Having the skill and knowledge to correctly estimate the amount of electrical wattage that will be required for a commercial application (also commonly known as the electrical load that a building or business will require) is one of the most important skills that you can acquire as a professional electrician. Commercial electric load calculations are not complex if you know what you are doing, but there are a number of different important steps that you must follow in order to properly perform the load calculation. Failure to do so correctly can lead to expensive service upgrades later on, or even to the potential for fire or other dangerous situations.
Voltage comes in a variety of different nominal voltages, such as 120, 220, 480 and 640. However, these are called nominal voltages for a reason -- they are not exact numbers. For example, a 220 volt circuit can range anywhere from 220 to 240 volts and still be considered correct. You will need to use a nominal voltage when estimating a commercial electrical load.
Proper System Sizing
When doing commercial electrical load calculation, it is important to size the system properly in order to prevent lack of power. One example of how to size a system properly is to calculate the maximum voltage required by all of the appliances in a business. This is called 100 per cent loading. For example, dryers in a laundromat will be running a lot, and you want to calculate the load required to run all of them at one time for 100 per cent loading. If all of the dryers will run continuously all day, then you may want to calculate for 125 per cent load on the electrical system, because of the increased heat and effort of the equipment.
Electric heat is another area that may need to run continuously during colder temperatures. If a commercial building is going to use this form of heat, then you would want to figure out the total voltage needed for the electric heat, and oversized that number by 125 per cent in doing your commercial load calculation. This will help you figure out your nominal voltage requirements at the meter.
National Electric Code
The National Electric Code (NEC) specifies a wide variety of different requirements for commercial buildings that you will want to meet with your load calculation to be sure that you are in compliance. If you have doubts about a particular requirement, then referring to the NEC will be the best way to make sure your load calculation is complete and correct.