Specifying a diesel generator based purely on connected load nameplate ratings is one of the most common mistakes made during the procurement process. While it is tempting to add up the kilowatt figures from equipment schedules and select a generator that covers the total, this approach routinely produces undersized or poorly matched systems that fail under real operating conditions.
Two electrical characteristics in particular are consistently underestimated during generator sizing: motor starting currents and harmonic distortion. Both can place significant additional stress on a generator, and neither is visible from a standard equipment list. Understanding how each factor influences generator specification is essential for any commercial or industrial backup power system intended to perform reliably throughout its service life.
Electric motors do not draw a steady current from the moment they are switched on. During the initial start-up phase, a motor can draw between five and eight times its normal running current for a brief period - sometimes referred to as inrush current or starting current. This transient demand places an immediate and significant load on the generator's alternator and engine governor.
If a generator is sized only for running loads, the starting sequence of even a single large motor can cause the generator to voltage-dip, trip protective relays, or in severe cases, stall entirely. For sites operating air conditioning compressors, pumps, conveyor systems, lifts or refrigeration plant, motor starting currents are a critical design consideration.
The issue becomes more complex when multiple motors are capable of starting simultaneously or in rapid sequence. A professional electrical load analysis will assess not just individual motor starting currents but the probable sequence of starting events and their combined effect on generator capacity. This is why generator specification for industrial generators, manufacturing facilities and commercial plant rooms requires detailed engineering input rather than a straightforward nameplate calculation.
Variable frequency drives (VFDs) can significantly reduce motor starting currents by ramping motor speed gradually, and their presence on site should be documented accurately during the site survey. However, VFDs introduce their own electrical challenges - specifically harmonic distortion - which must be addressed separately in the generator specification process.
Modern electrical installations increasingly contain equipment that does not draw current in a smooth, sinusoidal waveform. Variable frequency drives, LED lighting systems, uninterruptible power supplies (UPS), switch-mode power supplies and other electronic loads generate harmonic currents - distortions in the electrical waveform that can affect generator performance and connected equipment reliability.
Harmonics are expressed as multiples of the fundamental supply frequency. The third, fifth and seventh harmonics are typically the most significant in commercial and industrial environments. When harmonic currents are present in substantial levels, they can cause generator alternator heating, voltage waveform distortion, nuisance tripping of sensitive equipment and interference with communications systems.
A generator that has been sized for kilowatt demand alone, without consideration of the harmonic content of the site's electrical loads, may struggle to maintain stable voltage and frequency under real operating conditions. This is a particularly relevant concern for critical power systems supporting data centres, healthcare environments, precision manufacturing and any facility relying on sensitive electronic control systems.
Correctly accounting for harmonics during generator specification may influence alternator selection, the need for harmonic filters, or the choice of a generator with a lower subtransient reactance - all decisions that require engineering assessment of the site's actual electrical characteristics rather than generic assumptions.
Both motor starting currents and harmonic distortion underline why professional electrical load calculations are an essential part of generator specification, not an optional addition. A site survey that identifies connected loads without examining their electrical characteristics will produce incomplete data, leading to a generator that is technically undersized even if its kilowatt rating appears adequate on paper.
A thorough load analysis for standby power systems and prime power generators should include:
This level of analysis is what separates engineering-led generator consultancy from a straightforward sales quotation. Without it, the risk of specifying a generator that underperforms in service - or fails entirely when backup power is most needed - remains significant.
Once the full electrical picture of a site has been established, generator selection can be made on technical merit. Different engine and alternator combinations from manufacturers such as Perkins, Cummins, Volvo, Doosan and Baudouin offer varying performance characteristics in terms of transient response, voltage regulation and harmonic tolerance.
Working with an independent, multi-brand generator supplier means that equipment can be selected to match the specific electrical demands of the site rather than being constrained to the nearest available model within a single manufacturer's range. This independence is particularly valuable where motor starting performance or harmonic resilience are defining requirements of the specification.
A generator system correctly specified for motor starting currents and harmonic distortion will operate more reliably, require less corrective maintenance and provide more consistent performance throughout its service life - which for well-maintained equipment can exceed twenty years. Conversely, a system that has not accounted for these factors may appear to function adequately under light loads but prove unreliable precisely when backup power is most critical.
For commercial and industrial organisations where power continuity carries genuine operational and financial consequences, investing in accurate generator specification at the outset is not a cost - it is the most effective way to protect the value of the installation over the long term.
For more information on engineering-led generator sizing and specification, visit EA Power Systems.
