Live cable diversions are one of the more common surprises on UK construction sites. A routine site survey finds high-voltage or low-voltage cables crossing the plot, and before construction can proceed, those cables must be physically moved or protected. The process is specialist, expensive, and on the programme critical path, and sits alongside the broader scope of utility diversions on construction projects.
This guide covers what developers need to know about live cable diversions.
When a Diversion Is Needed
A cable diversion is needed when existing DNO cables:
Cross the building footprint.
Are too close to planned foundations.
Run under planned excavations.
Are in conflict with site roads or access.
Would be damaged by construction activities.
Are in locations that make future access impossible.
The first time this emerges is usually during pre-construction surveys and PAS 128 underground apparatus searches.
LV vs HV Diversions
Cable diversions fall into two categories with very different cost and complexity, mirroring the split between LV and HV electrical connections.
LV (low voltage, typically up to 1000 volts) diversions:
Service cables to nearby properties.
Street lighting connections.
Smaller main cables.
Cost: £2,000 to £15,000 per diversion typically.
Timescale: 4-12 weeks.
HV (high voltage, typically 11 kV, 33 kV, or higher) diversions:
Distribution cables serving larger areas.
Cables to customer substations.
Primary distribution cables.
Cost: £15,000 to £200,000+ per diversion.
Timescale: 3-9 months.
HV diversions dominate project costs and programmes when they occur.
The Process
A typical diversion runs:
Stage one: Identify the cable. Surveys, DNO records, physical identification.
Stage two: Engage with the DNO. Submit a diversion request with site plans.
Stage three: Site investigation by DNO. Physical verification of cable position and condition.
Stage four: Diversion design. DNO designs the new route and works.
Stage five: Quote. DNO provides cost and timescale.
Stage six: Acceptance. Developer accepts quote and pays.
Stage seven: Outage planning. DNO plans outages needed for the diversion.
Stage eight: Diversion works. Excavation, new cable installation, jointing, testing.
Stage nine: Commissioning and live working. Final connection to the network.
Stage ten: Old cable decommissioning. Old cable isolated and sometimes removed.
Outages
Cable diversions almost always require outages of the affected cable.
Planned outages: scheduled weeks or months ahead with affected customers notified.
Switched outages: short outages using switching to maintain supply to other customers.
Permanent isolation: for cables being replaced entirely.
Outages cause disruption to connected customers and must be coordinated carefully.
For HV cables feeding many customers, outage planning can extend timescales significantly.
Cost Drivers
Several factors drive diversion costs.
Cable length being diverted. Longer diversions are more expensive.
Voltage level. HV diversions cost much more than LV.
Ground conditions. Rock, contaminated land, or running water increase excavation costs.
Site access. Restricted access increases labour and plant costs.
Traffic management. Roads requiring TM add significant cost.
Reinstatement. Road reinstatement can be substantial.
Outage costs. Extended outages can attract compensation costs.
Network reinforcement. Sometimes the diversion requires upgrading cable capacity.
Timescales
Typical timescales for HV diversions:
Initial enquiry to quote: 8-12 weeks.
Quote to site survey and design: 4-6 weeks.
Design to outage planning: 6-10 weeks.
Outage planning to works: 4-8 weeks.
Works: 2-8 weeks.
Total: 6-9 months for a typical HV diversion.
LV diversions are typically 4-12 weeks.
Safety and Compliance
Live cable work is specialist. Requirements include:
Authorised engineers working to Electricity at Work Regulations.
DNO-specific procedures.
Permit-to-work systems.
Testing and verification procedures.
CDM regulations compliance.
Third-party safety management.
DNOs use their own engineers or authorised contractors for live cable work.
Early Discovery Is Valuable
The earlier a diversion need is identified, the better.
At site acquisition: surveys at due diligence identify problems before purchase.
At planning: design can avoid diversion where practical.
At feasibility: programme and budget can include the diversion.
At construction: discovery during construction causes delays and costs.
Routine PAS 128 surveys at feasibility or pre-construction stage should identify most cable conflicts.
Design Options to Avoid Diversion
Sometimes design can avoid the diversion.
Adjust building footprint to avoid the cable.
Provide easements to preserve DNO access.
Design foundations to allow cable to remain in place.
Adjust site levels to maintain cable cover.
Provide protective measures (cable tiles, concrete protection).
Working with the DNO at design stage can identify these opportunities.
Easements and Wayleaves
If the cable is being left in place, easements and wayleaves protect DNO rights.
Easements: permanent legal rights for the DNO over the land.
Wayleaves: agreements for the DNO to maintain cables on private land.
New development often requires renewed or new easements and wayleaves.
These are part of the legal work on any development with DNO apparatus.
Who Pays
The developer usually pays for diversions triggered by their development.
Exception: if the diversion also benefits the DNO (for example upgrading aged cable), the DNO may contribute.
Exception: if the DNO is responsible for the conflict (poor records), cost sharing is possible.
Most disputes are resolved commercially; formal dispute with Ofgem is available but rarely needed.
Common Pitfalls
Several issues come up repeatedly.
Late discovery. Finding cables during excavation causes major delays.
Underestimating HV cost. Developers budget for simple diversion, face six-figure costs.
Poor survey information. DNO records are not always accurate; physical surveys are essential.
Outage coordination. Failing to coordinate outages with wider DNO programme causes delay.
Assuming the DNO will rush. DNO timescales are driven by operational constraints and customer impact; they cannot be rushed without reason.
Third-party impacts. Diversions affecting neighbours need coordination, especially where NRSWA-compliant street works run across adopted highway.
Cable Diversion Alternatives
Before committing to a full diversion, consider alternatives.
Cable encasement: protecting the existing cable in concrete or protective sleeve.
Lifting and relaying: moving the cable locally without full rerouting.
Cable replacement: replacing the existing cable with new cable on the same route but at different depth.
Easement renegotiation: if the cable can remain, formalising the right to do so.
These are typically cheaper and faster than full diversions.
The Bottom Line
Live cable diversions are a common feature of UK construction sites, especially on brownfield and inner-city developments such as those we cover in our London utility connections work. The cost and programme impact can be substantial, particularly for HV diversions. Early identification through pre-construction surveys, engagement with the DNO at design stage, and exploring alternatives to full diversion, are the keys to managing the risk. For any development of meaningful scale, budgeting time and money for possible diversions is sensible, even when none is yet identified. Cable diversions are one of the utility-related items most likely to surprise a project, and the developers who prepare for them deal with them more successfully.