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Calculate Electrical Circuit Load Capacity

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Male electrician making adjustment to fuse box, rear view Anderson Ross/Photodisc/Getty Images
Question: Calculate Electrical Circuit Load Capacity
Electrical circuit load capacity is the total amount of power that your home actually will use. In order to decide how big of an electrical service is needed in your home, one has to do a little math homework. Calculating how much power both you and your electrical appliances use is necessary to calculate this number. I'm often asked how to figure this load. It really is pretty simple if you know what to look for and how to add up the loads.
Answer:

The first thing to know is that circuits should only be loaded at 80% of the total circuit load. 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 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.

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.

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 too 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.

Once you've determined the total load for your home, you'll know what size electrical service that you need.

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