In short. In the mid-range, roughly 60 to 500 kVA, temporary power is governed by motors and the order in which loads come on, not by the average demand. A set that carries the final running load comfortably can still fail during a direct-on-line compressor start, a simultaneous restart after an outage or the energisation of a transformer. Size for the hardest operating moment, and consider whether staged switching solves the problem more cheaply than a larger generator.
What makes the mid-range different
Below this band, one pump or one air-conditioner usually decides the set, as described under small generator rental. Above it, the project becomes a temporary power plant with synchronising and multi-layer distribution, covered under large generator rental. In between sit most industrial and commercial hires in Hyderabad: pumping stations, compressors, HVAC plant, lifts, packaging and machining lines, pharmaceutical utilities, and a factory block while its transformer is out. These loads share a characteristic: modest running demand and severe transient demand.
Three dynamic events that trip an adequately sized set
Direct-on-line motor starting
An induction motor started across the line draws about six times its full-load current at a power factor near 0.3 to 0.4. The alternator sees a large reactive surge, voltage dips, and if the dip is deep enough the motor fails to accelerate, contactors drop out or electronic loads reset. Generator manufacturers publish the kVA a set can start for a given dip; the starting kVA guide explains how to read those figures against motor data.
Simultaneous load restoration
After an outage, every chiller, compressor and pump whose contactor held in tries to restart together when supply returns. A set sized for one motor start at a time meets the sum of all of them, and stalls. Restart sequencing through timers or the plant control system is part of the rental design.
Transformer energisation
Where the rental set feeds through a transformer, the inrush on energisation can reach eight to twelve times the transformer’s rated current for a few cycles. A set that can run the transformer’s load for a year can still trip on protection or collapse voltage at the moment of closing. Running capacity is not proof that the set can energise the transformer; energise before the load is connected and check the sequence.
The rental engineering path
| Step | What is established | Typical effect on the hire |
|---|---|---|
| Operating load | Real running kW and kVA across the shift | Sets the floor for the rating |
| Largest step | Biggest single motor or transformer and what else is on when it starts | Usually sets the actual rating |
| Starting method | DOL, star-delta, soft starter or VFD on each large motor | Can halve the transient demand |
| Cable distance | Route length and conductor size to the panel | Adds to voltage dip during starts |
| Voltage | 415 V direct or stepped through a transformer | Adds inrush and a second protection layer |
| Generator configuration | One set, or two smaller sets for staging and redundancy | Changes cost, footprint and resilience |
| Distribution | Temporary boards, outgoing protection, earthing | Defines what can be isolated or shed |
| Sequencing | Order and timing of load application and restart | Often the cheapest fix for a transient problem |
The path produces a system requirement rather than an equipment list. The voltage dip guide gives a method for estimating the dip from motor starting kVA and generator transient reactance; as an illustration (illustrative), a 75 kW compressor started direct-on-line on a 250 kVA set with 60 kW already running can produce a dip of 25 to 30 percent, while the same compressor on a soft starter limiting current to three times full load keeps the dip near 10 percent.
Better switching before a bigger generator
When a transient drives the rating up by a size or two, the alternatives are worth costing. A soft starter on the one offending motor, a time-delay relay that staggers restarts after an outage, or a change in the order in which the operator brings the plant up may allow a set one size smaller, which saves hire cost and keeps the engine better loaded. Sometimes the most effective rental solution is better switching, not more kVA. A control that prevents two large motors from starting within the same ten seconds is inexpensive and protects the permanent installation as well.
Keeping the design true over the hire
A configuration approved at mobilisation is based on a defined demand. Mid-range hires often run for weeks, and additional equipment is connected over that time without anyone recalculating: a second compressor, a welding set, extra lighting for a night shift. Loads assumed to be non-simultaneous begin to run together, and the set that was correctly sized becomes overloaded by degrees. Operations planning should include a method of approving material load changes and a weekly check of the generator’s load meter at the busiest point in the shift. Where the hire feeds several areas, temporary distribution panels with outgoing protection let the operator isolate or shed a lower-priority feeder rather than lose the whole supply.
Transfer arrangements in the mid-range
Most mid-range hires connect through an open-transition changeover or ATS, with a short dead interval between sources. Some projects lengthen that interval so motor fields decay before reconnection; that is a project decision, not a default. Closed-transition transfer, where the sources overlap briefly, requires synchronising and is far more complex than an ordinary rental changeover, so it is specified only where the process cannot tolerate even a short break. The rental and temporary power overview covers the site and duration questions common to all hires.
Common mistakes
- Selecting the set from the plant’s average monthly kW. The average hides the start of the largest motor and the set trips or stalls at the first real test.
- Allowing all motors to restart together after a transfer. The combined inrush exceeds anything the set can supply and the supply collapses just when it was needed.
- Energising a transformer with the load already connected behind it. Inrush plus load current trips the generator breaker and the sequence has to be repeated.
- Ignoring the cable run when estimating voltage dip. The dip at the motor terminals is worse than at the generator and the motor fails to accelerate.
- Letting the connected load grow unchecked during a long hire. The set that started at 70 percent load is quietly running at 100 percent by the fourth week.
What to send us
Send a load list with each motor’s kW and starting method, the total running load, whether a transformer sits between the set and the load, the cable route length, and how the plant is brought back up after an outage. Nameplate photographs of the largest motors and the incoming panel, and a note on duration and operating hours, allow us to check starting capability and propose staging. Use the generator rental enquiry.
Questions engineers ask
What size rental generator do I need for a 45 kW compressor?
If the compressor starts direct-on-line with other load already running, a set of around 200 to 250 kVA is commonly needed to hold the voltage dip within limits, even though the running load is under 60 kVA. With a soft starter or star-delta start, a set of roughly half that rating may be adequate. The exact answer depends on the generator’s transient data and on what else is on the bus.
Is it better to rent one 250 kVA set or two 125 kVA sets?
Two sets give redundancy and let one run efficiently at light load, but they need synchronising equipment to share a bus. One set is simpler and cheaper for most mid-range hires. Choose two when a single failure would stop a continuous process, or when the load varies so much that one large set would run very lightly loaded for long periods.
Why does the generator trip when the plant restarts after a power cut?
Because every motor whose contactor stayed in tries to restart at once, and the combined starting current far exceeds the set’s capability. Fit staggered restart timers or sequence the restart manually, and check that any protection relay on the generator outgoing breaker is not set so tightly that it trips on normal motor inrush.
How much voltage dip can my plant tolerate?
Most contactors hold in down to about 80 percent of rated coil voltage and most motors will still accelerate at 80 percent terminal voltage, so a dip of 15 to 20 percent at the motor terminals is a common design limit. Electronic loads and some drives are less tolerant. Confirm with the equipment data for the sensitive items on the bus.
Can the rental set feed my plant through our existing transformer?
Yes, provided the generator voltage matches the transformer’s winding, the inrush at energisation is within the set’s capability and the protection is set accordingly. Energise the transformer with the downstream load off, then bring loads on in sequence. Tell us the transformer kVA, vector group and impedance so this can be checked.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.