How to Calculate Electrical Load

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Electrical circuits have many applications, including household, automotive and electronics. The electrical principles apply regardless of the application. You have a number of components distributed on the circuit that constitute the circuit load. You have a power source. You want to know the characteristics of the load components. You have Kirchoff’s Law, which essentially states that the sum of the load voltage equals the sum of the source voltage. You don’t want to damage any of the components on the circuit, so you calculate the load.

Calculating an Electrical Load in a Simple Circuit

    Calculate the electric load for a simple linear circuit having a source voltage of 9 volts and two resistors in series, each of 330 ohms. The second resistor has a lead that goes to ground. Calculate according to the following equations. Let Power = Voltage * Current (P=VI). Let Current = Voltage/Resistance (I=V/R).

    Apply Kirchoff’s Second Law, that the sum of the voltages around a circuit is zero. Conclude that the load voltage around the simple circuit must be 9 volts. Calculate that the load voltage is evenly distributed across each of the resistors, since they have equal resistance, and that the voltage over each must be 4.5 volts (or -4.5 for purposes of Kirchoff’s Law).

    Calculate I=V/R (current calculation), so that I=4.5/330=13.6mA (milliamps). Calculate P=VI=9*.0136=.1224 Watts. Note that all the load characteristics (Voltage, Resistance, Current, and Power) are now known. Be safe and choose resistors rated at .5 Watts.

    Use a linear circuit simulator online to simulate simple circuits and calculate the load characteristics. Utilize the linear circuit simulator described below called “Linear Technology Spice.” Create a sample circuit and experiment with different load components. Calculate the load characteristics using the voltage, current, resistance (or inductance) and power equations.

Calculate Household Electrical Load

    Calculate the load for a typical single family house using an online electrical load calculator. Utilize an online “Single Family Dwelling Electrical Load Calculator.”

    Enter the square footage of your house. Enter the number of “small appliance circuits” and “laundry circuits,” and refer to an electrical diagram if necessary. Use the default values if the information is unavailable. Enter values for “Fastened Appliances,” “Cooking Appliances,” “Heating or Cooling” and “Largest Motor.” Press “Calculate Load.”

    Note the “Total Computed Load,” the “Computed Amperage,” the “General Neutral Load,” the “Total Neutral Load,” and the “Total Neutral Amperage.”


    • Kirchoff’s Second Law: The sum of the source voltage and the load voltages is zero. For the two resistor circuit above, the source of 9 volts and the load voltages of -4.5 volts and -4.5 volts (across the two resistors) add to zero.


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