IEEE 802.3 PoE types and limits
What each PoE type guarantees at the powered device, what the sourcing equipment must supply, and why the two differ.
Limits by type
| Type | Pairs | PSE max | PD guaranteed | PD min voltage | PSE output | Max current |
|---|---|---|---|---|---|---|
| 802.3af - Type 1 (PoE) | 2 | 15.4 W | 12.95 W | 37.0 V | 44–57 V | 350 mA |
| 802.3at - Type 2 (PoE+) | 2 | 30.0 W | 25.50 W | 42.5 V | 50–57 V | 600 mA |
| 802.3bt - Type 3 (PoE++) | 4 | 60.0 W | 51.00 W | 41.1 V | 50–57 V | 1200 mA |
| 802.3bt - Type 4 (PoE++) | 4 | 90.0 W | 71.30 W | 41.1 V | 50–57 V | 1920 mA |
Why the PSE figure and the PD figure differ
The gap between what the sourcing equipment supplies and what the powered device is guaranteed is the cable. Type 1 budgets 15.4 W at the port and promises 12.95 W at the device: the missing 2.45 W is dissipated in the copper.
That number is not arbitrary, and checking it is a good way to verify any PoE calculation. At the Type 1 worst case — 44 V at the port, 20 Ω of channel loop resistance, 350 mA — the loss is exactly 0.35² × 20 = 2.45 W, and the voltage arriving is 44 − 0.35×20 = 37.0 V, which is precisely the published PD minimum. Every figure in the row closes on every other one.
Loop resistance
PoE returns its current through the cable, so both directions count. In 2-pair mode one pair carries the current out and one returns it, giving two conductors in parallel each way. In 4-pair mode there are four each way, which halves the loop:
Constant power, not constant current
A powered device draws constant power, so the current depends on the voltage that survives the cable, which depends on the current. Calculators using I = P / V under-report the drop. The operating point is the lower root of:
Authoritative values are in IEEE 802.3, Clause 33 (Types 1–2) and Clause 145 (Types 3–4). Related tool: PoE voltage drop and power budget.