There is not a single correct generator sizing solution. Following are several methods that, when mixed with good judgment, should result in an appropriately sized generator. Remember to consider load growth, seasonality, and effects of starting motors.
Never add Amps when sizing a generator. Convert Amps to kW and add kW to determine the required generator size. Power factors for various motor loads vary widely. Adding Amps without properly accounting for the power factor and/or mixing voltages will result in improperly sizing the generator. See our tools page for conversions and other resources.
When motors start they create a current surge that step loads the generator and creates a voltage dip. After selecting a generator, reference the generator’s surge capability. Verify that voltage dip is adequate for the application.
Some applications utilize an uninterpretable power supply (UPS) to back up critical loads. Please read sizing guide for this load type.
Use a clamp-on Amp meter or power analyzer to measure facility load levels. Clamp each lead separately and take the measurement during peak usage levels.
- 240v 1 Ø Applications: To determine peak usage in kW, add the highest Amp readings from the two legs, multiply by 120 and divide by 1,000.
(L1 + L2)120/1000
Size the generator 10 to 20% larger than the peak measured load.
- 3Ø Applications: Add the peak Amp readings from all three legs and divide by 3 to determine peak Amps. Multiply peak Amps by volts, multiply the result by 1.732 (square root of 3), then divide by 1000 to convert Amps to kW.
Peak Amps = (L1+L2+L3)/3
kW = [(Peak Amps x Volts) x 1.732]/1000*
*Assumes power factor of 1.0
Size the generator 20 to 25% larger than the peak measured load.
Determining Existing Loads/Billing History Method (220.87 NEC 2008)
Many customers have a utility rate structure that has a peak demand charge. Assuming no material changes in your electrical devices during the year, using a year’s worth of electric bills, size the generator 25% larger than the largest peak demand. When using this methodology, you should always confirm your conclusions by checking peak loads with a clamp-on Amp meter as suggested above.
Verify motor and UPS load compatibility.
Load Summation Method
- Calculate Total Electrical Loads =
(Running kW of all peak motor loads minus the largest motor) + (kW of all peak non-motor loads) + (Starting kW of largest motor load)
- Add 20 to 25% to total kW
- Confirm that voltage dip is within acceptable limits by comparing motor LRA to generator surge capability
- Confirm UPS compatibility
System Capacity – Load Calculation
If the local municipality or state you are in has adopted the 2008 NEC Code, you may be required to use this step. Article 702 of the 2008 NEC includes a new requirement for sizing (702.5B). If no other method for sizing is acceptable, sizing of the generator shall be made in accordance with Article 220 of the NEC.
Ball Park Estimates
Estimate based on 60% service size
240 Volts, 1 Ø:___________Amps x .15 = ____________kW
208 Volts, 3 Ø:___________Amps x .22 = ____________kW
240 Volts, 3 Ø:___________Amps x .25 = ____________kW
480 Volts, 3 Ø:___________Amps x .50 = ____________kW
Estimate based on square footage
- kW = 30 kW +5 watts/sq. ft
Amps to kW Rule of Thumb
For 480 volt systems: Amps = kW x 1.5
For 208 volt systems: Amps = kW x 3.5
For 240 volt 3 Ø systems: Amps = kW x 3
For 240 volt 3 Ø systems: Amps = kW x 4