What is cable ducting?
Cable ducting is a system of protective underground pipes, usually made from polyethylene or uPVC, installed to carry electric power cables or telecommunications cables through the ground. The duct is the passageway; the cable runs through it. Installing the ducting separately from the cable lets contractors prepare the civils infrastructure first, pull cables through later, and in time replace or upgrade them without digging up the route again.
Cable ducting is distinct from the cable itself, from cable trays used above ground, and from HVAC ductwork used for heating and ventilation. It is also distinct from cable conduit, though in UK civils the terms “duct” and “conduit” often overlap, with “conduit” leaning electrical-installation and “duct” leaning buried-service. British Standard BS EN 61386-24 covers both under a single framework.
In practice, the cable ducting discipline splits cleanly into two verticals: electric cable ducting (governed primarily by the Energy Networks Association) and telecoms or fibre cable ducting (governed primarily by Openreach and the PIA regime). The sections below cover each in turn.
How cable ducting works
A cable duct network is installed during the civils phase of a project, before any cables go in. The typical sequence: trench excavated to the required depth, bedding material laid, ducts laid in the trench with draw ropes pulled through, marker tape laid approximately 75 millimetres above the duct, trench backfilled. Joint boxes and chambers are installed at access points. Cables are pulled through the ducts later by the network operator or their appointed contractor.
For electric cable ducting, depth depends on the voltage. A typical low-voltage duct sits 450 to 600 millimetres below finished surface level. High-voltage ducts sit deeper. For telecoms, Openreach specifies 250 millimetres minimum depth for duct to premises, with runs kept as straight as reasonably possible and no more than one pre-formed 90-degree bend per run.
The draw rope left in place during installation is what allows cables to be pulled through after the fact. Without the rope, a cable pull becomes significantly more difficult on anything beyond the shortest straight runs. Pre-installation surveys per PAS 128:2022 and safe digging practice per HSE HSG47 cover the ground-detection side of the work before any excavation begins. The civils trench and chamber work that cable ducting sits inside is a discipline in itself, with its own depth, separation, and reinstatement rules.
Electric cable ducting
Electric cable ducting in the UK is governed by ENA Technical Specification 12-24, currently Issue 4 dated February 2021. This is the Energy Networks Association specification every Distribution Network Operator (DNO) references when approving plastic duct for buried electric cables. It sits alongside British Standard BS EN 61386-24, the European standard covering underground conduit systems for cable management.
ENA TS 12-24 classifies ducts into four performance classes. The class defines the compression strength the duct must hold at a specified temperature.
ENA TS 12-24 electric cable duct classes
| Class | Compression strength | At temperature | Typical use |
|---|---|---|---|
| Class 1 | 450 N | 75°C | High-voltage cables, including direct exposure to heated surfaces |
| Class 1+ | 450 N | 75°C | HV routes installed by horizontal directional drilling (HDD). PE100 material required |
| Class 2 | 450 N | 50°C | Low-voltage cables in standard trench installations |
| Class 3 | 450 N | 23°C | General-purpose ducting, service connections, street lighting |
Materials in common use are HDPE (high-density polyethylene, flexible coilable duct), uPVC (unplasticised polyvinyl chloride, rigid lengths), and PE100 (higher-grade polyethylene specified for HDD applications). Colour identification follows Street Works UK Volume 1 Issue 10 (covered in the Standards section below). Electric ducting is always black, red, or both.
For contractors working with an ICP, IDNO, or DNO on a new connection or network reinforcement, the duct class is usually specified upfront in the scheme drawings. For self-lay electric schemes where the civils contractor prepares the infrastructure and a NERS-accredited party pulls the cable, the duct class, diameter, and burial depth form part of the adoption specification. McFadden’s ducting work for developers and ICPs sits within this self-lay electric scope.
Telecoms and fibre ducting
Telecoms cable ducting in the UK is governed primarily by Openreach specifications, because the majority of the access network either is Openreach infrastructure or connects to it. Alternative network operators (altnets) building their own FTTP networks often install their own duct, but many use Openreach’s existing ducts and poles under the Physical Infrastructure Access (PIA) regime regulated by Ofcom.
Openreach’s developer specification for new-site duct is set out in the New Sites Fibre Handbook (August 2024 edition). The headline technical parameters:
- Minimum duct depth to premises: 250 millimetres
- Duct runs kept as straight as reasonably possible, with pre-formed bends rather than sharp kinks
- Maximum of one 90-degree pre-formed bend per duct run
- Marker tape approximately 75 millimetres above the installed duct
- Colour: green, per Street Works UK Volume 1
Approved modular chambers for Openreach adoption include the QuadBox (Radius Systems) and the JBF104 and JBF106 footway boxes (LN712 specification, 150 millimetre deep twin-wall HDPE). Chamber specification is not a free choice: Openreach names two approved suppliers and the colour scheme is strict.
