Specifying Cable Ducting: A Practical Guide to the Pathway

Of everything that goes into a trench, the ducting is the part that outlives all the rest. Cables get upgraded, pulled out and replaced. The duct around them stays in the ground, and the pathway it creates is what decides, years later, whether a network can add capacity through an existing route or has to dig the road up again. Get the ducting right and the route keeps giving for decades. Get it wrong, undersized, crushed, or the wrong class for the load above it, and the cheapest way to fix it is to excavate.

That makes cable ducting one of the highest-leverage specifications in a buried build and one of the least discussed. The choice of duct system, compression class, colour, and whether it carries a sub-duct inside it all shape what the route can do for the next twenty years.

Ducting does two jobs, not one

The obvious job of a duct is protection: a rigid or semi-rigid pipe around the cable that takes the crushing load of backfill, traffic and future excavation so the cable does not. The second is a reusable pathway. Once it is in the ground, cables can be pulled or blown through it, removed and replaced without touching the surface again. On a fibre network, spare duct or sub-duct makes it possible to add capacity at a fraction of the cost and disruption of a fresh dig.

The main duct types and when to use each

Twinwall duct is the workhorse for power and heavier utility routes. Its corrugated outer wall and smooth inside give high strength while remaining light to handle. Rigid twinwall in stick form, such as Ridgiduct Power, suits medium- and high-voltage applications, while coiled twinwall, such as Ridgicoil, reduces joints and can be laid quickly in long lengths. Single-wall duct, including Polyduct and uPVC high-voltage duct, suits applications where a smooth, rigid pipe is needed.

Sub-duct and microduct are smaller HDPE pathways installed inside a larger duct or directly in the ground for blowing fibre. Split duct wraps around an existing live cable for retrofit work without disconnection. Multipath systems provide several internal ducts inside a pre-configured outer duct for multi-way telecoms and data routes.

The number that matters: compression class

Compression, or crush, resistance is the rating that separates a duct that survives from one that fails quietly in the ground. BS EN 61386-24 is the standard for underground conduit systems, and a duct rated to 450N compression class is built for a normal buried environment. For power networks, ENATS 12-24 defines Classes 1 to 3 that DNOs expect. Ingress protection matters too: an IP4X interference-fit socket keeps dust and larger debris out, while an IP68-rated sealed socket is specified where the joint must be watertight.

Colour is identification, and it ties into safe digging

Duct colour is not cosmetic. Under the NJUG scheme, the colour tells whoever next opens the ground which utility it carries before they reach the cable inside. Power ducts are typically black or red, with high-voltage systems often using a red duct or inner wall. A correctly coloured duct, matching marker tape and, on the highest-risk routes, a protection tile work together as a layered safe-digging defence.

Sub-duct, microduct and the blowing strategy

For fibre, the duct network is a network of pathways within pathways. Smoothwall sub-duct and microduct give cable a dedicated low-friction bore, often with several microducts inside one larger duct so a route can serve multiple cables or operators independently. The design decision that matters at specification stage is leaving spare capacity: a spare microduct is a future cable added by blowing rather than by digging.

Handling, joints and retrofits

The best duct specification accounts for how the route goes in. Coiled twinwall reduces joints compared with cut sticks. Integral couplers and factory-installed drawcords reduce on-site work and leak points. Purpose-made bends preserve compression rating, sealing rings protect joints where ground water is a risk, and split duct lets a live run be protected or upgraded in place without an outage.

A specifier’s checklist for cable ducting

Size the bore for today’s cable plus realistic future capacity. Choose twinwall for power and heavy routes, sub-duct or microduct for blown fibre, split duct for live retrofits, and multipath for multi-way telecoms. Confirm BS EN 61386-24 and the ENATS 12-24 class required by the DNO. Match ingress protection to the joint, coordinate NJUG duct colour with marker tape and tiles, leave spare ways, specify proper bends and couplers, and check adoption approval where a DNO or Network Rail will take the route on.

Why the pathway is the asset worth protecting

Cable ducting is the quietest long-term decision in a buried build and one of the most consequential. The cable inside it will be replaced at least once in the life of the route; the duct will not. The pathway it creates is what lets a network add capacity, swap technology and recover faults without returning to the road each time. Specify the right class, the right size and the room to grow, and the route keeps giving for decades.

Talk to Mountrel about cable ducting

Mountrel supplies cable ducting for UK power, telecoms and utility networks, including Ridgiduct twinwall, Ridgicoil coiled twinwall, Polyduct single-wall, uPVC HV duct, sub-duct and microduct, split duct and multipath systems. Our team can help match the duct system, class and accessories to your utility, your ground and your future capacity.