PoE is usually discussed as a switch feature. It is equally a cabling decision: the pair count, the conductor gauge, the bundle size, and the temperature rating all decide whether a 60 W device gets 60 W at the far end.
Pair count
| Standard | Pairs used | Max at PSE |
|---|---|---|
| 802.3af (Type 1) | 2 pairs | 15.4 W |
| 802.3at (Type 2) | 2 pairs | 30 W |
| 802.3bt Type 3 | 4 pairs | 60 W |
| 802.3bt Type 4 | 4 pairs | 90 W |
Type 3 and 4 require all four pairs. A cable with a damaged pair can pass data at 1 Gbps on two pairs and fail to deliver power on four — a fault that looks like a switch problem.
Heat is the hidden limit
Current through a conductor produces heat. In a bundle, cables heat each other. Standards derate ampacity based on:
- Number of cables in the bundle
- Ambient temperature
- Cable temperature rating (60 °C, 75 °C, 90 °C)
- Installation (conduit, tray, insulation)
A 48-cable bundle of Cat6A delivering 60 W per port can exceed the cable's rated temperature in a closed riser. The failure is not immediate; insulation degrades over years.
CCA cable is not Ethernet cable
Copper-clad aluminum has higher resistance than copper and fails PoE at distance, sometimes melting at the connector. It is sold cheaply and looks identical. Check the conductor before installing hundreds of runs.
Voltage drop
| Gauge | Resistance (Ω/100 m, loop) | Drop at 0.6 A |
|---|---|---|
| 24 AWG | ~18.8 | ~11 V |
| 23 AWG | ~14.6 | ~8.8 V |
At 60 W and 50 V, current is 1.2 A, and the drop doubles. That is why the PD receives 51 W rather than 60 W, and why a marginal run can push a device below its minimum.
Worked example: 60 W at 90 m
- Device: 802.3bt Type 3, 51 W at the PD.
- Cable: Cat6A, 23 AWG, 90 m.
- Loop resistance: ~13 Ω.
- Current at 51 V: ~1.2 A.
- Drop: ~15.6 V — far too much.
The arithmetic shows why high-power PoE wants short runs and 23 AWG or better, and why the standard's "100 m" is a data limit, not a power guarantee.
What breaks in production
- CCA cable, failing at distance and heating at the connector.
- Long runs at high wattage, delivering less than the device needs.
- Dense bundles in conduit, exceeding the temperature rating.
- Damaged pairs, passing data and failing power.
- Cheap patch cords in the path, adding resistance at the worst point.
- No derating, so a 90 °C cable is treated like a 60 °C one.
Checklist
- Conductor is copper, not CCA.
- Gauge is 23 AWG or better for high-power runs.
- Bundle size and ambient temperature are considered.
- Cable temperature rating matches the installation.
- High-wattage runs are kept short where possible.
- Patch cords match the permanent link's rating.
Next
The cable is installed. Continue with Racks, Labeling, and Standards for the closet it lands in.