Temporary Site Power

Definition

Temporary site power is the complete electrical installation that serves a construction site before permanent energisation: the supply source (utility connection or generators), main distribution boards, step-down transformers to reduced low voltage (110 V in the UK, or protected 230 V with RCDs elsewhere), final circuits, leads and earthing. It is designed, installed, inspected and maintained like any electrical installation — because it is one, operating outdoors, in mud, around water, under constant mechanical abuse.

Why It Matters

Power is the invisible dependency in every programme. An undersized generator that trips when two tower cranes slew together costs a day's lifting every time it happens. A drowned cable joint in a haul route is an electrocution hazard at ankle height. And the loads change constantly — welfare cabins multiply, winter lighting arrives, dewatering starts, the tower crane arrives before the mains upgrade. Sites that treat temporary power as "the electrician's problem" discover it is actually a programme, cost and safety system that needs a load schedule, a growth plan and a named owner like any other temporary work.

How to Plan and Run It

1. Build a load schedule — every foreseeable load with demand and diversity factors: cranes, hoists, pumps, batchers, welfare, lighting, tools, heating.
2. Size for starting currents — direct-on-line motors draw six to seven times running current; the largest motor starting on a running system sets the generator size.
3. Design for growth — boards with spare ways, generator sets with headroom, cable routes that accept a second run.
4. Protect critical circuits separately — dewatering, fire pumps and safety lighting on dedicated breakers with their own backup feed.
5. Install for the environment — armoured cable or cable ramps on haul routes, IP-rated enclosures, leads elevated or buried and marked.
6. Inspect and test on a regime — weekly RCD tests, monthly inspections, thermographic checks on loaded boards, records kept.

Real-World Example

A city-centre frame job ran two tower cranes, two hoists and the dewatering off a single 500 kVA generator that the load schedule said was adequate — on paper, with a hopeful diversity factor. Every morning at 07:00, both hoists started together as the workforce went up, the voltage dipped, and the crane VFDs tripped on undervoltage. Twenty minutes of reset, twice a day, quietly costing a lifting hour daily. The fix was not a bigger generator; it was a ten-line staged-start procedure and moving the hoists onto a soft-start setting, agreed in one meeting with the crane supplier. The load schedule was updated with the measured starting currents, and the next project's temporary power design started from those numbers, not the brochure ones.

Practical Lessons Learned

• kVA is not kW. At a 0.8 power factor a 500 kVA set delivers 400 kW — and motor loads pull the power factor down further. Size on real power, then add starting margin.
• Winter doubles the load. Lighting, heating and drying arrive together in October; the schedule that worked in June fails in November.
• Dewatering never shares. Put it on its own breaker and its own backup supply; a nuisance trip elsewhere must never stop the pumps.
• Cable routes are logistics. Every haul route crossing needs ramps or burial before the first delivery, not after the first shredded lead.
• The electrician needs programme visibility. Crane arrivals, pump starts and cabin moves are electrical events; put them in the look-ahead with a power impact flag.

Expert Tips

• Clamp-meter the actual currents for a week before finalising generator size; brochure figures and real starting behaviour differ enough to matter.
• Colour-code and label every board and transformer with circuit, source and the phone number of the responsible electrician — in an emergency, nobody reads a schematic.
• Fit hour meters and fuel logs to generators; consumption trending catches both underloading (wet stacking) and the cabin heater someone wired in without asking.
• Keep a single-line diagram laminated at the main board and updated at every change. It is the first thing the utility engineer, the fire service or the investigator will ask for.
• Run generators at 60–80% load where possible; chronically light-loaded diesels glaze, smoke and fail early — arrange load banks if the site is between phases.

Common Mistakes

• Sizing the generator on connected load × diversity wishful thinking, then discovering motor starting currents the hard way.
• Daisy-chained splitters and domestic-grade leads feeding half the site from one 32 A socket.
• Cables across haul routes unprotected, or buried without marking and struck by the first excavation.
• No RCD test regime — the devices that prevent electrocution silently failed months ago.
• Treating the temporary supply as fit-and-forget, with no named owner, no inspections and no record of the fifty modifications made since installation.

Key Takeaways

• Temporary power is a designed installation with a load schedule, growth plan and named owner — not a commodity hire.
• Size on measured real power and starting currents; brochure arithmetic trips cranes.
• Critical loads — dewatering, fire, safety lighting — get dedicated circuits and independent backup.
• Protect cables mechanically and inspect the installation on a documented regime.
• Put electrical events — crane arrivals, pump starts, winter loads — into the programme look-ahead.

Related Concepts

Pairs with Site Mobilization Plan, Site Logistics Plan, Temporary Works Register, and Dewatering System Operation.