How to use this calculator
- Enter the total connected load in watts. This is the sum of all electrical devices expected to run simultaneously.
- Enter the starting load in watts. This accounts for surge currents from motors or compressors starting up.
- Enter the power factor of your load, typically between 0.8 and 1.0. If unknown, use 1.0 for purely resistive loads.
- Click "Calculate" to get the recommended generator size in kVA and kW.
- Use the recommended size to select a generator that meets or exceeds this rating.
Complete Guide to Generator Sizing Calculator
Generator sizing is a critical step in ensuring reliable power supply for residential, commercial, or industrial applications. The generator must be capable of supplying the total connected load, including transient starting currents, without excessive voltage drop or overload.
The total connected load is the sum of all devices expected to run simultaneously, measured in watts. Starting loads, especially from motors and compressors, can be significantly higher than running loads and must be included to avoid undersizing.
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amperes). Since generators are rated in kVA, the PF is used to convert watts to kVA. A lower PF means the generator must supply more apparent power for the same real power.
A safety margin of 25% is added to accommodate transient loads, future expansion, and to ensure the generator operates within safe limits. The final recommended size is rounded up to the nearest standard generator rating.
Proper generator sizing improves reliability, prevents damage, and ensures compliance with electrical codes and standards.
Safety & Common Mistakes
Warning: Electricity is dangerous and can cause serious injury or death. Always consult a licensed electrician or electrical engineer before installing or modifying electrical systems.
Common mistakes include undersizing the generator, ignoring starting loads, and using incorrect power factor values. These errors can lead to generator overload, equipment damage, and unsafe operating conditions.
Always verify your calculations and consider future load growth. Use proper protective devices and follow local electrical codes and standards.
Never attempt to install or service generators without proper training and safety precautions.
Real World Example
Running a 100 ft run of 12/2 Romex cable to power a workshop with a total connected load of 8,000 watts, including a 2,000 watt motor starting load, and a power factor of 0.9.
- Step 1: Enter Connected Load: Input the total connected load of 8,000 watts into the calculator.
- Step 2: Enter Starting Load: Input the motor starting load of 2,000 watts to account for surge current.
- Step 3: Enter Power Factor: Input the power factor of 0.9, typical for mixed resistive and inductive loads.
- Step 4: Calculate: The calculator computes the total load, applies the power factor, adds a 25% safety margin, and rounds up to the nearest standard generator size.
Result: Recommended generator size is 15.0 kVA / 13.5 kW, ensuring reliable operation with margin for starting loads and future expansion.
Frequently asked questions
Why do we add a 25% safety margin when sizing a generator?
A 25% safety margin is added to account for transient loads such as motor starting currents, future load expansions, and to ensure the generator operates within safe limits without being overloaded. This margin helps improve reliability and longevity of the generator by preventing frequent overload conditions.
What is the significance of power factor in generator sizing?
Power factor (PF) represents the phase difference between voltage and current in an AC circuit and affects the real power delivered. Since generators are rated in kVA (apparent power), the power factor is used to convert real power (kW) to apparent power (kVA). A lower power factor means the generator must supply more apparent power for the same real power load, impacting sizing.
How do starting loads affect generator sizing?
Starting loads, especially from motors and compressors, can draw significantly higher current than their running load, sometimes 3-7 times the rated current. Including starting load wattage in the calculation ensures the generator can handle these transient surges without voltage drops or damage.
Can I use this calculator for three-phase generators?
Yes, this calculator provides the total generator size in kVA/kW regardless of single or three-phase systems. However, for three-phase systems, ensure the connected load input represents the total load across all phases. Further phase-specific calculations may be needed for detailed design.
What happens if I undersize my generator?
Undersizing a generator can cause frequent overloads, voltage drops, and potential damage to both the generator and connected equipment. It may also lead to nuisance tripping of protective devices and reduced generator lifespan. Proper sizing is critical for safe and reliable operation.
References & additional resources
Allowable Ampacities of Insulated Conductors.
Recommended Practice for Electric Power Distribution for Industrial Plants.
Comprehensive guide on sizing generators for various applications.