Electrical Systems

Parallel Circuit Calculations 311

Parallel Circuit Calculations provides a comprehensive overview of the rules and formulas used to calculate electrical quantities and capacity. Using a direct current parallel circuit as an example, it explains the basic rules for electrical variables and how they relate. In a parallel circuit, voltage is the same across each branch. Current in a parallel circuit adds up to the total current value. In any one branch of a parallel circuit, current and resistance are inversely proportional. The total resistance is always less than the smallest individual resistance.

After taking this class, users should have a strong understanding of the methods used to determine electrical values in parallel and combination circuits. Understanding these rules and formulas is important for working with electrical systems because they make it possible to predict and calculate electrical quantities. This is especially important for parallel circuits because they are so commonly used.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 15

  • Language English


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Course Outline
  • DC Parallel Circuits
  • Current
  • Voltage
  • Parallel Circuit Basics Review
  • Resistance
  • Determining Resistance: Resistors of Equal Value
  • Determining Resistance: Product over Sum
  • Determining Resistance: Reciprocal Formula
  • Resistance Review
  • Rules for DC Parallel Circuits
  • The Power Wheel
  • Solving a DC Parallel Circuit
  • Combination Circuits
  • Solving a Combination Circuit
  • Final Review
  • Describe a DC parallel circuit.
  • Describe current in a DC parallel circuit.
  • Describe voltage in a DC parallel circuit.
  • Describe resistance in a DC parallel circuit.
  • Calculate the resistance of a DC parallel circuit using the resistors of equal value method.
  • Calculate the resistance of a DC parallel circuit using the product over sum method.
  • Calculate the resistance of a DC parallel circuit using the reciprocal formula method.
  • List the rules for DC parallel circuits.
  • Describe the power wheel.
  • Calculate missing variables in a DC parallel circuit.
  • Explain how to solve a combination circuit.
  • Calculate missing variables in a combination circuit.
Vocabulary Term


A. A unit of measurement that describes both an amount of electricity and the time it takes to travel a certain distance. One ampere, or amp for short, equals one coulomb per second.


A portion of a parallel circuit's path that contains a load. Each branch has its own electrical load that determines the electrical values of the circuit.


The amount of electricity that can safely flow through a wire without the wire overheating. Capacity is an extremely important electrical quantity.

combination circuit

An electrical system that has elements of both series and parallel circuits. Combination circuits may be thought of as large series circuits with parallel sub-circuits or vice versa.


A component in a circuit that controls the flow of electricity. The control determines when a circuit is energized.


I. The flow of electricity. Current is measured in amperes (A).

current add rule

A law that states that the total current of a parallel circuit is the sum of the currents through each individual branch, regardless of the number of branches. The current add rule is derived from Kirchhoff's Laws.


The number in the bottom part of a fraction, below the fraction bar. The denominator expresses the quantity of total parts into which the whole number has been divided.

direct current

DC. Current that travels in one direction. Direct current does not reverse the direction of flow.

inversely proportional

A relationship in which a number either increases as another decreases or decreases as another increases. Inversely proportional is the opposite of directly proportional.

Kirchhoff’s Laws

A set of universal truths that describe the flow of electricity in a circuit. Kirchhoff's Laws imply that what goes into a circuit must come out of it.


A component in a circuit that converts electricity into light, heat, or mechanical motion. Examples of loads include light bulbs, resistors, and motors.


The number in the top part of a fraction, above the fraction bar. The numerator indicates the number of parts of the whole.

Ohm’s Law

The universal truth that describes the relationship between voltage, amperage, and resistance. Ohm's Law states that one volt equals one amp times one ohm.


Ω. A unit of measurement for electrical resistance. One ohm is required for one volt to produce one amp.

parallel circuit

An electrical system that has multiple paths for the flow of electricity. Loads connected in parallel can operate independently of one another.


A conductor that directs electricity in a circuit. The path is often made of copper wire.


The rate at which a device converts electrical energy into another form of energy, such as heat or light. Power is measured in watts (W).

power wheel

A round chart of equations used to calculate current, voltage, resistance, and power. The equations on the power wheel are derived from Ohm's Law and Watt's Law.


The result of multiplying two or more numbers. The product over sum method of finding resistance divides the product of two circuit branches by their sum.

product over sum method

A process used to determine the total resistance of a parallel circuit by finding the product of two resistors and dividing it by their sum. For circuits with more than two branches, this process is repeated until all branches have been incorporated into the calculation.


The inverse of a given value. The reciprocal of a number is equal to 1 divided by the number.

reciprocal formula method

A process used to determine the total resistance of a parallel circuit by finding the reciprocal of the sum of the reciprocals of the individual branches. The reciprocal formula method can be used for any parallel circuit.


R. The opposition to current flow. Resistance is measured in ohms (Ω).


A device used in circuits to limit current flow. Resistors can be used to prevent overheating.

resistors of equal value method

A process used to determine the total resistance of a parallel circuit with resistors that have the same value. Total resistance is equal to the value of one individual resistor divided by the number of branches.

scientific calculators

A small electronic device used to calculate advanced mathematical problems. Scientific calculators include a reciprocal button labeled "1/X."

series circuit

An electrical system that has only one path for the flow of electricity. Series circuits are limited because, for any load to work, every load in the circuit must be switched on.


A component that provides electrical power to a circuit. The source is the origin of electricity, such as a power plant.


The total amount resulting from adding two or more numbers together. The sum of a parallel circuit's individual current values is equal to its total current.


E. A measure of electromotive force. Voltage is measured in volts (V).

voltage drop

A decrease in voltage along a conductor through which electricity flows. Voltage drops occur when electricity passes through resistance.


V. A unit used to measure electromotive force or pressure, which is called voltage. One volt of force is needed to force one coulomb to do one unit of work.

Watt’s Law

The universal truth that describes the relationship between wattage, amperage, and voltage. Watt's Law states that one watt equals one amp times one volt.


P. The power used in a circuit. Wattage is measured in watts (W).


W. A unit of measurement for the wattage or power used or produced by a circuit. One watt is used when one volt forces one amp through a load.