Sizing Guide for Commercial Generators

Commercial generators supply power for manufacturing, retail and other commercial applications during periods when the power from the local electric utility is interrupted. Power interruptions occur during unplanned outages and when the utility has a contract with the commercial entity that allows them to interrupt the supply during periods high power usage.

The loss of power to refrigeration units, heating units and other applications is a critical problem during a power outage. Standby commercial generators provide power to prevent losses associated with outages, keep businesses operating, and allow critical building services to continue.

Sizing Considerations

Considerations for sizing a commercial generator include choosing single- or three-phase power, voltage selection, and total power output. Most commercial applications require a greater reserve or surge capability, especially when large motors are involved or a number of units operate independently of each other and turn on and off frequently.

The inrush current of large motors can exceed 200 percent of the normal full-load current for short periods, and lack of sufficient reserve capacity can make starting motors difficult or cause damage to the motor. Include the allowable voltage drop for any device when calculating the reserve capacity required for the generator. A momentary voltage drop of 30 percent might be fine for some devices, but not others.

Large-scale commercial applications usually require an engineering plan and an engineer’s supervision during installation to ensure correct generator sizing and compliance with National Electrical Code (NEC) requirements.

The 2008 NEC includes articles 700, 701, 702 and 708 which govern the sizing, installation and use of commercial generators and the critical systems that operate during an emergency to ensure safety. Safety systems include alarms, fire pumps, health/safety systems, and any other system designed to protect life during an emergency. Consult your local commercial building code authority for details and implementation in your area, and what systems are required.

Generator Sizing

Full Load Capacity by Measurement

Estimate the required generator capacity by taking full-load current measurements during peak usage at the service panel. Use a clamp-on ammeter on each leg of the electrical service and add the measurements together. This provides the total amps used by the facility.

Divide the total amps by three for three-phase current, and by two for single-phase current, then multiply the result by the supply voltage, and again by 1000 for Kilowatts(kW) Required.

Add the power in kilowatts used by each emergency safety system according to articles 700, 701, 702 and 708 of the NEC to the kilowatts required to obtain Full Load Kilowatts.

Full Load Kilowatts = Total Amps x Supply Voltage x 1000.

Reserve Capacity = Full Load Kilowatts x 0.25.

For 100 percent power, generator size = Full Load Kilowatts + Reserve Capacity.

Full Load Capacity by History

Utilize your utility company’s billing system to find your maximum power usage. Most companies will include your peak demand on your monthly bill. Analyze the power usage over the previous year to find the highest peak demand and then add 25 percent for reserve capacity.

Full Load Capacity with Extensive Motor Use

Find the starting current for the largest motor that turns on and off and multiply it by the voltage for the number of watts required. For all other motor and non-motor load, multiply the current by the voltage for watts. Total the watts required for the largest motor and all remaining motor and non-motor loads for the total power and multiply by 1000 for kilowatts. Add 25 percent for reserve/surge capacity and size the generator according the result.

Square Footage Measurement

The square footage sizing method is most frequently used in retail applications such as grocery stores, restaurants, convenience stores, and other commercial applications. Use the following calculations for determining generator size.

Retail application: 50 kilowatts + 10 watts per square foot.

Other commercial application: 50 kilowatts + 5 watts per square foot.

*These Sizing Guides are intended for estimating purposes only. Actual power requirements of high demand appliances vary widely by brand, model and capacity. Always have an authorized dealer or qualified electrician analyze your specific requirements before making a purchase decision.