In short. Construction power is temporary utility infrastructure, not a generator parked at the gate. The load changes with every project phase, from site cabins and dewatering to tower cranes, batching plants and commissioning tests. Plan the generator, distribution, earthing and fuel for each phase, size for the largest motor start rather than the connected total, and agree who may change the installation. Reddy Generators supports sites across Telangana and Andhra Pradesh with staged rental and purchase arrangements.
Why construction power changes with the programme
A permanent building has a defined load schedule. A construction site does not. In the first month the demand is a few site cabins, lighting and a dewatering pump. Six months later the same site has a tower crane, a batching plant, a bar-bending yard with welding sets and a dozen contractor distribution boards. At the end, the permanent chillers, lifts and fire pumps are commissioned on temporary power because the DISCOM connection is still weeks away. A single generator chosen at mobilisation is wrong for most of that period, either oversized and wet-stacking at 20 per cent load, or undersized and tripping on the crane.
The practical answer is a phased power plan. For each phase, list the loads that genuinely run together, identify the largest motor and how it starts, estimate the hours and duration, and decide whether rental, purchase or a combination serves it best. The construction-site rental page covers the commercial side, and the mining and quarrying page covers crusher starts; agree at the same time who may request additional load, who authorises changes, and what triggers a fresh engineering review.
| Project phase | Typical loads | Sizing driver | Usual arrangement |
|---|---|---|---|
| Site establishment | Cabins, lighting, small pumps, hand tools | Running kW, nonlinear lighting | One small set, 15 to 62.5 kVA |
| Excavation and foundations | Dewatering pumps, piling rig auxiliaries, compressors | Pump starting and 24-hour duty | Dedicated pump set plus site set |
| Structure | Tower cranes, hoists, batching plant, concrete pumps, welding | Crane and mixer motor starts, poor power factor | Medium set or two sets in parallel |
| MEP and finishing | Many small loads across floors, lifts on temporary supply | Distribution and voltage drop | Set plus several sub-distribution boards |
| Commissioning | Chillers, AHUs, fire pumps, lifts under test | Largest permanent motor | Large rental set for a short window |
Sizing for cranes, batching plants and welding
The three loads that most often catch a site out are the tower crane, the batching plant and the welding fleet. A tower crane nameplate might read 60 kW, but the hoist motor is what matters. Older cranes use direct-on-line hoist motors drawing six to seven times full-load current at start; newer cranes use variable-frequency drives that limit inrush but inject harmonics. Either way the generator must hold voltage within the limit the crane controls accept, usually a dip of 15 to 20 per cent, or the contactors drop out mid-lift.
Starting kVA = Motor kW × starting multiplier ÷ (efficiency × running power factor)
Here the starting multiplier is the ratio of starting current to full-load current for the chosen starting method, and efficiency and power factor are the motor’s running values. For a 30 kW direct-on-line hoist motor at 0.9 efficiency and 0.85 power factor with a multiplier of 6, the starting demand is roughly 235 kVA (illustrative). A set chosen on the 60 kW nameplate alone, about 75 kVA, would not survive the first lift. The motor starting guide explains how alternator transient capability is read from manufacturer data, and the voltage dip guide covers acceptable limits.
Batching plants combine a mixer, conveyors, a cement screw, a compressor and a pump, started in sequence by the plant PLC, so ask the supplier for the sequence and the largest single motor rather than the total. Welding sets are intermittent and draw current at a power factor of 0.5 to 0.7; eight or ten machines on one small generator produce lighting flicker and AVR hunting.
Distribution, cable routes and earthing on a changing site
The generator is rarely the item that fails on a construction site. The cable from the generator to the work front is. Routes that were safe this month cross an excavation next month, and a cable laid on the ground is run over by a transit mixer. Treat distribution as part of the scope: armoured cable on defined routes, lifted on poles or buried in marked trenches where plant moves, with sub-distribution boards placed close to the work rather than long flexible leads.
Voltage drop deserves a calculation, not a guess. A 415 V feed over 200 metres of 35 mm² copper carrying 100 A loses roughly 2.5 to 3 per cent, which is acceptable; the same run in 16 mm² aluminium is not. The cable sizing guide has the method. Every board feeding portable tools needs 30 mA residual current protection in line with the CEA safety regulations and IS 3043, and the generator neutral must be earthed at the set with a measured electrode. The earthing guide explains why a mobile source needs its own earth reference.
Tunnelling, metro and pumping work
Tunnel and metro work is not ordinary open-site power. Ventilation fans, dewatering pumps, lighting and communication inside a tunnel are life-safety loads, and loss of ventilation with diesel plant underground is an evacuation event. Such sites need two generators with automatic changeover for the safety loads, with fans and pumps on separate feeders. Monsoon dewatering is similar in consequence: an excavation that floods overnight because a pump set ran dry costs a week of programme. Where pumping is continuous, specify prime rating, 24 hours of fuel and a low-fuel alarm that reaches someone.
Fuel, hours and operating cost
Construction sets run long hours at variable load, so fuel is the dominant cost. A modern diesel set consumes roughly 0.2 to 0.25 litres per kWh at 70 to 100 per cent load, rising sharply below 30 per cent load where the engine also risks wet stacking. A 125 kVA set carrying a steady 75 kW for ten hours a day uses about 160 to 190 litres a day (illustrative); a 250 kVA set kept on site for one crane start burns more for the same work. A two-set arrangement, a small set for continuous site load and a larger set for the structure phase, often costs less over a 24-month programme than one large set. The fuel consumption guide gives figures by load band.
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Common mistakes
- Sizing the generator from the connected kW of the crane and batching plant instead of the largest motor start. The set holds the running load but trips on the first hoist.
- Running a large set at very low load through the finishing phase. Carbon builds in the exhaust and turbocharger and the engine needs an unplanned top overhaul.
- Leaving the generator neutral earthed only through the reinforcement or not at all. Earth-fault protection cannot operate and a damaged tool lead becomes a shock hazard.
- Leaving cables where they were laid while the site moves around them. Mechanical damage causes most site power faults and stops every contractor at once.
What to send us
Send the programme with the major plant per phase, the nameplate data and starting method for the tower crane hoist, batching plant and largest pumps, and a site layout showing where the generator can stand and where power is needed. Include expected daily hours, the duration of each phase and whether the site is in a residential area with noise limits. Send your construction power requirement.
Questions engineers ask
What size generator does a tower crane need?
It depends on the hoist motor and its starting method, not on the crane’s total kW. A 30 to 45 kW direct-on-line hoist usually needs a 160 to 250 kVA set to hold voltage during the start, while a VFD-driven crane of the same size may run from a set of half that rating. Ask the crane supplier for the hoist motor data.
Should a construction site rent or buy its generator?
For a site of under 18 months with changing load, rental usually wins because the set can be swapped as the phase changes and maintenance is included. Contractors running several sites often buy a core fleet of small sets and rent the large ones for the structure and commissioning phases.
What earthing does a site generator need?
The generator neutral must be solidly earthed at the set through a measured electrode, with the frame bonded to the same earth, and every board feeding portable tools needs 30 mA residual current protection. Reinforcing steel is not an acceptable earth electrode on its own.
Does CPCB IV+ apply to construction site generators?
New diesel generators up to 800 kW supplied in India must meet CPCB IV+ norms wherever they are used. Older rental sets built to earlier norms can still operate, subject to state pollution control board conditions.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.