Calculate Electrical Circuit Load Capacity

Electric Circuit Breaker
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Electrical circuit load capacity is the total amount of power that your home actually will use. In order to decide what size electrical service is needed in your home, one has to do a little math homework. Older homes often only had a 60-amp electrical service, connected to a fuse panel. Newer homes have 100- or 200-amp electrical services.

Understanding Electrical Loads

Calculating how much power both you and your electrical appliances use is necessary to calculate this number. As technology continues to advance, it seems we add more and more electrical loads to our home. To figure out your electrical loads you need to know what to look for and how to add up the loads. 

The first thing to know is that circuits should only be loaded up to 80% of the total circuit capabilities. Having said that, it doesn't mean that you should keep adding additional loads until you get to *% capacity. Instead, aim for a more reasonable amount, say %0- 60% load if at all possible, allowing for future additional loads. It's better to have too many circuits in a home than too few.

elecrtical circuit load capacity
Illustration: Nusha Ashjaee © The Spruce, 2018

Calculating Capacity

To help you understand the concept, if you have a 15-amp circuit, the safe operating amperage would be no greater than 12 amps. The total wattage would be 1,800 watts, meaning the safe wattage usage would be 1,440 watts. If you had a 1,100-watt hairdryer plugged into this circuit, you can see that just one device uses almost the entire desired load capabilities.

If you have a 20-amp circuit, the safe operating amperage would be no greater than 16 amps. The total wattage would be 2,400 watts, meaning the safe wattage usage would be 1,920 watts. In this instance, you could actually have a hairdryer, radio, and electric razor running on the same circuit, but not much else. That's why there should be additional bathroom circuits to cover lighting, exhaust fans, and heat lamps for drying. 

On a 30-amp circuit, the safe operating amperage would be no greater than 24 amps. The total wattage would be 3,600 watts, meaning the safe wattage usage would be 2,880 watts. This information comes in handy with central air conditioners, electric dryers, electric ranges, and electric ovens.

To determine the wattage, you take the voltage times the amperage. Check the tags on all of your appliances for the required amperage rating. Add all of the lighting load by adding the total wattage of the light bulbs in your home. Look at the light bulbs and read the wattage that is printed on them.

Your home will likely also have 240-volt appliances like water heaters, air conditioners, electric dryers, and electric ranges. These will have an amperage rating label and the wattage can be calculated. The voltage, 240 volts, times the amperage, say 30 amps, will equal the wattage requirements.

Main Circuit Breaker Panel and Sub-Panels 

Once you've determined the total load for your home, you'll know what size electrical service you need. Most homes have either a 100-amp or a 200-amp circuit breaker. There may also be additional sub-panels feeding off of the main circuit breaker panel. People often put sub-panels on the second floor for easy access to breakers in the event of trouble.

With a large enough main circuit breaker panel, you can add sub-panels to your garage or outbuildings as well. A good rule of thumb is to always have more outlets and service space available for adding circuits and electrical loads. Who knows what might be the next electrical load that you will add in the future? If there is anything in your home that needs to be over-sized, it's your electrical service. Over the years you will grow into it.