PoE (802.3af)
Power over Ethernet · 9 specs
Direct Answer
Power over Ethernet (PoE, IEEE 802.3af Type 1) is a 2003 networking standard that delivers up to 15.40 W of direct current (DC) power from a Power Sourcing Equipment (PSE) port, guaranteeing 12.95 W of usable power at the Powered Device (PD) over two twisted pairs.
Source: IEEE 802.3af-2003 (incorporated in IEEE 802.3 Clause 33) · Last reviewed Aug 28, 2026
Specs
| IEEE Standard | IEEE 802.3af-2003 |
| PoE Type | Type 1 (PoE) |
| PSE Output Power (Max) | 15.40 W |
| PD Input Power (Guaranteed) | 12.95 W |
| PSE Output Voltage Range | 44.0–57.0 V DC |
| PD Input Voltage Range | 37.0–57.0 V DC |
| Max Current | 350 mA |
| Twisted Pairs Energized | 2 pairs (Alternative A or B) |
| Minimum Cable Category | Category 3 (Cat 5/5e standard) |
Alternative A vs Alternative B Power Modes
IEEE 802.3af supplies DC power using two distinct pin configurations:
- Alternative A (Endspan): Power is transmitted over data pairs 1-2 and 3-6 using phantom feeding, where DC bias is injected onto the center taps of Ethernet isolation transformers.
- Alternative B (Midspan): Power is delivered over the spare, unused pairs 4-5 (+) and 7-8 (-).
Powered Devices (PDs) must support both Alternative A and B modes to ensure full interoperability regardless of switch or midspan injector design.
Power Budgeting and Losses
The difference between 15.40 W at the PSE switch port and 12.95 W at the remote device accounts for standard resistive line dissipation across a maximum 100-meter loop resistance of 20 ohms on Cat 3/5 cable.