How Long Can an Ethernet Cable Run Actually Be?

The number everyone half-remembers is right: twisted-pair Ethernet is engineered around a 100 meter (328 ft) channel — formally 90 m of permanent cable plus 10 m of patch cords at the two ends. Gigabit over decent Cat5e/Cat6 genuinely works at that distance. The interesting questions are at the edges.

Where the rule bends

• 10 Gigabit shortens Cat6. Regular Cat6 holds 10G only to roughly 55 m; Cat6A restores the full 100 m at 10G. Wiring for the future? That's the argument for 6A.
• Thin cable derates. Slim 28AWG patch cords trade reach for flexibility — fine at patch-field lengths, wrong for a 90 m pull. Bulk runs should be 23–24AWG solid copper (why solid matters).
• Cat8 is short on purpose. 40G at up to 30 m — a rack standard, not a building one.
• Cheap copper lies. CCA (copper-clad aluminum) cable fails well before 100 m. Pure copper or nothing.

When the run is just too long

Past 100 m you have three honest options. Put a small switch mid-span (each switch port restarts the 100 m clock — often the cheapest fix if power exists en route). Or step up to fiber: multimode OM3/OM4 carries 10G for 300–400 m, and OS2 single mode goes kilometers — our fiber guide picks the grade. Between buildings, fiber wins for a second reason: it electrically isolates the two ends, protecting equipment from ground-potential differences.

Two habits that save re-pulls

Measure the actual path — up walls, around obstacles — not the straight line, then add 10–15% slack. And test the run before you close the wall. A $30 tester finding a bad crimp today beats diagnosing "intermittent drops" for a month.