How to Size a Generator for Commercial Building
When it comes to installing a generator for a commercial building, size matters. To determine the appropriate size for your commercial generator, you’ll have to consider single- or three-phase power, voltage selection, and total power output. The following is a concise guide to helping determine the best fit for you.
Before You Begin
Before calculating the appropriate size for a generator for a commercial building, you may want to think about the following…
- Voltage drop – When motors start, they create a current surge that step loads the generator and creates a voltage drop. Most commercial applications should be limited to about 15% voltage drop.
- Never add amps when sizing a generator. Convert amps to kW and add kW to determine the required generator size. When sizing a generator for a commercial building, you want to calculate the total number of power output rating in kW you would need in order to ensure that your generator can provide power to all required equipment and machinery. Simply adding the needed amps to the total kW value will provide inaccurate power calculation needs and result in the purchase of a generator that does not meet or exceeds your total power requirements.
- Resources to help convert amps to KW and power calculation formulas can be found here:
- Ensure that your commercial generator and installation are compliant with the National Electrical Code (NEC).
Method 1: Full Load Capacity
Merely follow these mathematical steps to figure out the right size for your commercial generator!
- Step 1: Estimate the mandatory 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.
- Step 2: Divide the total amps by three for three-phase current, and by two for single-phase current, then multiply that by the supply voltage, and again by 1000 for Kilowatts (kW) Required.
- Step 3: 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.
- Step 4: Remember, full load kilowatts = total amps x supply voltage / 1000. Reserve capacity = full load kilowatts x 0.25.
- Step 5: For 100 percent power, generator size = full load kilowatts + reserve capacity.
Method 2: Square Footage
If you run a retail store or restaurant, this may be the best and simplest method for you.
- For a retail application: 50 kilowatts + 10 watts per square foot.
- For a commercial application: 50 kilowatts + 5 watts per square foot.
Next Steps
Still uncertain? Be sure to contact Woodstock Power and a member of our expert staff would be thrilled to help!
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