How to size a 750 kVA standby diesel generator for a Saudi commercial mall

## TL;DR

Procuring a standby generator for a Saudi commercial mall requires balancing heavy HVAC loads against strict SEC and SASO compliance. For a 750 kVA unit, the buyer must account for the high ambient temperatures in the Kingdom, which can reach 50°C, necessitating significant engine derating. A 750 kVA rating (Standby) typically supports a continuous running load of approximately 600 kVA (480 kW) at a 0.8 power factor, with the remaining capacity reserved for motor starting surges from chillers or elevators. In Saudi Arabia, any installation must adhere to SEC (Saudi Electricity Company) connection standards and SASO safety regulations. Failure to account for the starting current of large HVAC motors (often 5-7 times the running current) is the most common cause of system collapse during a power outage. This guide provides the exact calculation steps to ensure your 750 kVA specification is robust enough for the local climate and load profile.

## Calculating the standby diesel generator duty point

To calculate the duty point for a 750 kVA generator in a mall environment, you must first aggregate the 'Total Connected Load' and apply a diversity factor. For a commercial mall, not all loads (lighting, escalators, HVAC) peak simultaneously. However, in Saudi Arabia, HVAC often accounts for 60-70% of the load. The sizing equation is: kVA = (Running kW / Power Factor) × 1.25 safety margin. To this, you must add the 'Surge Allowance' for the largest motor. For example, if your running load is 450 kW and your largest motor is a 75 kW chiller starting Direct-On-Line (DOL), the surge can exceed 400 kVA momentarily. The 750 kVA rating is a 'Standby' rating, meaning it is intended for use during the duration of a utility power outage. It should not be loaded at 100% for extended periods; typically, a 70-80% average load factor over 24 hours is recommended to maintain engine longevity and prevent 'wet stacking' caused by under-loading.

## Standards and Saudi Arabia codes that apply

In Saudi Arabia, the primary regulatory body is SASO (Saudi Standards, Metrology and Quality Organization), which enforces standards based on IEC 60034 for rotating electrical machines. The generator must be SASO-certified and IECEE-recognised for import. Furthermore, the Saudi Electricity Company (SEC) specifies the requirements for the Automatic Transfer Switch (ATS) and the physical housing of the unit to ensure it doesn't back-feed into the grid. ISO 8528 is the international benchmark for diesel gensets; for a mall, a 'Class G2' performance level is usually sufficient for lighting and general power, but 'Class G3' may be required if the mall houses sensitive data centres or high-end telecommunications equipment. NFPA 110 (Emergency and Standby Power Systems) is often adopted by Saudi consultants for life-safety systems like fire pumps and smoke extraction fans. Compliance ensures that the generator can assume the full life-safety load within 10 seconds of a utility failure.

## Common procurement traps for commercial mall

Ambient derating is the most frequent procurement error in the GCC. Most manufacturers rate their 750 kVA units at 25°C at sea level. In Riyadh or Jeddah, where ambient temperatures hit 50°C, the engine’s ability to cool itself decreases, and the air density drops. This can result in a 10-15% derate in actual available kW. Another trap is ignoring 'Harmonic Distortion' from non-linear loads. Malls are filled with LED lighting and Variable Frequency Drives (VFDs) for AHUs, which produce harmonics that cause heat build-up in the alternator windings. Buyers should specify an alternator with a 2/3 pitch winding to mitigate these effects. Finally, failing to specify the fuel tank capacity for at least 8-12 hours of full-load operation can lead to operational failure during extended grid instability, which is a critical risk for commercial tenants relying on refrigeration and security systems.

## Worked example for a 750 kVA commercial mall

Let us calculate the requirements for a medium-sized mall.

1. **Running Load:** 440 kW (Lighting, small power, escalators).

2. **Conversion to kVA:** 440 kW / 0.8 pf = 550 kVA.

3. **Safety Margin:** 550 kVA × 1.25 = 687.5 kVA.

4. **Motor Surge:** The largest motor is a 90 kW HVAC compressor (Star-Delta start). The starting surge is approx. 3.5 times the running current.

- Running kVA of motor = 90 / 0.8 = 112.5 kVA.

- Surge kVA = 112.5 × 3.5 = 393.75 kVA.

5. **Total Requirement:** Using a step-loading analysis, we ensure the 750 kVA unit can handle the 550 kVA base load plus the transient dip caused by the 90 kW motor.

6. **Final Selection:** A 750 kVA (Standby) / 680 kVA (Prime) unit is selected. After applying a 10% derating for 50°C ambient heat, the effective standby capacity is 675 kVA, which comfortably covers our 550 kVA running load with room for motor starting.

### How much should I derate a 750 kVA generator for Riyadh's summer?

For Riyadh, you should typically allow for a 10% to 15% derate from the standard ISO rating due to ambient temperatures reaching 50°C and the city's elevation. Always request the manufacturer’s derating chart for the specific engine and radiator package.

### Does a 750 kVA generator require a special permit in Saudi Arabia?

Yes, installations usually require approval from the Civil Defence for fuel storage safety and must comply with SEC (Saudi Electricity Company) regulations for the ATS connection to ensure no parallel operation with the grid occurs without synchronization equipment.

### Can a 750 kVA generator start a 150 kW chiller motor?

It depends on the starting method. A 150 kW motor using Direct-On-Line (DOL) starting may cause a voltage dip exceeding 30%, potentially tripping the generator. Using a VFD or Soft Starter is highly recommended to keep the 750 kVA unit within its transient response limits.