How to Calculate Electrical Load Step by Step

Before you calculate electrical loads for residential dwellings, you must know the difference between the load and the capacity.

• Electrical capacity is the maximum amount of electricity a circuit, breaker box or overall residential system can safely withstand. A home’s overall electrical capacity is measured in amps.
• Electrical load is any part of a circuit that consumes electricity or, in a residential system, the amount of electricity the home uses. A home’s electrical load is measured in watts and determined by the demand of its appliances and applications such as lighting.

A home’s electrical load should never be more than 80% of the electrical capacity. If a load exceeds 80% of capacity, the breaker generates excessive heat and becomes more likely to trip. Breakers that frequently overheat or trip can lead to electrical fires. Power outages can cause connected devices to lose data.

Determining electrical load and capacity requires using common units of measurement for electrical power: volts, amps and watts.

• Volts (or voltage) measure the speed of electrons as they pass through a circuit, such as an electrical outlet.
• Amps (or amperage) measure the volume of electrons as they pass through a circuit. Electrical capacity is measured in amps.
• Watts (or wattage) measure the rate of electrical power flow and are determined by multiplying volts and amps. The electrical load, or the power used by appliances and other applications, is measured in watts.

Calculating electrical loads usually involves comparing amperage, wattage and voltage in electrical systems. The following are useful equations when determining volts, amps and watts:

• Volts x Amps = Watts
• Watts / Volts = Amps

You can estimate the amperage of a home’s electrical capacity based on its age:

• Homes built before 1950 with screw-in fuses and knob-and-tube wiring usually have a 30-amp capacity.
• Homes built in the 1950s typically have a 60-amp capacity.
• Homes built between 1960-1980 typically have a 100-amp capacity.
• Homes built after 1980 usually have a 200-amp minimum capacity.

Most homes have two types of electrical loads to consider.

• The total connected load is the amount of energy the home would use if every appliance was running simultaneously. It does not reflect the actual electrical usage of a home.
• The demand load is the amount of electricity the home uses on a regular basis. It is a portion of the total connected load.

Different types of appliances have different electrical loads.

• Appliances with non-continuous loads use electricity for brief periods and include toaster ovens, blenders, vacuum cleaners and comparable devices. Use their listed wattage when calculating a residential load.
• Appliances with continuous loads use electricity for three or more hours at a time and can include water heaters, furnaces during winter and air conditioning units during summer.

The National Electrical Code (NEC) offers a method for determining the load of a single-family home. There are eight basic steps for calculating electrical panel load.

Begin by determining the square footage of the home’s living space.

Assume 3 watts per square foot of living space for the home’s lighting and general purpose circuits.

Assume 1,500 watts for each 20-amp small appliance circuit, as well as the laundry circuit.

Determine the electrical usage of the major appliances. The wattage is usually printed on an appliance label or nameplate and listed in watts (W) or volt-amperes (VA). These are some common electrical loads of appliances:

• Electric clothes dryer: 5,000 watts
• Electric range/oven: 8,000-12,000 watts
• Electric water heater: 4,500 watts
• Refrigerator: 300-800 watts

Combine the total appliance wattages with the other amounts for the home’s total connected load.

Adjust the total connected load to determine the demand load, the amount of electricity the home will regularly use. These are the steps for determining the demand load:

• Use 100% for 10,000 watts of the total, or "the first 10,000 watts."
• Use 40% of the remaining wattage.
• Add these two amounts.
• Add the wattage of the air conditioning or the heating system, whichever is greater. This final total will be the demand load.

After calculating the electrical demand load in watts, determine the appropriate capacity in amps. Divide the demand load by 240 volts, the standard service voltage of U.S. homes.

Finally, use the home’s capacity to determine the size of the electrical system to install. Homes have such standard electrical service ratings as 60, 100, 125, 150, and 200 amps. Round up to the next-highest standard amperage to make sure the system has enough capacity.

Follow the steps to calculate a sample home’s load and capacity.

• For a house of 1,800 square feet, the lighting and general use circuits would total 54,000 watts. 1,800 x 3 watts = 5,400 watts
• If the house has two small appliance circuits and a laundry circuit, that makes 4,500 watts. 1,500 x 3 = 4,500 watts
• The appliances for this house include a water heater (5,500 watts), an electric dryer (5,600 watts), a dishwasher (1,500 watts), a garbage disposal (600 watts) and an electric range (15,000 watts). Combined with the above, this makes a total of 38,100 watts.
• Apply the formula for the demand load to 38,100 watts. Calculate the first 10,000 watts at 100% and the remaining 28,100 at 40% (28,100 x 0.4 = 11,240). Add those amounts to get the total demand load: 10,000 + 11,240 = 21,240 watts.
• Add 5,000 watts for the central air conditioner for a total 26,240-watt electrical load for the house.
• Determine the capacity by dividing the load by 240 volts. The electrical load of 26,240 volts divided by 240 yields 109.33-amp electrical capacity for the home.
• The next standard service rating higher than 109.33 is 125, meaning the home will need an electrical service of 125 amps.

Pro Tip: Some appliances list power in volt-amperes (VA), not wattage. While the terms are not identical, they can be used interchangeably for calculating load.

Determining electrical load requires precision. Underestimating and overestimating the load can both cause difficulties.

Underestimating the load means the electrical use may exceed the system’s safe capacity. These can lead to the following issues:

• Circuit overloads lead to tripped breakers and possible fire hazards.
• Voltage drops damage some types of electronic equipment or lead to a loss of data.
• Increased system deterioration leads to higher maintenance costs and reduced life of the equipment.
• Failure to comply with the electrical code can have insurance consequences in case of an accident.

Overestimating the load can bring other problems. The system will be safe but may include a larger and more expensive electrical panel, circuit breakers and other equipment than the home needs. You could end up overbidding for a job based on overestimating the electrical equipment costs or overspending on items that are unnecessary or more expensive than your needs.

In some cases, calculating the home’s present electrical load and capacity correctly may not be enough. Depending on the customer, you may need to overestimate the load to make room for future expansion needs. When upgrading a customer’s system, find out if they have plans for additions like EV chargers and add those devices’ electrical load to your total.

Pro Tip: Check with the local power company to see if the meter provides demand data. Smart meters can provide detailed information on peak electrical demand, which can lead to more accurate load estimates.

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