Schematic of a simple electrical circuit (C) Carson Dunlop Associates Electrical Circuit Basics for Homeowners

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Electrical circuits & short circutis, home electrical wiring basics & definitions:

This article answers basic questions about electricity and electrical wiring such as "What is an electrical circuit" and what is a "short circuit".

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What is an Electrical Circuit?

Basic example of electricity (C) Carson Dunlop AssociatesSketches at page top and at left are provided courtesy of Carson Dunlop Associates

[Click to enlarge any image]

At the top of this page we show a very simple electrical circuit. Electrical energy (here very roughly defined as a flow of electrons) moves through a circuit to perform work such as lighting a light or heating a room.

At left we show a common analogy using water flowing in pipes to help understand electrical circuits. Some people explain volts as similar to water pressure in a pipe, and amps as water current or total quantity flow. We discuss volts and amps below and in detail at this website.

A closed path of electron movement is required to put electrical energy into use.

This path is called a circuit.  Sketch courtesy of Carson Dunlop Associates.

Every electric circuit has four basic parts:

  1. Electrical Energy Source - a source of electrical energy, AC (alternating current, used in most homes) or DC (direct current)
  2. Electrical conductors - electrical wires to carry electrical energy from its source to the point of use
  3. Electrical Load - an electrical device that uses energy such as a light bulb, computer, or electric heater
  4. Electrical Switches - controls to turn on or off electrical energy being supplied to an electrical load or device. In a home or other building electrical safety devices which protect the building from unsafe conditions such as a fire or electrical shock include circuit breakers or fuses which are really acting as an electrical "switch" to turn off electrical power should an unsafe condition occur.

Where does Electricity in a Building Come From?

Sketch of electrical service to a home (C) Carson Dunlop Associates

The real source electrical power at most buildings is an electric utility company which operates an electrical generator (a power station). The electric utility brings power from its power generators into a neighborhood where power is to be used by means of electrical transmission lines or power lines (big heavy wires).

For efficiency, electrical power is usually delivered into a neighborhood at very high voltage levels. In the neighborhood high voltage is converted by local power transformers (those big boxes or "cans" you see on some electric utility poles) to the lower voltage levels (240V or 120V) used in most buildings.

From the overhead (or in some communities buried) power transmission lines and transformer, a local power distribution wire (also overhead or buried) brings electrical power close to the building being served with power.

A service entry cable (SEC) (which is the responsibility of the building owner) connects the local power distribution wire to an electric meter mounted on or close to the building, and from the electrical meter, the service entry cable continues in to one or more main electrical panels on or inside the building. Sketch courtesy of Carson Dunlop Associates.

The electrical panel provides a place for mounting of fuses or circuit breakers which protect the building wiring from overheating and short circuits as a way to reduce fire risk. Inside of a building electrical power is distributed to various rooms through individual circuits, each of which is fed from the electrical panel(s).

However, for practical purposes, the source of electricity at an individual home or other building may be thought of as the circuit breaker panel or fuse box from which electrical power is distributed throughout a home or other building. Junction boxes and wall outlets may be thought of as secondary sources of electrical power since they convenient points at which a building occupant can connect electrical devices anywhere in a building.

See ELECTRICAL DEFINITIONS for definitions of "Volts", Watts, Amps, etc.

What are Electrical Conductors

Electrical Conductors are the wires that carry the electric energy to the point at which it will be used. Conductors offer little (but not zero) resistance to current flow because atoms in the metals they are made of have lots of free electrons and therefore can transport it efficiently.

What are the Definitions of Electrical Load, Amps, Watts

Sketch explaining electrical AMPS (C) Carson Dunlop Associates

The electrical load is the amount of electrical energy that any device (a light bulb, your toaster or your washing machine, for example) uses to perform some work. The electrical load (unlike the electrical conductors) offers resistance to current flow.

This resistance makes it possible for the device to convert electric energy to another form (heat, light, mechanical movement, etc.).

The load resistance determines how much current will flow in a circuit. Increase the resistance in the load and you automatically decrease the current flowing in the circuit.

If the voltage applied to a circuit is multiplied by the current in amperes, the result is in units called watts. The watt is a unit of power and indicates the rate at which the load is consuming power.

The electrical load on a circuit is measured in Amps, a measure of electrical current.

See ELECTRICAL DEFINITIONS for detailed definitions such as "amps", "volts", and "watts". Sketch courtesy of Carson Dunlop Associates.

The Function of Electrical Switches

The fourth element in an electrical circuit is a means of controlling the energy. This is a device that can interrupt and restore current flow as desired by the user. Switches used in home electric systems control current flow by inserting a very high resistance (air gap) in the circuit to stop the current flow and by) removing the resistance (closing the air gap) to start current flowing again.

A basic principle of electrical wiring is that switches should always be wired into the hot (black or red) line leading directly to a device or outlet. When switches in the hot line are turned off, no hot line power is present in the load device. This greatly reduces the possibility of injury or damage resulting from accidental grounding.

What is an Electrical Short Circuit?

Sketch of an electrical short circuit (C) Carson Dunlop Associates

An electrical short circuit is a fault that occurs when a low resistance path exists between the hot lead and some grounded points.

When an electrical short circuit occurs, heavy current will flow in the circuit for as long as it takes for the circuit breaker to trip to OFF or for the fuse to blow.

Because a properly-working circuit breaker or fuse cuts off the current flow quickly, the circuit wiring will not have time to overheat to the point where fire may occur.

Sketch courtesy of Carson Dunlop Associates.

Safety Warning: an over-sized or improperly-installed fuse, and also some circuit breakers do not work reliably; a circuit breaker that is damaged by rust, corrosion, modification, or a circuit breaker which was poorly designed in the first place may not trip - resulting in a fire.



CIRCUIT BREAKER FAILURE RATES for some examples of problem circuit breakers.

What is an Open Circuit?

An open circuit is a fault that cuts off current flow in a circuit. The break that causes the open circuit may be in the hot line, the neutral line, or the load. An open circuit appears to be off. The break that causes an open circuit may be quite small. Vibration or even changes in temperature may suddenly turn the circuit on again. When looking for the cause of an open circuit, be sure power is off at the source.

Open electrical panels are dangerous (C) Daniel Friedman

Watch out: Safety Warning: Do not attempt to work on your electrical wiring, switches, or outlets unless you are properly trained and equipped to do so. Electrical components in a building can easily cause an electrical shock, burn, or even death. 

Even when a hot line switch is off, one terminal on the switch is still connected to the power source. Before doing any work on the switch, the power source must be turned off by set­ting a circuit breaker to OFF or removing a fuse.



What are the definitions of AC and DC Electrical Current?

The electrical energy that flows in circuits and operates loads can be generated in either of two different forms. One form is called alternating current (AC); the other form is called direct current (DC).

Alternating current is almost universally used for home electric power and is, therefore, the kind this article is primarily concerned with. In an AC circuit, the amount of voltage applied to the circuit is constantly changing from zero to a maximum and back to zero in one direction and then from zero to maximum and back to zero in the other direction. The maximum voltage is set by the generating plant.

Direct current is most commonly found in homes in the form of electrical energy stored in batteries. In a DC circuit, the amount of voltage and the direction of application are constant. The amount of voltage is determined by the type and size of battery. The direction of current flow is also constant and, as in AC circuits, the amount of current flow is determined by the load resistance.

DEFINITIONS of ELECTRICAL TERMS defines alternating current and direct current in more detail.

This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions.


Continue reading at ELECTRICAL DEFINITIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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