Electrical Circuits – Series and Parallel

serving the Ottawa area for over 30 years!


• Electrical circuits have closed paths

• Two methods of connecting 

• Series circuits

• Parallel circuits



Electrical energy is distributed throughout a home using wire circuits. An electrical circuit is a closed path, through which current can pass and deliver power to a load. Picture an electrical circuit as a water system. Water from a lake gets piped to your home. It is then used, returned through the drain ready to repeat the cycle all over. Electrical current is like water and must return to its source. A path must be established to allow the current from an electric utility such as Ontario Hydro to return to the service. This path consists of transformers, panels, wires, connections, junction boxes, and convenience outlets. 


As mentioned earlier, there are two distinct methods of connecting circuit components. The first is to join them in series or secondly to connect components parallel to each other. Each method of connection has its advantage or use. You need to be familiar with each process of connection and the reason for connecting circuit components in a particular way. A third means of connection combines both series and parallel circuits. Not surprisingly, these are called combination circuits.



This method of connection is one which contains only one path for current to flow from the source to the load and back to the source. The primary use of series circuits is for control of a load. Since there is only one path, a break anywhere along the path will stop current flowing. This action explains how the light switch functions. It breaks the completed circuit to the fixture. The source of power gets connected to one screw on the switch. A second wire gets connected to the second screw on the switch. By flipping the switch, you open and close the circuit to the light. The light requires a neutral so that the conductor is continuous to the light fixture without any switching. This is the most popular type of circuit used throughout a house.


Switching from two locations (using switches commonly referred to as 3-way switches) are other forms of switching circuits in the house. The switches get connected one after the other, or in series. This type of control is standard on stairs. Always check the operation of this switching as it is sometimes done incorrectly, a sure warning sign that the wiring has been tampered with by an inexperienced person. To check the operation or the 3-way switching, turn on the light at either switch. Go to the second switch and turn the light off, now return to the first switch, and it should be able to turn the light back on. Four-way switching gets rarely used due to the cost of the 4-way switches which can be five to ten times the cost of a 3-way switch. Testing this type is the same as a 3-way configuration. Four-way switches turn on lights from more than two locations.


Parallel circuits connect devices between the live conductor (black) and the return conductor (white). Each device acts independently of all other devices connected to the circuit. Each new device, such as a fixture will come on and off as required and will have no effect on any additional load. With this type of circuit, all the lights and receptacles get connected throughout a house. This method requires that the circuit and service equipment deliver more and more current as each independent circuit is turned on. The larger the house, the more electrical devices will be installed, resulting in a more significant electrical service. The lights must get controlled independently, and circuits must get protected from overload. These requirements mean that purely parallel and series circuits are impractical. A realistic circuit would be one containing a combination of series and parallel-connected devices.



A combination of connections must be employed to accomplish protection and control. A typical lighting circuit would come through the circuit breaker or fuse which will protect the circuit conductor.

Current would then flow to a junction box from which a fixture hangs. From here, a parallel connection could be made to supply a fixture in another room. From this same junction box, a cable will go to the switch to control the light (the switch is in series).


Household circuits get wired in parallel, such as receptacles. Switches are wired in series to interrupt the current flow to light fixtures and equipment. If multiple light fixtures get connected to the same switch, they get linked in parallel after the switch. Control wiring is usually in series, and that would include the circuit for the furnace in the house.


 A formula for calculating resistance in a parallel circuit is as follows:

Resistance = (R1 x R2) + (R1 + R2).

Resistance total in a parallel circuit has to be smaller than any resistance in the entire circuit.

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Ottawa ON

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