becomes 60 + 30 + 30.8 = 120.8 W, over budget by 0.8 W. In practice you hit a harder wall first: those ports stop at 30 W, so the new access point either comes up with radios turned down or does not come up. The fix is a switch with 802.3bt ports. On one with a 240 W budget, the Class 6 access point, the PoE+ access point, both cameras and the phone add up to 127.8 W, which fits with 64.2 W to spare.

Watching PoE once it is in

A managed PoE switch with SNMP usually exposes its budget and its measured draw through POWER-ETHERNET-MIB, defined in RFC 3621, sometimes alongside a vendor MIB with more detail. The objects worth polling:

• pethMainPsePower: the nominal PoE budget of the switch, in watts.
• pethMainPseConsumptionPower: the measured draw right now, in watts.
• pethPsePortDetectionStatus: per port, whether it is searching, delivering power or in a fault state.
• pethPsePortPowerClassifications: per port, the class the device announced.

With SNMPv3 credentials in shell variables, a walk of the main table looks like this (add the MIB file to your MIB directory first if snmpwalk does not know the names):

snmpwalk -v3 -l authPriv -u monitor -a SHA -A "$AUTH_PASS" -x AES -X "$PRIV_PASS" \
  -m +POWER-ETHERNET-MIB 192.168.10.2 POWER-ETHERNET-MIB::pethMainPseTable

Graph the consumption next to the budget and alert when it crosses your usable figure (96 W in the example), not the full budget. Watch the per-port status for faults too: a port that flips between delivering and searching usually points at a bad cable or a device that browns out at night when its heater or infrared lights come on. The interface types guide covers how power and sensor readings sit next to traffic graphs in a monitor.

PoE watts are also load on your UPS, on top of the switch itself. Size the UPS and its shutdown thresholds for the full draw, as in UPS monitoring with NUT, and if the cameras live on their own VLAN, the home VLAN plan shows a clean way to split them off.

Before you buy or rearrange, go through the same short list. Read the class on the device datasheet, not the "typical power" line. Add up the PSE column, never the PD column. Keep about 20 % free. Check the per-port standard as well as the total. Then put the numbers into the PoE budget calculator and let it tell you whether the plan fits.