How to Calculate Power Supply Requirements

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When you are building a new house or upgrading the electrical system in your current one, you need to have an accurate estimate of what your electrical demand will be on paper. The building inspector may also ask to see a copy before issuing your permit. If you have a copy of the National Electrical Code, you can find the formulas for calculating residential load requirements in Appendix D.

Things You'll Need

  • Calculator
  • Figure the square footage of the house, not including uninhabited areas such as the garage. Multiply the square feet by 3 to obtain the required watts.

  • Add 1500 watts for each small appliance circuit in the kitchen. Two circuits are required by code. Add another 1500 watts for the laundry room, not counting the dryer.

  • List all your major appliances and add their power ratings, except for your central air conditioner and heat supply. Get the numbers from the nameplates on each appliance. "Major appliances" means energy-hungry appliances such as your stove, dishwasher, water heater, dryer and well pump.

  • Multiply the total watts in step 3 by 40 percent and add the result to the results from steps 1 and 2. For example, 30,000 watts multiplied by 40 percent equals 12,000 watts. The reason you are taking the smaller number is because all the appliances won't be running at the same time.

  • Add the load from either your central air conditioner or your heating unit, whichever is the largest. Add it to the results from steps 1, 2 and 4 and divide the total by the service voltage (240 volts) to find out how many amps you will use. For example, 42,000 watts divided by 240 volts equals 175 amps.

  • Consider any possible future loads when calculating your needs. In the step 5 example, 175 amps is rather close to a 200-amp service limit, so it would be a better idea to install 300-amp service instead. It will be much more expensive if you have to increase the service in the future after everything has already been wired.

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