Quick Answer: Dig the duct before you think about category. For a buried Ethernet run, keep 450 mm of cover under grass or a footway and 600 mm under anything a vehicle crosses, both rules of thumb to check against the local highway specification. Give the duct an internal diameter at least twice the cable diameter, hold installed bends to a radius of 10 times that diameter, and place a draw pit every 30 m and at every turn. Pull at no more than 110 N, then choose Cat 6a at 500 MHz for 10GBASE-T to 100 m.
Buried ducting is a civil job with cable rules attached: depth and bend radius come first, category comes last. A trench 200 mm too shallow, or a bend formed over a brick, will damage a Cat 6a cable on the day it is pulled, and no screen or pair separator will recover the link afterwards. Design the route as a structure and let the cable follow it.
Depth is set by traffic, not by the cable
Cover runs from the finished surface to the top of the duct, not to the cable inside it. The figure follows what passes overhead: a lawn sees a spade, a driveway sees a wheel load, a road sees a roller during resurfacing.
Grass, borders and footways
450 mm is the working rule of thumb for a garden or a footway. It clears most hand tools, sits below the frost zone of a British winter, and leaves a trench two people can dig by hand in a day. Bed the duct on 50 mm of sand.
Driveways, yards and roads
600 mm under a driveway, and deeper under an adopted road, where the highway authority sets its own figure and inspects before backfill. A duct at 300 mm will ovalise under a delivery van, and an oval duct grips the cable when a replacement is pulled.
Duct size, fill ratio and bend radius
Two numbers decide whether the cable survives the pull: how much of the duct it fills, and how tightly the route turns.
Internal diameter and fill
Aim for an internal diameter at least twice the cable diameter, with the cable filling no more than a third of the cross-section. That spare capacity is what lets a second cable and a rope pass without binding.
Sweeps, not elbows
Hold the installed bend radius at 10 times the cable diameter, and 20 times while pulling. Form bends with a sweeping jig or a pre-formed sweep, never by warming the duct. One sharp elbow is enough to push a 60 m pull past the tension limit.
Worked figures for a 60 m garden run
One duct from a house wall to a garden office, one cable, one spare draw.
| Element | Figure | Why it matters |
|---|
| Cover under lawn | 450 mm | Clears hand tools and frost movement |
| Cover under driveway | 600 mm | Spreads wheel loads before the duct |
| Duct bore | 50 mm internal | Room for a second cable later |
| Installed bend radius | 75 mm, or 10 x 7.5 mm | Keeps pair geometry intact |
| Draw pits | Every 30 m and at each bend | Short pulls, re-pull without digging |
Pulling tension and the change of sheath
Tension accumulates, so the figure that matters is at the cable head, not the winch. Four-pair cables are commonly rated at 110 N; pull by hand and stop when the sheath necks. The buried run ends at a sleeve through a wall, and beyond it the route is inside a building, where fire performance applies: transition at a box and continue with an in-wall Ethernet cable.
Category comes last
With the duct right, the cable choice is short. Cat 6a carries 10GBASE-T to 100 m at 500 MHz, while Cat 6 reaches 55 m. Screening is about the earth path at each end, not the duct, as set out in Cat 6a shielded versus unshielded. At the surface, follow our guide to network cable conduit.
Practical scenarios
- Situation: A 60 m run to a garden office. Use 450 mm cover, one 50 mm duct, draw pits at both ends plus one at 30 m, and pull Cat 6a by hand below 110 N.
- Situation: A 4 m crossing under a block-paved drive. Drop the duct to 600 mm, sleeve it for the crossing, and lay warning tape 150 mm above.
- Situation: Three turns in 40 m. Fit a draw pit at each turn rather than tight sweeps, so no single pull exceeds 15 m.
- Situation: A duct already buried at 300 mm. Do not force a shielded cable through it; re-route the leg instead.
FAQ
Can Ethernet cable be buried without duct?
Only with cable made for it. Direct-burial constructions carry a flooded core and a thick polyethylene sheath, and they still need 450 to 600 mm of cover over a sand bed. Standard indoor Cat 6 takes in water at the first sheath nick, and the link fails as the copper oxidises.
How deep must a duct be under a road?
600 mm is a working minimum, but an adopted road is not yours to specify. The highway authority publishes its own cover figure, often deeper, and inspects before backfill. Get that figure in writing before the trench is opened, not afterwards.
Does ducting remove the need for Cat 6a?
No. Ducting protects the cable and lets you replace it; it does nothing for bandwidth. If the run must carry 10GBASE-T to 100 m, the cable still has to be Cat 6a at 500 MHz. Ducting does make a later move to fibre easier, which is the reason to oversize the bore now. Leave a draw rope in the duct as well.
