How to Size a Generator for a Commercial Building

how to size a generator for a commercial building

Learning how to size a generator for a commercial building in Alabama involves more than just selecting the most powerful one. You must consider the unique features of Alabama’s climate, as well as, the energy needs of your building and company. This approach will help you prevent unnecessary costs and guarantee uninterrupted power supply during extreme weather conditions such as thunderstorms, hurricanes, or tornadoes, which frequently occur in Alabama. Ultimately, finding the right balance between capacity and cost-efficiency is crucial. Think of it as finding a well-fitted suit: the generator should meet your building’s demands and provide sufficient power during peak usage without being excessively large.

To begin, you need to understand your building’s full-load capacity. This involves measuring the full-load current when your building uses the most power. Use a clamp-on ammeter to measure the electrical service on each leg; these preliminary numbers will give you a starting point. Using this information, calculate the total kilowatts needed.

It’s important to plan for future expansions or increased power demands. Your generator should have about 20-25% more capacity than your initial calculations. This extra capacity ensures your generator can handle additional load without issues to ensure your business’s continued productivity.

As Alabama is extremely vulnerable to power interruptions due to severe weather, it is essential to have a dependable generator to guarantee both operations and safety. By factoring in specific local conditions like usual weather trends and the length of power outages, you can assure that the generator you select will enable your business to function seamlessly even during difficult circumstances.

Determining Your Power Needs

When sizing a generator for your commercial building, it’s essential to get a clear picture of your power consumption, particularly during peak times, to ensure you’re adequately prepared for any power outage.

Assessing Peak Usage and Total Power Requirements

Start by identifying the highest peak demand of your building. This isn’t about averaging out your power use; it’s about knowing what the maximum power usage is when all systems are operational.

To begin:

  1. Measure the total amps drawn during peak periods using a clamp-on ammeter on each leg of your electrical service.
  2. Calculate full load kilowatts (kW) by using the formula: Total Amps x Supply Voltage / 1,000.
  3. Add the power in kilowatts used by critical systems, such as emergency safety systems, security systems, and other essential safety systems. Articles 700, 701, 702, and 708 of the NEC might be relevant for your commercial or retail application.

Keep in mind, you’re looking for total power requirements, not just what you use on a day-to-day basis. This will form the basis for the generator capacity you need.

Analyzing Equipment and Circuits

Now, break down your usage:

  • Largest Motor: Start by identifying the largest motor load you have because it will dictate a significant portion of your startup power needs.
  • Non-Motor Load: Add up the kW of your non-motor load.
  • Factor in the amount of power needed for all other operations in your building.

From lights to computers, to HVAC and production equipment, every bit adds to your overall power demands.
Don’t forget about natural gas usage if applicable, as it could affect which generator you choose.
Finally, calculate a reserve capacity, typically adding 25% (0.25) to your full load kW. This provides a cushion to cover unexpected increases in power usage or additional future needs.

Selecting the Right Generator

Choosing the right generator for your commercial building involves considering fuel types, sizing carefully based on your power needs, and ensuring safe installation practices.

Understanding Generator Types and Fuel Options

When choosing a generator, it’s important to know the difference between portable and standby types. Portable generators are flexible and affordable but usually offer less power. On the other hand, standby generators are great for businesses because they can automatically provide backup power in an emergency.

Fuel choices like diesel, natural gas, and propane are also key considerations. Diesel generators are popular in commercial settings due to their strength and efficiency. Additionally, the local availability and cost of fuel should influence your decision on the best type of generator for your needs.

Sizing Guidelines and Considerations

Here’s where it gets technical. To find the right size generator, you need accurate current measurements from your service panel during peak usage. You can use a clamp-on ammeter to measure the full-load current on each leg of the electrical service. Your formula should look like this:

  • Full load kW = Total amps x supply voltage / 1,000
  • Reserve capacity = Full load kW x 25% (for surge capability)
  • Generator size = Full load kW + Reserve capacity

Keep in mind that places like grocery stores or convenience stores, with more refrigeration needs, might require larger generators than office buildings.

Installation and Safety

The installation of generators requires a high level of importance placed on safety. It is crucial to have an engineer oversee the installation process to adhere to the National Electrical Code (NEC). To prevent hazardous voltage dips and safeguard both the generator unit and the electrical system of the building, it is essential to have an automatic transfer switch integrated into the generator. Additionally, for commercial buildings that utilize three-phase power, it is advisable to consider using three-phase generators for improved power balance and efficiency.


When sizing a generator for your commercial building in Alabama, what should you know?

Asking the right questions is key to ensuring your backup power is reliable in Alabama’s unique conditions.

How can you figure out the best generator size for a commercial building in Alabama?

To find the right generator size, you need to determine your full-load capacity. This involves measuring your peak electricity usage at the service panel during the highest consumption periods, which can be critical in Alabama’s storm-prone environment.

What are the steps to calculate the proper generator size for a commercial property in Alabama?

Start by using a clamp-on ammeter to measure the current on each leg of your electrical service during peak usage, often during severe weather. Combine these measurements to estimate your generator capacity, and remember to include additional loads that might not be active during the measurement.

Is there an easy way to calculate the size of a generator for a commercial building in Alabama?

Yes, there are online calculators available that can help you estimate the needed generator size. These tools typically ask for details like square footage and essential electrical load information, which are crucial given Alabama’s frequent power outages.

What size diesel generator is suitable for a medium-sized business in Alabama?

For a medium-sized business in Alabama, a generator between 50 to 100 kilowatts usually works well. However, your specific requirements may vary based on your power consumption and critical load needs, especially during storm seasons.

How can you determine if a standard kVA generator is sufficient for your commercial needs in Alabama?

To verify if a standard kVA generator is enough, compare its rating with your calculated full-load capacity. This helps ensure your critical systems stay operational during Alabama’s frequent power outages.

What should you consider when choosing a generator for a business in Alabama?

When picking a generator for your Alabama business, think about local regulations, fuel availability, and seismic considerations. Also, plan for high-usage times like heat waves or severe storms that can lead to extended power outages.

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