Quick answer: A 20 ft ethernet cord is about six metres, far inside the length where twisted pair is comfortable. What decides whether it works is not distance but three details: the bend radius at a corner, the strain relief behind the plug, and whether the cord shares a path with mains wiring. A stranded cord with moulded boots, kept off sharp edges and tested before it vanishes behind a wall, holds gigabit. A flat cord stapled tight does not.
Six metres of cable looks harmless, which is why it gets installed badly. It gets stapled to a skirting board, folded behind a drawer, laid along the same path as a mains extension, then pushed through a hole drilled too small. The link comes up, the speed test looks fine, and three weeks later the port starts renegotiating. The fault sits in the physical layer, not in the settings.
What six metres costs the signal
At 20 ft, insertion loss is small. Twisted pair is specified for channels of 100 m, so length alone is not the problem. What matters is that a stranded patch cord uses thinner copper than solid horizontal cable, so resistance per metre is higher, and that a tight bend or a crushed jacket adds a return loss fault which a short run will happily hide.
Bend radius and strain relief at the plug
Most failures in a cord this length live within 25 mm of a plug. The plug is rigid, the cable is not, and every force applied to the cord arrives at that joint.
The radius rule that matters
Twisted pair should not be bent tighter than four times its jacket diameter, and a stranded cord should not be folded flat round a corner at all. A tighter bend distorts the pair geometry and changes the impedance at that point. Leave a small loop wherever the cable changes direction.
Strain relief that actually holds
A moulded boot spreads the load over the jacket; a bare crimp plug passes it straight to the contacts. Where the cord will be pulled, add a hook and loop tie a hand width behind the plug so the force lands on the tie rather than the latch.
Shielded or unshielded beside mains cable
A foil and braid shield with a drain wire blocks capacitively coupled noise from a mains cable running parallel to it. The question is whether that noise exists and where the drain wire ends up.
When the shield earns its place
It helps where the cord shares a conduit, lies along a fluorescent fixture or crosses a distribution board, and where the parallel length is measured in metres rather than centimetres.
When it makes things worse
A shield bonded at both ends creates a loop that carries current between two points at different earth potential, which injects hum. In a home run, bond one end and leave the other floating.
Proving the link before the wall closes
Connect the real equipment and watch the port counters rather than the link light. Errors, retransmits and renegotiations at idle mean a physical fault. A qualification tester reports margin against the category, while a continuity tester only tells you the pairs are joined.
Routing and securing the cord
Keep the cord clear of sharp metal and anything that slides, and use a floor duct wherever it crosses a walkway. Where it passes through a stud, drill a hole at least twice the jacket diameter and cross mains cable at a right angle. A cord tied every 300 mm with tight cable ties has been bent at every tie.
| Path condition | Cord to use | Bend radius | Symptom if ignored | Fix |
|---|
| Desk to wall plate | Stranded, moulded boot | Four times diameter | Latch broken, cord unplugs | Service loop |
| Beside mains lead | Shielded, one end bonded | No flat fold | Retransmits under load | Cross at right angles |
| Through a stud | Solid core or slim patch | Hole twice diameter | Open pair after weeks | Re-drill and re-terminate |
| Under a carpet edge | Flat cord, no staples | No compression | Link drops when walked on | Floor duct |
Practical scenarios
- Bedroom to hallway router: a 6 m moulded cord behind a skirting board holds gigabit, provided it is not pinched at the door frame.
- Workshop bench: the cord runs with a mains lead for two metres, so a shielded cord bonded at one end removes the noise the plain one picked up.
- Cord through a stud wall: a hole cut at the jacket diameter damaged one pair and the link trained down to 100 megabit until it was re-drilled.
- Before the plasterboard goes on: a qualification test with the final cord attached proves the run; a continuity beep proves almost nothing.
FAQ
Is 20 ft too long for gigabit?
No. Gigabit over twisted pair is specified for 100 m channels, so 6 m is a fraction of the allowance. Plug quality and routing decide the outcome here.
Can I use a flat cord under a carpet?
Only where nothing presses on it. Flat cords use thin conductors with little pair spacing, so a compression point can fail one pair while the others still pass.
Does a shielded cord need a special socket?
The socket must bond the shield, otherwise the drain wire has nowhere to go and the cord behaves like an unshielded one with a floating conductor attached.
Next step
Before ordering, match the cord to the permanent run with the Cat6A specification overview, check construction differences in Cat6 versus Cat6A, and follow the RJ45 termination quality guide so both ends match.
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