PoE++ Cable Requirements: Current, Heat and Bundle Derating
Quick answer / TL;DR: Short answer: 802.3bt Type 4 supplies 90 W at the PSE and guarantees 71 W at the powered device over all four pairs. Use 23 AWG or heavier, keep DC resistance low, and derate for bundles because heat accumulates. A 28 AWG slim patch cord is not suitable for Type 3 or Type 4 loads.
Three PoE adapters: injector adds power, splitter removes it, midspan sits in the run.Power over Ethernet began as a way to avoid a power outlet for a telephone. Type 4 of 802.3bt delivers 90 W at the source, which is enough for access points, pan-tilt-zoom cameras and even displays, and it turns a data cable into a power conductor carrying roughly 1.7 A per pair set.
That current changes the cable requirement. Conductor gauge, DC resistance, temperature rise in a bundle and connector rating all become constraints that do not appear at 15 W.
This article sets out the four power types, the gauge arithmetic, and the bundling rule that most installations ignore.
PoE is specified in IEEE 802.3af, 802.3at and 802.3bt. The types differ in source power, device power and how many pairs carry current.
The Four Types
| Type | Standard | PSE power | PD power | Pairs used |
|---|
| 1 | 802.3af | 15.4 W | 12.95 W | 2 |
| 2 | 802.3at | 30 W | 25.5 W | 2 |
| 3 | 802.3bt | 60 W | 51 W | 4 |
| 4 | 802.3bt | 90 W | 71 W | 4 |
The gap between source and device power is the loss budget for the cable. At Type 4 the budget is 19 W, which is a large amount of heat to dissipate in a cable bundle and is the reason gauge matters more than it did at Type 1.
The Gauge Arithmetic
Power loss in the cable is current squared times resistance, and resistance falls as the conductor gets thicker. Doubling the cross-sectional area halves the loss.
A 28 AWG conductor has roughly 2.6 times the resistance of 23 AWG. On a 60 metre Type 4 run that difference is the difference between delivering 71 W and delivering considerably less, which manifests as a device that reboots under load rather than as a link failure.
- Specify 23 AWG or heavier for Type 3 and Type 4
- Treat slim and flat patch cords as data-only unless the datasheet states a Type 4 rating
- Include both patch cords in the loss budget, since two 28 AWG cords can dominate the channel
- Check the connector rating separately, because the jack and plug have their own current limit
Heat and Bundling
A bundle of cables each carrying power has no free surface to dissipate heat. The inner cables run hotter than a single cable in free air, and the temperature rise reduces the current the cable can safely carry.
TIA TSB-184-A addresses this by providing bundle derating guidance, and the practical consequence is that a cable rated for 90 W in free air may need to be derated in a 48-cable bundle. Where the design sits near the limit, splitting the bundle or using a heavier gauge is cheaper than discovering the problem after the ceiling is closed.
Real-World Applications
A warehouse ceiling with 60 Wi-Fi 6E access points on Type 3 power uses 23 AWG Cat6A in bundles of 24. The design derates the bundle and separates power-heavy runs from data-only runs to limit the temperature rise in the middle of the bundle.
A campus CCTV retrofit powers pan-tilt-zoom cameras at Type 4 over existing Cat5e. The runs are 45 metres, so the loss is acceptable, but the installer replaces the factory 28 AWG patch cords at both ends because they would add more resistance than the permanent link.
A retail display powers screens over Ethernet at Type 4 on 70 metre runs. The screens draw close to the device limit continuously, so the voltage at the far end is measured under load rather than assumed from the datasheet.
A building with a mix of Type 1 sensors and Type 4 cameras shares one tray. The sensors do not heat the bundle, but the cameras do, and the derating calculation uses the worst case present in the bundle rather than the average.
Standards and Verification
The power types are specified in IEEE 802.3af, IEEE 802.3at and IEEE 802.3bt. The cabling channel is specified in ISO/IEC 11801 and TIA-568, and bundle thermal derating is addressed in TIA TSB-184-A. Connector current ratings are separate from cable ratings and appear in the component datasheet.
Verification should include a channel certification, a measurement of the voltage at the powered device under full load, and a check that the PSE budget covers the sum of the connected loads plus cable loss. A link that comes up proves negotiation, not power delivery margin.
Where the installation is dense, an infrared check of the bundle after a day of operation shows whether the thermal assumption was right. It is a five-minute check that prevents a mid-summer outage.
Related reading: Cat5e PoE support — blogs/news/cat5e-poe-support
Related reading: Cat6A ethernet cable guide — blogs/news/cat6a-ethernet-cable-guide
Related reading: Solid vs stranded Cat6 — blogs/news/solid-vs-stranded-cat6
Type 3 and Type 4 use all four pairs. A cable with a damaged pair can still pass data on two pairs while losing half its power capacity, which produces a device that works until it draws full power.
Frequently Asked Questions
Can I run PoE++ over Cat5e?
Yes, if the cable is 24 AWG or heavier and the run is short enough that the loss budget is respected. At Type 4 the loss budget is 19 W, so gauge and length matter more than the category label.
Are slim patch cords suitable for PoE++?
Generally no. A 28 AWG slim cord has roughly 2.6 times the resistance of 23 AWG, which consumes a large part of the Type 4 budget and heats up in a bundle.
What is the maximum PoE++ distance?
100 metres for the channel, the same as any Ethernet link. The practical limit is often shorter because the power loss budget is exhausted before the distance limit is reached.
Why does my device reboot when it draws full power?
Usually voltage drop under load caused by thin conductors, long runs, or both patch cords being slim. Measure the voltage at the device under load rather than at idle.
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