3 Phase Generator Size Calculator
Calculate the appropriate size three-phase generator needed for commercial, industrial, or large residential applications by entering your electrical loads below.
System Information
Motor Loads
Added Motors
Lighting and Receptacle Loads
HVAC Loads
Special Loads
Add Custom 3-Phase Loads
Added Custom Loads
About 3 Phase Generator Sizing
Selecting the right size three-phase generator for commercial or industrial applications requires careful calculation of all connected loads, with special consideration for motor starting requirements and power factor. Our calculator helps you determine the appropriate generator capacity based on your specific electrical system requirements.
3 Phase Generator Size Guide
| Generator Size | Application | Typical Coverage |
|---|---|---|
| 30-50 kW | Small Commercial | Retail stores, small offices |
| 60-100 kW | Medium Commercial | Restaurants, medical offices, schools |
| 125-250 kW | Large Commercial | Hotels, supermarkets, data centers |
| 300-500 kW | Industrial | Manufacturing, warehouses |
| 600-2000 kW | Large Industrial | Hospitals, factories, high-rises |
Common 3-Phase Power Requirements
| Equipment | Running kW | Starting kVA | Power Factor |
|---|---|---|---|
| 20 HP Motor (Direct Start) | 15 kW | 90 kVA | 0.85 |
| 50 HP Motor (Soft Start) | 37 kW | 75 kVA | 0.87 |
| 100 Ton Chiller | 90 kW | 300 kVA | 0.82 |
| Elevator (10 HP) | 7.5 kW | 45 kVA | 0.75 |
| Welding Machine | 25 kW | 30 kVA | 0.65 |
| LED Lighting (1000 fixtures) | 15 kW | 15 kVA | 0.95 |
Essential Considerations for 3-Phase Generators
1. Motor Starting Methods
Different motor starting methods significantly impact generator sizing:
| Starting Method | Starting Current | Impact on Generator | Typical Applications |
|---|---|---|---|
| Direct Online (DOL) | 600-800% FLA | High surge requirement | Small motors, pumps |
| Star-Delta | 200-300% FLA | Reduced surge | Medium motors, fans |
| Soft Starter | 150-300% FLA | Gentle ramp-up | Conveyors, compressors |
| Variable Frequency Drive | 100-150% FLA | Minimal surge | Precision control systems |
2. Power Factor Considerations
Understanding power factor is crucial for proper generator sizing:
- Definition: Ratio of real power (kW) to apparent power (kVA)
- Typical Values:
- Motors: 0.8-0.9
- Lighting: 0.95-1.0
- Welding: 0.5-0.7
- Computers: 0.6-0.7
- Generator Impact: Low power factor loads require larger generators
- Correction: Capacitor banks can improve power factor
3. Voltage Dip Requirements
Critical systems have strict voltage dip limitations:
| Application | Max Voltage Dip | Recovery Time | Generator Considerations |
|---|---|---|---|
| Data Centers | 10% | <100ms | UPS required |
| Medical Facilities | 15% | <500ms | High inertia generator |
| Manufacturing | 20% | <1s | Soft starters recommended |
| Commercial Buildings | 25% | <2s | Standard generator acceptable |
Frequently Asked Questions
How does three-phase generator sizing differ from single-phase?
Three-phase generator sizing has unique considerations:
- Power calculations use √3 (1.732) factor for line voltage
- Motor starting currents are more critical
- Load balancing across phases is important
- Power factor has greater impact on sizing
- Harmonic distortion may need consideration
What's the difference between standby and prime power ratings?
Generator ratings vary by application:
- Standby: For emergency use, limited run time
- Prime: Continuous operation, unlimited hours
- Continuous: Same as prime but with 10% overload
- Standby ratings are typically 10-15% higher than prime
How do I account for harmonic loads in generator sizing?
For non-linear loads with harmonics:
- Add 20-30% capacity for moderate harmonic distortion
- Consider derating generator by 15-20% for high THD
- Specify generator with low subtransient reactance
- Install harmonic filters if THD > 10%
What size generator do I need for multiple motors?
For multiple motor applications:
- Sum all running kW requirements
- Add largest starting kVA to running kVA of others
- Consider staggered starting sequence
- Add 25% margin for future expansion
How does ambient temperature affect generator sizing?
Temperature impacts generator performance:
- Above 40°C (104°F): Derate 1% per 1°C over 40°C
- High altitude: Additional derating required
- Humidity: Affects cooling efficiency
- Enclosure type: Impacts cooling capability
Should I size for peak or average demand?
Generator sizing should consider:
- Peak demand for worst-case scenario
- Diversity factor for non-coincident loads
- Load sequencing possibilities
- Future expansion requirements
How do I calculate generator size for data centers?
Data center generator considerations:
- N+1 redundancy for critical loads
- 2N configuration for high availability
- Include UPS recharge current
- Account for cooling system loads
What's the impact of voltage regulation on generator sizing?
Voltage regulation affects:
- Motor starting capability
- Sensitive equipment performance
- Generator transient response
- Need for voltage regulators
How do I size a generator for a chiller plant?
Chiller plant considerations:
- Include all chiller components (compressors, pumps)
- Account for across-the-line or soft starter
- Consider lead/lag sequencing
- Add cooling tower fan loads
What's the difference between kVA and kW in generator sizing?
Understanding power measurements:
- kW (kilowatts): Real power doing work
- kVA (kilovolt-amperes): Apparent power
- Relationship: kW = kVA × power factor
- Generators are typically rated in kVA
How do I account for future expansion in generator sizing?
Planning for growth:
- Add 20-25% capacity margin
- Consider modular generator systems
- Plan for paralleling capability
- Ensure physical space for expansion
What's the impact of load sequencing on generator size?
Load sequencing benefits:
- Reduces required generator size
- Manages inrush currents
- Prioritizes critical loads
- Can be automated with PLC controls
How do I size a generator for a healthcare facility?
Healthcare specific requirements:
- Life safety and critical branches
- Equipment branches per NEC 517
- Strict voltage regulation needs
- Redundancy requirements
What's the impact of generator transient response on sizing?
Transient response considerations:
- Affects voltage dip during motor starts
- Subtransient reactance (X"d) important
- Higher inertia rotors better for motor loads
- May require oversizing for sensitive loads
How do I calculate generator size for a manufacturing plant?
Manufacturing plant specifics:
- Inventory all motor loads
- Consider production line sequencing
- Account for welding loads separately
- Include compressed air systems