For altnet work, duct specification follows the operator’s own adoption rules, but the base Street Works UK guidance on burial depth, colour, and positioning still applies. McFadden’s telecoms ducting work is scoped for Openreach and altnet civils including FTTP rollout and multi-utility corridor schemes.
UK standards and specifications
Cable ducting work in the UK sits at the intersection of electrical installation standards, civil engineering specifications, and street works legislation. The cross-reference that matters in practice:
Core UK standards for cable ducting
| Standard | Publisher | What it covers |
|---|---|---|
| ENA TS 12-24 Issue 4:2021 | Energy Networks Association | Plastic ducts for buried electric cables. Class 1 / 1+ / 2 / 3 performance, dimensions, colour codes |
| BS EN 61386-24 | BSI / CENELEC | Underground conduit systems for cable management. 25 mm to 250 mm sizes. Type N450 and N750 impact ratings |
| BS 7671:2018+A2:2022 | IET / BSI | IET Wiring Regulations. Regulation 522.8.10 requires buried conduits and ducts to be suitably identified |
| Street Works UK Vol 1 Issue 10 | Street Works UK (formerly NJUG) | Positioning and colour coding of underground utilities. Depths, separation, marker systems |
| Openreach New Sites Fibre Handbook 2024 | Openreach (BT Group) | Duct laying, chambers, and FTTP civils specification for developers |
| HSG47 | Health and Safety Executive | Avoiding danger from underground services. Detection and safe digging |
| PAS 128:2022 | BSI | Underground utility detection, verification and location. Pre-installation survey spec |
| NRSWA 1991 | Department for Transport | Statutory framework for street works, reinstatement, inspections |
Colour coding under Street Works UK Volume 1 Issue 10:
- Black: low-voltage electricity (below 1 kV)
- Red: high-voltage electricity
- Yellow: gas
- Blue: water
- Green: telecoms (Openreach and most altnets)
- Orange: street lighting
- Purple: motorway communications
A common misconception, including in some AI-generated answers, is that orange identifies power. It does not. Orange is street lighting. Black is LV power, red is HV power. Getting this right at site inspection level matters when crews from different disciplines are working a shared trench. For the underlying engineering detail on buried conduits and ducts, the IET Wiring Matters article on BS 7671 Regulation 522.8.10 is the accessible reference.
Cable ducting vs HVAC ductwork
Cable ducting vs HVAC ductwork
| Attribute | Cable ducting | HVAC ductwork |
|---|---|---|
| Purpose | Protect and route buried cables | Move air for heating and ventilation |
| Material | HDPE, uPVC, PE100 | Galvanised steel, flexible insulated |
| Standards | ENA TS 12-24, BS EN 61386-24 | BS EN 1505, BS EN 1506 |
| Typical installer | Civil engineering or utility contractor | Mechanical or HVAC contractor |
| Location | Underground | Above ground, usually in ceiling voids |
If you searched for cable ducting and landed on HVAC content, or vice versa, this is why. The terms are adjacent linguistically but the disciplines are separate trades with separate standards and separate installers. Cable ducting is the domain of civil engineering and utility contractors. HVAC ductwork is the domain of mechanical and air-handling contractors.
Cable ducting in practice
McFadden's cable ducting work sits within self-lay electric schemes. Civils only, to the spec set by the adopting body. Cable pulling and energising runs separately, handled by the appropriate ICP or DNO after civils handover.
Typical delivery
On a typical scheme we handle the full civils package: trenching, ducting install, draw pits, chambers, joint bays, substation bases, backfilling, and reinstatement to SROH. Ducts go in to the spec set out by the adopting body, ENA TS 12-24 class and diameter for electric, Openreach spec for telecoms. Draw rope pre-installed. Marker tape laid to depth. Chambers sited per drawing.
The duct install runs alongside the rest of the utility civils, which means one contractor on site rather than coordinating multiple trades. Typical schemes: new-build developer estates, substation base civils, multi-utility corridor works for civil engineering contractors, and ICP-led connections where we carry the civils side.
Recent project
Ducting installation for new substation, January 2023. Full duct network install for a new substation base, including chambers and draw pits. Civils completed and handed over to the cable pulling contractor.