Ethernet Cable Length: The 100 Metre Rule and Its Arithmetic
Quick answer / TL;DR: Short answer: keep the total channel to 100 metres, which includes patch cords at both ends. The permanent link in the wall should not exceed 90 metres, leaving the remainder for cords. Higher categories do not extend the limit: Cat6a still stops at 100 metres. Beyond that, use fibre. For Power over Ethernet, long runs also lose voltage, so heavy loads at full distance may need heavier gauge or a closer power source.
Length tiers: patching, desk, room, cross-room, floor – each with its own SKU.Copper Ethernet is limited to a 100 metre channel. That figure is not arbitrary: it comes from the signal budget defined when the standards were written, and it has held across five orders of magnitude of speed increase.
The number is a channel limit, not a cable limit, and the distinction matters because the patch cords at each end count against it. A 95 metre permanent link plus two 5 metre cords is 105 metres and outside specification.
Channel and Permanent Link
The channel is equipment to equipment; the permanent link is the installed cable between outlets. The standards allocate the difference to cords, which is why 90 metres is the design figure for the wall.
| Element | Limit | Notes |
|---|
| Permanent link | 90 m | installed cable between outlets | Patch cords, both ends | 10 m total in the model | 5 m per end is the standard assumption | Channel | 100 m | the figure that must not be exceeded | Fibre beyond 100 m | varies | 400 m on OM4 at 10G, 10 km on single-mode |
In practice most installations use much shorter runs and 1 to 3 metre cords, leaving headroom. The limit becomes real in large buildings and in warehouse or campus layouts where a run crosses several rooms.
Why the Limit Exists
Insertion loss and crosstalk both grow with length. The standards define a channel budget that a compliant cable can meet at 100 metres, with margin for connectors and for temperature.
- Higher categories do not extend the distance, only the bandwidth at that distance
- Cat6a provides 10G at 100 m; Cat8 still stops at 100 m for 10G and 30 m for 25G and 40G
- Ambient temperature affects loss, which matters in ceilings and risers
- PoE adds voltage drop on long runs, separate from signal loss
Real-World Applications
Office desk run
Typical runs are 30 to 70 metres, comfortably inside the limit. Cat6 or Cat6a handles the requirement, and the constraint is conduit capacity rather than distance.
Warehouse or factory floor
Runs frequently exceed 100 metres. The correct answer is fibre to a local switch or media converter, with copper only for the last segment.
Detached building
Fibre, for distance and for ground isolation between buildings. Copper between structures creates a path for ground potential differences and for lightning-induced transients.
Where the limit is exceeded unknowingly
A run that measures 105 metres including cords may still work at 1G and fail at 10G, or work until the temperature rises. Length compliance is not optional because a link happens to function.
Related reading: Cable categories and their limits — blogs/news/ethernet-cable-types
Related reading: Fiber for longer runs — blogs/news/fiber-optic-ethernet-cable
Standards and Verification
Channel and permanent link limits are defined in ANSI/TIA-568.2-D and ISO/IEC 11801-1, with component requirements in IEC 61156. The 100 metre channel and 90 metre permanent link figures are consistent across categories.
Ethernet speeds are defined in IEEE 802.3, including 1000BASE-T in 802.3ab and 10GBASE-T in 802.3an. Those standards assume the 100 metre channel, which is why a longer run is outside specification regardless of what it measures.
Verify with a certifier that reports length as well as transmission parameters. Length is measured electrically from the cable's propagation delay, so a coiled excess shows up in the result even when the route appears short.
Do not use excess cable coiled at both ends to consume length. The coil contributes to the channel, blocks airflow and adds alien crosstalk in a bundle, and it is visible in the certifier's length reading.
Frequently Asked Questions
What is the maximum Ethernet cable length?
100 metres for a channel including patch cords, or 90 metres for the permanent link in the wall.
Does Cat6a allow longer runs?
No. Cat6a increases bandwidth at 100 metres, not the distance. The 100 metre limit applies to all copper categories.
Why 100 metres specifically?
It is the channel length for which the signal budget was defined, accounting for insertion loss, crosstalk and margin for connectors.
What do I use beyond 100 metres?
Fibre. Multimode reaches hundreds of metres at 10G and single-mode reaches kilometres.
Does PoE reduce the maximum length?
Signal-wise no, but voltage drop increases with length, so a high-power device at full distance may need heavier gauge cable.
Sourcing cables for a project, a store, or a private-label line? Contact our B2B sales team for specifications, certification evidence, and volume terms – FOB and CIF options available. Request a quote or sample.
