How To Estimate Electrical Load

The amount of electricity your house requires may be determined by adding a safety margin to the amperage load of all the different appliances and fixtures. It is often advised to limit the load to no more than 80% of the electrical service’s capability.


You must comprehend the correlation between watts, volts, and amps in order to utilize the math. There is a mathematical link between these three widely used electrical words, which may be stated in a few different ways:


Watts = Volts x Amps


Amps = Watts/Volts


These equations may be used to determine the electrical service’s overall capacity and load as well as the capacity and loads of individual circuits. For instance, a 20-amp, 120-volt branch circuit may handle 2,400 watts in total (20 amps x 120 volts). The 20-amp circuit has a reasonable capacity of 1920 watts since the typical advice is for the load to total no more than 80% of the capacity. Therefore, the combined power consumption of all the light fixtures and plug-in appliances on this circuit should not exceed 1,920 watts in order to prevent the risk of circuit overload.


To evaluate if a circuit is going to overload, it is pretty simple to examine the wattage ratings of the lightbulbs, televisions, and other appliances on the circuit. For instance, if you regularly put a 1500-watt space heater into a circuit and use the same circuit to power many 100-watt light fixtures or lights, you have already used up most of the safe 1920-watt capacity.


The entire electrical service capacity of the home may be calculated using the same approach. The calculation is as follows since a home’s main service is 240 volts:


100 amps at 240 volts equal 24,000 watts.


19,200 watts is equal to 80% of 24,000 watts.


To put it another way, a 100-amp electrical service should only be able to provide up to 19,200 watts of power demand at a time.

Load Calculation

The capacity of each circuit and the total electrical capacity of the house may then be compared with the load, which can be determined by simply summing the wattage ratings of all the different fixtures and appliances that will be using electricity simultaneously.


You could believe that this entails calculating the combined wattage of all the equipment, including hard-wired and plug-in, and comparing it to the overall capacity. For example, you wouldn’t run the heater and the air conditioner at the same time, nor is it probable that you would be vacuuming when the toaster is going. However, it is uncommon for all electrical appliances and fixtures to operate at the same time. Because of this, electricians often use alternate techniques to establish the proper size for the electrical service. 

A straightforward guideline is offered by other electricians:


A 100-amp service is often sufficient to run one or two electric appliances, such as a stove, water heater, or clothes dryer, in addition to the basic branch circuits of a small to the moderate-sized house. If the heating appliances are gas-powered, this service would be enough for a house that is less than 2,500 square feet.


In residences up to around 3,000 square feet in size, 200-amp service will be able to carry the same load as 100-amp service in addition to electric appliances and electric heating/cooling equipment.


Large residences (more than 3,500 square feet) with electric heating and cooling systems and all-electric appliances are advised to have a 300- or 400-amp service. Where the anticipated electric heat demand is above 20,000 watts, this service size is advised. Installing two service panels—one delivering 200 amps and the other providing another 100 or 200 amps—usually results in a 300- or 400-amp service.

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