How to size a 750 kVA standby diesel generator for a Qatari manufacturing plant

## TL;DR

Selecting a 750 kVA standby diesel generator for a Qatari manufacturing facility requires precision in calculating load profiles and environmental derating. In Qatar, the primary challenge is the combination of extreme heat (up to 50 °C) and high humidity, which can significantly reduce an alternator's efficiency and an engine's cooling capacity. A 750 kVA rating (Standby) usually supports a continuous running load of approximately 500-600 kW, providing a buffer for motor starting surges. Procurement managers must ensure compliance with Kahramaa regulations and QGEW standards. The sizing process must account for the power factor (typically 0.8) and a 1.25 safety margin to prevent the generator from running at 100% load, which shortens its lifespan. This guide breaks down the essential kVA formula, regional electrical codes, and a step-by-step calculation for a manufacturing environment with significant motor loads, ensuring your facility maintains power continuity during grid outages.

## Calculating the standby diesel generator duty point

The sizing of a generator starts with the total connected load in kilowatts (kW). For industrial plants, we convert this to kVA using the formula: kVA = (Running kW / Power Factor) × 1.25. The 1.25 factor provides a 25% safety margin for future load expansion and transient stability. However, the most critical factor in manufacturing is the 'starting kVA' of the largest motor. Motors started Direct-on-Line (DOL) can draw 6 to 7 times their running current. If your plant has a large 100 kW compressor, the surge could trip a marginally sized generator. Therefore, the duty point is the larger of: (Total Running kVA + Safety Margin) OR (Steady State kVA of all loads minus the largest motor + Starting kVA of the largest motor). For a 750 kVA set, you are typically looking at a sustained running load of no more than 600 kVA to ensure the engine operates within its optimal thermal envelope in the Qatari climate.

## Standards and Qatar codes that apply

In Qatar, the electrical infrastructure is strictly regulated by Kahramaa (Qatar General Electricity and Water Corporation). All standby generators must comply with Kahramaa’s 'Regulations for the Installation of Electrical Wiring, Electrical Equipment and Air Conditioning Equipment.' This includes specific requirements for the Automatic Transfer Switch (ATS) and earthing systems. Internationally, the generator must meet ISO 8528, which defines the performance classes (G1, G2, G3) for voltage and frequency regulation; manufacturing plants typically require G2 or G3 for sensitive machinery. Additionally, IEC 60034 governs the rotating electrical machines (the alternator). In some Qatari industrial zones, environmental regulations also dictate noise levels (dBA) at the boundary, necessitating sound-attenuated enclosures that meet Qatar Civil Defence (QCD) fire safety requirements for fuel storage and ventilation.

## Common procurement traps for manufacturing plant

The most expensive mistake in Qatar is ignoring site-specific derating. A generator rated 750 kVA at 'Standard Reference Conditions' (25 °C, sea level) will not produce 750 kVA at 50 °C in Mesaieed or Lusail. You must apply derating factors for altitude (minimal in Qatar) and temperature (significant). Another trap is the 'Leading Power Factor' caused by modern LED lighting and UPS systems, which can destabilize the alternator's voltage regulator. Furthermore, procurement often overlooks the 'Minimum Load' requirement; running a 750 kVA generator at less than 30% load (wet-stacking) leads to unburnt fuel in the exhaust and engine damage. Finally, ensure the ATS is 4-pole to comply with local earthing regulations, preventing stray currents during the transition from Kahramaa grid power to standby power.

## Worked example for a 750 kVA manufacturing plant

Imagine a factory with a total running load of 450 kW and a power factor of 0.8.

1. **Convert kW to kVA**: 450 / 0.8 = 562.5 kVA.

2. **Apply Safety Margin**: 562.5 × 1.25 = 703.125 kVA.

3. **Check Motor Surge**: The largest motor is a 75 kW DOL pump. Starting kVA ≈ 75 / 0.8 × 6 = 562.5 starting kVA.

4. **Transient Check**: When the motor starts, the load jumps. Total peak = (Total Running 562.5 - Motor Running 93.75) + Motor Starting 562.5 = 1031.25 kVA (momentary).

5. **Refinement**: While the peak is high, modern alternators can handle short-duration 300% surges. However, for steady-state reliability and Qatari heat derating, a 703 kVA requirement fits perfectly into the 750 kVA standard rating.

6. **Recommendation**: Procure a 750 kVA (Standby) / 680 kVA (Prime) unit with a Tropical Radiator to ensure continuous operation at 50 °C.

### Does Kahramaa require a specific type of ATS for generators?

Yes, Kahramaa regulations generally require a 4-pole Automatic Transfer Switch (ATS) to ensure that the neutral conductor is isolated during the switchover, maintaining the integrity of the earthing system.

### How much should I derate a generator for Qatar's summer?

Depending on the engine manufacturer, you should expect to derate the kVA capacity by 10% to 15% when ambient temperatures rise from 25 °C to 50 °C, unless the unit is equipped with a high-ambient cooling package.

### What is the difference between Standby and Prime ratings for a 750 kVA unit?

A 750 kVA Standby rating is for emergency use only during a utility outage (max 200 hours/year), whereas a Prime rating (usually 680 kVA) is for variable load applications where no grid is available.