PoE Cable Length: Two Limits in One Run
Quick answer / TL;DR: Short answer: keep PoE runs within the standard 100 metre channel, and check voltage drop for high-power devices. A 23 AWG cable at 90 metres with a 25 W load loses several volts, which matters for devices at the edge of their input range. Use 23 AWG or heavier, prefer Cat6 over thin Cat5e for high-power runs, and remember that 802.3bt devices draw more current and therefore lose more voltage over the same cable.
Three PoE adapters: injector adds power, splitter removes it, midspan sits in the run.A Power over Ethernet run faces the usual 100 metre signal limit and a second, separate constraint: voltage drop. The cable carries current as well as data, so its resistance reduces the voltage reaching the device.
That second constraint is why a device may work at 20 metres and fail at 90, even though the link trains normally and the switch reports no fault.
The Two Mechanisms
Signal loss and power loss both grow with length, but they are governed by different parameters and they fail differently.
| Mechanism | Governed by | Failure mode | Observable |
|---|
| Signal loss | bandwidth and length | link will not train | switch port reports no link | Voltage drop | current, resistance, length | device resets or fails to start | device cycles, switch reports power delivery |
The second row is the confusing one, because the network link is healthy while the device misbehaves. A camera that reboots at night is often a voltage drop problem rather than a network one.
What Affects the Drop
Three factors combine, and each is controllable at design time.
- Load: higher power means higher current, so bt devices lose more than af devices
- Gauge: 23 AWG loses less per metre than 26 AWG patch cable
- Length: the drop is proportional, so the last twenty metres matter most
- Temperature: bundled cables run warmer, which raises resistance slightly
- Connectors and joints: each adds resistance
Real-World Applications
Access point at the far end of a building
A 25 W access point at 85 metres on 23 AWG cable is within limits, but the margin is smaller than at 20 metres. Confirm the device's minimum input voltage against the calculated drop.
Camera with a heater on a long run
Heaters draw intermittently and can push a bt device to its class limit. This is the case where gauge and length matter most, and where a short local run with a midspan injector may be better.
Warehouse with runs near 100 metres
Design to 90 metres for the permanent link and use 23 AWG or heavier. If the device is high power, consider a local switch closer to it rather than a long PoE run.
Where length is not the problem
Devices drawing under about 7 W at short distances. At those currents, voltage drop is negligible and the signal limit is the only concern.
Related reading: PoE classes and power levels — blogs/news/poe-af-at-bt
Related reading: Cable selection for PoE bundles — blogs/news/poe-cable
Standards and Verification
PoE is defined in IEEE 802.3, with 802.3af, at and bt defining power at source and device. Cabling limits come from ANSI/TIA-568 and ISO/IEC 11801-1, with conductor resistance published in the wire tables those standards reference.
Verify by measuring the voltage at the device end under load. That single reading distinguishes a power problem from a network problem and is faster than substituting hardware.
For design, calculate the drop using the loop resistance of the cable at the length installed, then compare the result with the device's minimum input voltage. Leave margin, because the load varies with device activity.
Avoid thin patch cords at either end of a high-power PoE run. A 26 AWG patch cord adds disproportionate resistance for its short length, and it is a common reason a marginal run fails.
Frequently Asked Questions
Does PoE reduce the maximum cable length?
The signal limit remains 100 metres, but voltage drop increases with length, so a high-power device may not operate at full distance.
What gauge should PoE cable be?
23 AWG or heavier for high-power runs. Thin 26 AWG patch cords add resistance and are best avoided at the ends of long runs.
Why does my camera reboot at night?
Likely voltage drop combined with heater draw. Measure the voltage at the device under load.
Does 802.3bt need heavier cable?
It draws more current, so voltage drop is greater. Heavier gauge and shorter runs matter more than with af or at.
Can I exceed 100 metres for PoE?
No. Beyond the channel limit the link is outside specification, and PoE does not extend it.
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