PoE for IP Phones, Explained: 802.3af vs 802.3at, Classes, and Switch Budgets

Power over Ethernet is why an IP phone needs one cable instead of two — and it is also why phones mysteriously fail to boot at the far end of a switch full of other phones. The rules are not complicated, but they are specific: two standards, a class system that reserves more power than most phones actually use, and a total budget per switch that runs out silently. This guide covers 802.3af and 802.3at in plain terms, what the phone classes on Cisco, Yealink, and Grandstream datasheets actually mean, and how to budget a switch so every phone gets power.

The two standards that matter for phones

• IEEE 802.3af (PoE, Type 1): up to 15.4W per port at the switch (the PSE), of which 12.95W is available at the phone (the PD) after cable losses.
• IEEE 802.3at (PoE+, Type 2): up to 30W per port at the switch, with 25.5W available at the device.

Almost every mainstream desk phone runs comfortably inside 802.3af. PoE+ enters the picture with extras — key expansion modules (sidecars), some video phones under full load, and larger devices such as Wi-Fi access points and cameras that share your switch. A PoE+ port is backward compatible: plugging an 802.3af phone into a PoE+ switch is normal and safe; the phone only ever draws what it needs.

Classes: the reservation is not the draw

During the handshake between switch and phone, the phone announces a power class, and the switch reserves the top of that class range against its budget:

• Class 1: 4.0W reserved at the switch (device uses 0.44–3.84W)
• Class 2: 7.0W reserved (device uses 3.84–6.49W)
• Class 3: 15.4W reserved (device uses 6.49–12.95W)
• Class 4 (802.3at only): 30W reserved (device uses 12.95–25.5W)
• Class 0 is the default when a device does no classification, and it is treated as Class 3 for budgeting.

Here is the part that surprises people: the reservation counts against the switch budget, not the phone's real draw — and real draw is usually far lower. Yealink's datasheets make the point cleanly. The SIP-T46U and SIP-T54W are both 802.3af Class 3 phones, so a switch reserves 15.4W for each, yet their measured PoE consumption is 1.6–3.8W and 2.2–5.5W respectively. Cisco's own reported figures for the 8800 series tell the same story: about 6.5W for an 8811 or 8841, 9.8W for an 8851, and 14.2–14.7W for the larger 8861 and 8865. (One caution: the 8851's class itself varies by hardware revision — Cisco material lists both Class 3 and, for v08 and later hardware, Class 4. Check the unit or the switch's own report rather than assuming.)

Budgeting a switch, worked through

1. Find the switch's total PoE budget. Every PoE switch has one — a fixed pool of watts shared by all ports, set by its internal power supply. It is in the datasheet and, on Cisco switches, in the output of show power inline, which reports Available, Used, and Remaining power.
2. Budget by class, not by hope. Multiply the class reservation by the number of phones. Twenty-four Class 3 phones reserve 24 × 15.4W = about 370W on paper, even though the same phones may actually draw under 150W in total. Switches can negotiate the reservation down using LLDP/CDP once devices are connected — Cisco's own troubleshooting examples show a Class 3 device allocated 15.0W with an LLDP-negotiated 12.7W and a measured draw of about 6W — but plan against the class number and treat any negotiated savings as headroom, not budget.
3. Watch what happens when the budget runs out. The switch does not brown out every phone equally — it denies power to later ports. The symptom is specific: the newest phone, or the phone on the highest port, sits dark while everything else works, and Cisco switches count these events as Power Denied in the same show power inline output. If phones fail one at a time as you add them, suspect the budget before the phones.
4. Count the extras. A key expansion module can move a phone out of 802.3af territory: Cisco's support for an 8851 or 8865 with a KEM attached depends on the phone's hardware version, and in the configurations where 802.3af is not sufficient, the requirement is 802.3at or the local Power Cube 4 — check the expansion module's datasheet for your exact phone and hardware version before deploying. Access points, cameras, and paging speakers on the same switch draw from the same pool.

When a phone needs its power cube instead

• No PoE on that model at all. The plain Grandstream GRP2612 is the classic example — only the GRP2612P/W/G variants take PoE; the base model needs its 5V adapter. Check the exact model, not the family name.
• No PoE switch, or a switch port without power. A local adapter (or a PoE injector) covers a one-off desk.
• Expansion modules can push a phone past 802.3af. As above — where the module's datasheet calls for it, use PoE+ or the cube. For Cisco 8800 phones that is the CP-PWR-CUBE-4.
• Match the adapter to the model. Voltages differ even within one family: the Grandstream GRP2612/GRP2613 use 5V adapters while the GRP2614/GRP2615 use 12V, and Yealink specifies 5V/1.2A for the T43U against 5V/2A for the T46U/T54W. The unit label is the authority.

The short version

If every phone is a mainstream desk phone with no sidecar, 802.3af is enough per port — your real constraint is the switch's total budget, reserved by class. Add up Class 3 phones at 15.4W each, leave headroom for the day someone plugs in a camera, and verify with show power inline on Cisco gear (see Reef's Cisco PoE switches collection). Reach for PoE+ when expansion modules, video endpoints, or shared infrastructure devices enter the picture. Our per-model Knowledge Base guides — Cisco 8800, Yealink T4/T5, and Grandstream GRP in this series — list the class and power figure for each phone where the manufacturer publishes one.