How to Calculate Electrical Capacity


Circuit breakers control the electrical capacity for each area of your home and your objective in calculating electrical capacity is to ensure you do not overload one or more of the circuit breakers. For example, your home is monitored by a circuit breaker panel. Each breaker in the panel monitors the electrical outlets in a specific area in your home. Each breaker is sized, as with a 20-amp breaker size, to ensure the normal current can flow. However, when the current flow is higher than normal, as with the case of a short circuit, the breaker will open or "trip" to ensure the over current doesn't damage the appliances in your home.

Things You'll Need

  • Calculator
  • Notepad
  • Find the circuit-breaker panel in your home. Starting with the first breaker, record the amp rating of that breaker. Then multiply that value by 80 percent or 0.80. For example, if you have a 30-amp breaker, 80 percent of that will be 24-amps. In this example, 24-amps represent the safe operating level for that breaker. Next, determine the power-handling capacity of that breaker by multiplying the 80 percent value by 120-volts. Continuing with our example, 24-amps times 120 volts is 2,880 watts. Follow this procedure for all circuit breakers in the panel and write down the power handling capacity in watts associated with each circuit breaker.

  • Determine the area of the home each circuit breaker monitors. Turn off one breaker at a time and go inside to see which area in your home is without power. Note all of the electrical outlets that are inactive. Write them down and associate them with the circuit breaker you turned off. Repeat this step for all circuit breakers in the panel.

  • Check to load for each breaker. Starting with the first breaker, go to the electrical outlets supported by that breaker. Find the manufacturers nameplate of every appliance connected to the electrical outlets associated with that breaker. Record the watts of each appliance and add them together. Next, compare the total watts with the power handling capacity you calculated for that breaker in Step 1. If you are at or close to the maximum, consider downsizing the appliances or rearranging appliances to reduce the load. Follow this procedure for each breaker.

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