A temporary builders supply (TBS) is an electrical connection installed on a construction site specifically to power the building works: site cabins, tools, lighting, welfare facilities, and cranes. It is separate from, and installed before, the permanent electrical supply to the completed development.
This guide covers how TBS works, what it costs, and how to plan one properly.
Why You Need a TBS
Construction sites draw significant electrical loads from the earliest stages of the build. Site offices, welfare units, lighting towers, electric tools, and tower cranes all need power. Relying on diesel generators is expensive, noisy, and increasingly unacceptable for environmental and planning reasons.
A TBS provides mains power on site for the duration of the build, and is decommissioned once the permanent connection is live.
When to Install a TBS
The timing matters. A TBS that arrives too late delays site mobilisation. A TBS that is installed too early sits under-used while still attracting standing charges.
The right time is usually four to eight weeks before site mobilisation. That gives enough contingency for any civil works needed to install the TBS and ensures power is available on day one of the build.
For larger sites with significant early cut and fill works, earlier installation may be justified.
How a TBS Connects to the Network
A TBS is typically a single-phase or three-phase LV connection drawn from the local DNO network. It terminates at a site distribution board housed in a lockable meter box or small cabinet.
For smaller sites, a 63A or 100A three-phase supply is standard and serves most welfare and tool requirements.
For larger sites with tower cranes, concrete batching, or significant early works, a 200A to 400A supply may be needed.
For very large sites, an HV supply with a temporary transformer may be justified. This is unusual but not unheard of on major developments.
Load Sizing
Accurate load sizing for a TBS is important for two reasons. Undersizing leads to trips and disruption. Oversizing increases cost and standing charges unnecessarily.
A typical load profile for a medium construction site:
Site cabins and welfare: 5 to 15 kVA.
Tower crane: 30 to 80 kVA.
Tool circuits: 10 to 20 kVA.
Lighting towers: 5 to 10 kVA.
Concrete batching (if on site): 40 to 100 kVA.
Welding plant: 10 to 30 kVA.
Add a diversity factor of around 0.6 to 0.8 to get the realistic simultaneous demand. A typical medium-size site TBS sits in the 100 to 200 kVA range.
Costs
Temporary builders supply costs in the UK fall roughly into these bands:
Small TBS (single-phase 63A or three-phase 100A) on an existing street: £1,500 to £4,000.
Medium TBS (three-phase 200A) with modest cable run: £4,000 to £10,000.
Large TBS (400A or above) with longer cable run: £10,000 to £25,000.
Very large TBS with temporary transformer: £25,000 to £80,000.
In addition to the installation cost, you pay standing charges throughout the life of the TBS, which on a large site can run to £500 to £2,000 per month.
The Decommissioning Step
A TBS is temporary by definition. It must be decommissioned and disconnected once the permanent supply is live. Decommissioning is typically straightforward: the DNO removes the meter and disconnects the service.
Costs: typically £500 to £2,000 for decommissioning.
Two common problems at decommissioning. First, the TBS is forgotten and continues to draw standing charges after the permanent supply is live. Second, the site is still drawing power from the TBS beyond practical completion because the permanent supply has snagging issues. Plan the transition carefully.
Smart Strategies
On larger developments, consider using the TBS route to bring in the permanent HV supply. The HV cable is laid during the early build phase and a temporary transformer is used to provide LV for construction. When the permanent substation is ready, the transformer is swapped out and the same HV cable serves the completed building. This is one of the tactics covered in our housing development utility checklist.
This approach avoids double-digging for separate temporary and permanent cables, and often reduces total cost by 10 to 20 per cent.
Common Pitfalls
Underestimating the load. Site managers often start with one load figure and discover halfway through the build that tools, heaters, or welfare units are drawing more. Leave headroom in the initial sizing.
Forgetting the lead time. A TBS is a real electrical connection with a real design and install timeline that sits alongside the broader electricity connection programme. Ordering it two weeks before mobilisation rarely works.
Missing the meter reading at decommissioning. This causes billing disputes and delays the final account.
Poor cable protection on site. Construction activity around an exposed TBS cable risks strikes. Route cables through protective ducting and mark them clearly.
What Good Looks Like
A well-planned TBS arrives four to eight weeks before site mobilisation, is sized to the actual peak demand with reasonable headroom, is protected from construction activity, is decommissioned immediately once the permanent supply is energised, and leaves a clean paper trail for the final account.
Done well, it is a line item in the enabling works that nobody on site thinks about. Done badly, it is a source of daily frustration for the site manager.
The Bottom Line
A temporary builders supply is a small project in the context of a major development, but the impact of getting it wrong is immediate and daily. Get the load right, get the timing right, and specify a reputable provider with experience in construction site supplies. The TBS that nobody notices is the TBS that is working properly.