A temporary supply can look straightforward until several trades are sharing it: a mixer starts, battery chargers are left running, a saw cuts out and a lead has been dragged through mud. This site power distribution guide explains how to plan a practical temporary electrical set-up that keeps equipment supplied without treating safety protection as an afterthought.
For contractors, maintenance teams and serious DIY users, the aim is simple: get the right power to the point of work, protect people and equipment, and avoid downtime caused by overloaded circuits, damaged cables or unsuitable gear.
Start with the supply, not the extension lead
Power distribution starts at the source. That may be a permanent mains supply, a generator, or a temporary site intake. Before adding transformers, splitters and leads, establish what voltage and current are actually available, who is responsible for the supply, and whether it has been tested for site use.
On UK construction and outdoor work sites, 110V centre-tapped supplies are commonly used for portable tools. A site transformer reduces 230V mains to 110V, with the output arranged as 55-0-55V to earth. This reduces the severity of electric shock compared with using ordinary 230V portable equipment in harsh conditions. It does not make damaged tools, wet connections or poor cable management acceptable.
A 230V supply may still be necessary for fixed equipment, cabins, lighting, chargers or specialist machinery. Where this is the case, the circuit needs appropriate protection and should be planned by a competent person. Do not assume that because a plug fits a socket, the supply is suitable for the job.
Match the transformer to the real load
A common failure point is selecting a transformer based on the rating of one tool, then expecting it to run several items at once. Transformer capacity is usually stated in kVA. The useful question is not only, “What does this saw draw?” but, “What could be connected at the same time?”
Add up the likely running loads in watts, then allow for start-up current. Motors in tools such as mixers, compressors and larger saws can pull a higher current when starting than their normal running figure suggests. A transformer that is only just large enough on paper may trip, run hot or give poor tool performance in use.
For example, a 3kVA transformer may suit a single higher-demand 110V tool or several low-draw items, depending on their total demand. It is not a licence to connect every available socket to heavy equipment. Check the transformer rating, outlet rating and tool data plate. If there is any doubt, separate the loads across suitable supplies rather than forcing everything through one unit.
Transformer choice also depends on the working environment. A hard-wearing site transformer with protected outlets and a carry frame makes sense where it will be moved regularly. For a static temporary work area, positioning, ventilation and protection from weather can matter more than portability. Never cover a transformer or leave it where it can sit in standing water.
Plan distribution around the work area
Good distribution reduces trip hazards and prevents leads becoming the weakest part of the installation. Put the supply point as close as practical to the centre of the working area, then route cables along edges, overhead supports or protected cable routes where possible. Avoid running leads across access routes, doorways, vehicle paths and wet ground.
Use a distribution unit where multiple circuits are needed, rather than relying on a chain of extension reels and adaptor blocks. A proper distribution board can provide individual outlets, circuit protection and clearer control over what is connected. This makes faults easier to isolate and stops one problem from taking down the whole work area.
Keep lighting and power tools on separate circuits where the set-up allows it. If a tool trips while someone is working in an enclosed space, losing the only source of light creates a separate hazard. Temporary lighting should be suitable for the area, mounted securely and protected from impact.
Cable length is another practical consideration. Longer leads can cause voltage drop, particularly when feeding high-demand equipment. The tool may feel underpowered, motors can run hotter and nuisance tripping becomes more likely. Use the shortest suitable cable run with the correct conductor size. A reel that is still wound up can overheat under load, so fully unwind it before use.
Choose the right plugs, sockets and leads
Site electrical equipment is built around clear voltage identification. Yellow plugs and sockets are generally used for 110V systems, while blue industrial connectors are commonly used for 230V single-phase supplies. The colour is a useful visual check, but always confirm the voltage, current rating and pin configuration before connecting equipment.
Leads need to be fit for the environment, not merely long enough. A domestic extension lead is not a substitute for a site-rated cable assembly. Look for durable flexible cable, properly moulded or correctly fitted connectors, strain relief and an IP rating appropriate to the exposure. Outdoor or wet-area work calls for more protection than a dry internal refurbishment.
Inspect every lead before use. Cuts in the outer sheath, crushed sections, taped repairs, cracked plugs, loose cable grips and bent pins are all reasons to remove it from service. A damaged lead can still power a tool while presenting a real shock or fire risk. Replacing it is cheaper than losing time to an incident or damaging equipment.
Do not join leads with unsuitable adaptors or leave couplers lying in wet grass, mud or puddles. Raise connections off the ground where practical and keep them protected from direct rain, traffic and impact. If a connector is hot, discoloured or repeatedly trips a protective device, stop using it and investigate the cause.
Protection devices are part of the system
Overcurrent protection, RCD protection and earthing all have different jobs. A fuse or MCB protects the cable and circuit against overload and short circuit. An RCD monitors for earth leakage and disconnects rapidly when it detects a fault. An RCBO combines those functions for an individual circuit.
An RCD is valuable protection, but it cannot compensate for poor site practice. It will not prevent a cable from being cut by a saw, crushed by plant or overloaded beyond its rating. It also does not remove the need for correct earthing, suitable enclosures and regular inspection.
Test buttons on RCDs should be used in line with the manufacturer’s instructions and the site’s electrical procedures. Where equipment is subject to heavy use, a formal inspection and testing regime is sensible. Records help a site manager identify repeat faults, remove defective kit quickly and show that temporary electrics are being managed properly.
Generator-fed power needs extra checks
Generators are useful where mains power is unavailable, but they need the same load planning as a fixed supply. Check the generator’s continuous output rating rather than relying on a peak figure. A generator that is too small may struggle with motor start-up loads, produce poor supply quality or trip repeatedly.
Site transformers and distribution equipment must also be compatible with the generator arrangement. Earthing and neutral arrangements can affect whether RCDs operate correctly, so this is not a matter for guesswork. A competent electrician should assess the set-up, particularly where several circuits, cabins, metalwork or sensitive equipment are involved.
Position the generator on stable ground with adequate ventilation, safe refuelling access and protection from unauthorised use. Keep fuel, exhaust gases and noise away from occupied work areas. Never refuel a hot or running generator.
A quick pre-use check for temporary power
Before the shift starts, walk the set-up rather than waiting for faults to appear. Check that:
- the supply, transformer and distribution equipment are correctly rated for the expected load;
- cables are fully unwound, undamaged and routed away from traffic, water and sharp edges;
- plugs, sockets and couplers are secure, clean and protected from impact;
- RCDs and circuit protection are in place and site procedures for testing have been followed;
- only suitable tools and equipment are connected to each voltage system.
When to bring in a competent electrician
Portable 110V equipment and straightforward lead management can be handled by trained users following site rules. Altering fixed wiring, installing distribution boards, setting up generator earthing, diagnosing repeated tripping or connecting temporary cabins is different work. It should be carried out, inspected or verified by a competent electrician and in accordance with the relevant requirements, including BS 7671 where applicable.
For day-to-day site requirements, DYNATEX can be a practical source for site transformers, industrial plugs and sockets, electrical cable, temporary lighting and the supporting hardware needed to keep work moving. Buy for the load, the environment and the route the cable has to take, not solely for the lowest headline price.
Temporary power works best when it is treated as working equipment rather than a collection of spare leads. Set it up properly, inspect it often and remove damaged items without debate - the job will be safer, and the next task is far less likely to stop halfway through.