At AP Electric & Generators, we want to make it easy for you find the perfect generator for your needs. As such, we have developed this guide to walk you through the process.
Commercial Standby Generator
While prolonged power outages at home are a nuisance, power outages at work can represent devastating financial and contractual liabilities. In previous decades, it would have been feasible to ride out an outage at your business. Now, however, businesses rely heavily, often entirely, on electricity. If you have a business (or a large home) for which an outage is simply not an option, then an industrial generator is an essential investment.
These units are large, heavy-duty, stationary systems that function with a diesel-fueled engine, and generate single-phase or three-phase power. These units can generate a huge amount of power, and with an automatic transfer switch, they can immediately start your backup power when the normal electricity fails, and in doing so save your business.
Methods for Sizing a Commercial Generator
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 1,000 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 / 1,000.
- 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.