Nameplate rating alone does not show whether a generator set will operate stably at a specific site. Temperature, altitude, cooling and combustion air, discharge air, fuel or gas supply, load steps and motor starts all affect selection, installation and performance.
A generator set may appear large enough on paper yet reach its limits when a motor starts, a major load block is applied or site conditions are unfavourable. Steady-state load, transient performance and installation conditions must therefore be assessed together.
Which site conditions matter?
- maximum and minimum ambient temperature
- altitude and atmospheric pressure
- humidity, dust, salt, rain and direct solar load
- cooling-air inlet and outlet plus permitted restriction
- exhaust routing and heat rejection
- foundation, vibration isolation, space and service access
- fuel storage or gas pressure and quality
- noise limits and distance to sensitive locations
Manufacturer limits and project calculations govern. An enclosure can change the cooling capability of the set.
What is a load step?
A load step is a rapid change in electrical demand, for example when a large pump, compressor or process section is connected. Unlike the utility grid, a generator set is a finite power source. A step can therefore cause a temporary voltage and frequency response.
The magnitude, power factor and dynamics of the step, plus the tolerance of connected loads, all matter.
Why are motor starts demanding?
A motor can draw substantially more current while accelerating than during normal operation. Starting kVA, starting kW and starting power factor must be assessed separately from running values. Direct-on-line, star-delta, soft-starter and variable-frequency drive starting methods affect the generator differently.
A fixed multiplier is not sufficient for final selection. Motor data, starting method and driven mechanical load should be recorded.
How can load sequencing help?
Starting large loads sequentially instead of simultaneously can reduce the maximum transient demand. Critical loads are prioritised and non-essential loads can be delayed or enabled according to available capacity. Control and ATS/AMF logic must implement the sequence reliably.
What data belongs in the sizing study?
- running kW, kVA, current and power factor for each load
- starting current, starting kVA/kW and starting method
- coincidence and planned load steps
- permitted voltage and frequency dip
- sensitivity of controls, UPS and electronic loads
- site conditions including expected extremes
- current generator, engine and alternator data
Complex installations benefit from manufacturer-based sizing software or an engineering calculation.
Frequently asked questions
How should motor starting currents be considered when sizing a generator?
Record at least the motor data, starting method, starting current, starting kVA, starting kW, starting power factor, acceleration time and mechanical load. Then check the worst credible combination and starting sequence against the permitted voltage and frequency dip and the specific transient data for the load, alternator and generator set; a fixed multiplier is not sufficient.
Why can a motor start overload a generator that looks large enough?
Because short-term starting demand and starting power factor can differ substantially from the steady-state rating.
Does a soft starter always reduce the required generator size?
It can affect starting current, but the result depends on settings, motor, load torque and generator response and must be checked.
Which site data should be available early?
At least temperature range, altitude, ventilation, exhaust routing, dust and humidity, space, noise limits and fuel or gas supply.
Record load profiles as a timeline
An equipment list is insufficient. Record base load, start time, starting method, inrush, inertia, acceleration time, the next load block and cyclic or regenerative loads. For each relevant motor, provide rated power, voltage, phases, efficiency, power factor and mechanical load type.
Soft starters and VFDs do not automatically reduce generator requirements. Settings, harmonics, torque profile and transitions must be checked against the exact generator set.
Sources and technical basis
The linked manufacturer sources provide technical context. Current datasheets, manuals and agreed specifications govern each specific project.
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Send us the load profile, operating duty and site conditions. We support technical sizing and a suitable configuration.
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