Accurate generator sizing is critical for ensuring reliable power supply and operational efficiency. It involves calculating the generator capacity based on the installed electrical load.
This article explores generator sizing methodologies aligned with IEEE and IEC standards, including practical calculators, formulas, and real-world examples. Readers will gain comprehensive insights into selecting the right generator size for various applications.
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- Calculate generator size for a 50 kW industrial load with 0.8 power factor.
- Determine generator capacity for a residential building with 120 kVA installed load.
- Find required generator rating for a hospital with 200 kW peak load and 0.9 power factor.
- Estimate generator size for a commercial complex with 350 kVA total connected load.
Common Values and Parameters for Generator Sizing Based on Installed Load
| Parameter | Typical Range / Value | Unit | Notes |
|---|---|---|---|
| Installed Load (Connected Load) | 1 – 1000+ | kW / kVA | Sum of all electrical loads connected |
| Demand Factor | 0.6 – 1.0 | Unitless | Ratio of maximum demand to installed load |
| Power Factor (PF) | 0.7 – 1.0 | Unitless | Ratio of real power to apparent power |
| Starting Load Factor | 1.0 – 1.5 | Unitless | Accounts for motor starting currents |
| Safety Margin | 10% – 25% | % | Additional capacity for future expansion and contingencies |
| Voltage Level | 230 / 400 / 415 | Volts | Common low voltage levels for generators |
| Frequency | 50 / 60 | Hz | Standard power system frequencies |
Typical Demand Factors by Application (IEEE and IEC Guidelines)
| Application | Demand Factor | Reference Standard | Notes |
|---|---|---|---|
| Residential Buildings | 0.4 – 0.6 | IEC 60364-8-1 | Lower diversity due to non-simultaneous usage |
| Commercial Complexes | 0.6 – 0.8 | IEEE Std 141 | Moderate diversity, includes lighting and HVAC |
| Industrial Plants | 0.7 – 1.0 | IEEE Std 242 | High simultaneous load, motor starting considered |
| Hospitals | 0.8 – 1.0 | IEC 60364-7-710 | Critical loads, minimal diversity allowed |
Essential Formulas for Generator Sizing Based on Installed Load
Generator sizing requires calculating the required apparent power (kVA) considering installed load, demand factor, power factor, starting load, and safety margin. The following formulas are fundamental:
| Formula | Description |
|---|---|
| Generator Size (kVA) = (Installed Load × Demand Factor) / Power Factor × Starting Load Factor × (1 + Safety Margin) | Calculates the required generator capacity considering all load factors and margins. |
| Installed Load (kW) = Σ (Individual Load kW) | Sum of all connected loads in kilowatts. |
| Demand Factor = Maximum Demand / Installed Load | Ratio representing the actual maximum load compared to total connected load. |
| Power Factor (PF) = Real Power (kW) / Apparent Power (kVA) | Ratio of active power to total power, typically 0.8 to 1.0 for sizing. |
| Starting Load Factor = 1 + (Motor Starting kVA / Running kVA) | Accounts for transient high currents during motor starts. |
| Safety Margin (%) = 0.10 to 0.25 (10% to 25%) | Additional capacity to cover future load growth and uncertainties. |
Explanation of Variables
- Installed Load: The total connected electrical load in kilowatts (kW) or kilovolt-amperes (kVA).
- Demand Factor: A coefficient less than or equal to 1, representing the ratio of maximum actual load to installed load.
- Power Factor (PF): The cosine of the phase angle between voltage and current, indicating load efficiency.
- Starting Load Factor: Multiplier to account for motor starting currents, typically between 1.0 and 1.5.
- Safety Margin: Percentage added to the calculated size to ensure reliability and future expansion.
Real-World Application Examples
Example 1: Industrial Plant Generator Sizing
An industrial plant has an installed load of 500 kW with a demand factor of 0.85. The power factor is 0.9, and the starting load factor is 1.3 due to several large motors. A safety margin of 15% is required. Calculate the required generator size in kVA.
Step 1: Calculate the adjusted load
Adjusted Load = Installed Load × Demand Factor = 500 kW × 0.85 = 425 kW
Step 2: Convert to apparent power (kVA)
Apparent Power = Adjusted Load / Power Factor = 425 kW / 0.9 ≈ 472.22 kVA
Step 3: Apply starting load factor
Starting Load Adjusted = 472.22 kVA × 1.3 ≈ 613 kVA
Step 4: Add safety margin
Final Generator Size = Starting Load Adjusted × (1 + Safety Margin) = 613 kVA × 1.15 ≈ 705 kVA
Result: A generator rated at approximately 705 kVA is recommended for this industrial plant.
Example 2: Hospital Emergency Generator Sizing
A hospital has an installed load of 300 kW with a demand factor of 0.95. The power factor is 0.95, and the starting load factor is 1.1 due to medical equipment. A safety margin of 20% is applied. Determine the generator size.
Step 1: Calculate the adjusted load
Adjusted Load = 300 kW × 0.95 = 285 kW
Step 2: Convert to apparent power
Apparent Power = 285 kW / 0.95 ≈ 300 kVA
Step 3: Apply starting load factor
Starting Load Adjusted = 300 kVA × 1.1 = 330 kVA
Step 4: Add safety margin
Final Generator Size = 330 kVA × 1.20 = 396 kVA
Result: A 400 kVA generator is suitable for the hospital’s emergency power needs.
Additional Technical Considerations for Generator Sizing
- Load Diversity: IEEE and IEC standards emphasize considering load diversity to avoid oversizing. Diversity factors reduce the total load based on non-simultaneous usage.
- Motor Starting Currents: Large motors can draw 5-7 times their rated current at startup. The starting load factor accounts for this transient demand.
- Harmonics and Power Quality: Non-linear loads can affect generator sizing due to harmonic distortion. IEEE Std 519 provides guidelines for harmonic limits.
- Altitude and Temperature: Generator output decreases with altitude and temperature. Correction factors must be applied per manufacturer data.
- Fuel Type and Efficiency: Diesel, gas, or dual-fuel generators have different performance characteristics affecting sizing and runtime.
- Standby vs Prime Rating: Standby generators handle emergency loads intermittently, while prime-rated generators support continuous operation. Sizing criteria differ accordingly.
References to IEEE and IEC Standards
- IEEE Std 141-1993 (Red Book) – Electric Power Distribution for Industrial Plants
- IEEE Std 242-2001 (Buff Book) – Protection and Coordination of Industrial and Commercial Power Systems
- IEC 60364-8-1 – Electrical Installations of Buildings – Selection and Erection of Electrical Equipment
- IEC 60364-7-710 – Electrical Installations in Medical Locations
- IEEE Std 519-2014 – Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
Summary of Best Practices for Generator Sizing
- Always start with an accurate inventory of installed loads, including all motors and sensitive equipment.
- Apply demand factors based on the type of installation and usage patterns as per IEEE and IEC guidelines.
- Consider power factor correction to optimize generator sizing and efficiency.
- Include starting load factors for motor-intensive applications to prevent undersizing.
- Incorporate a safety margin to accommodate future load growth and unforeseen conditions.
- Consult manufacturer data and standards for altitude, temperature derating, and fuel considerations.
- Validate sizing with real-world load testing and monitoring where possible.
By following these detailed methodologies and leveraging IEEE and IEC standards, engineers can ensure optimal generator sizing that balances cost, reliability, and performance.