What is the common wire? A guide to electrical wiring in your home

What is the common wire? A guide to electrical wiring in your home



Present-day households cannot function without electricity. It powers light fixtures, A/C, TVs, computers, refrigerators, and all of your other electrical equipment. A basic understanding of your home's electrical system is always a good idea because it helps you diagnose appliance or electricity-related problems faster.

In this article, we explain what common wires are, their purpose, and how you can identify them. In addition, we discuss how a home’s electrical system works and other key electrical components and terms to understand.


What is the common wire?

In a typical household, there are three types of house wiring: common wires, ground wires and hot wires. A common wire, also called a C-wire or a neutral wire, is a part of your heating system.

What is the purpose of a common wire?

A common wire permits the completion of a circuit initiated by a hot wire. It goes from a low-voltage heating system and supplies constant power to the thermostat. A thermostat is a device that detects any temperature changes. It is specifically designed to maintain the temperature in a closed area.

How to identify the common wire

Identifying a common wire isn’t so easy, so we simplified it for you:

  • Unscrew the cover of the thermostat, and take a hard look at the wires inside. Note the number of wires and their colors.
  • If you live in the U.S., as per electrical codes, a common wire should be either white or gray. White common wires are seen more often.
  • On the other hand, blue is a commonly used color code for common wires in European counterparts.

Unfortunately, many homes have wrongly colored wires, which can be dangerous if misinterpreted. So, while color is a good rule of thumb, there is one surefire way to identify a common wire —  by testing the electrical system.

It is advisable to take the help of an electrician because this involves testing hot wires, which can be dangerous. If you happen to go the DIY route, here’s how to identify a common wire:

  1. Turn off the power to your home.
  2. Cap the electrical wires that will not be tested.
  3. Turn the power back on.
  4. Take a multimeter and set the voltage to the highest DC (direct current) range.
  5. Take the red probe of your multimeter and connect it to the wire you want to test.

If you see a reading on the multimeter, you have the hot wire. If there is no reading at all, you have found the common wire.

The difference between a ground wire, hot wire and common wire

In this section, we define hot and ground wires and understand how they differ from common wires.

Hot wire. Put simply, a hot wire directly transfers electrical current from its source (in the home) to various bulbs and switches mounted across the home. It acts as the receptacle for transmitting power to the circuit. Hot wires are dangerous because they always carry electricity and are covered with a black case. In some instances, they come in red and yellow color codes too.

On the contrary, a common wire does not necessarily always carry electrical current. 

Common wires need to be handled with caution, but experts should always handle hot wires.

A key difference between these wires is the common wire returns the electrical current to its sources, whereas the hot wire only transmits power. 

Ground wire. A ground wire is installed for electrical safety, especially when there is instability or sudden surges in electrical current. It rarely carries any electrical power. Whenever there is any discrepancy in the electrical flow, it directs the flow of electricity to the ground rather than other areas of the home. It prevents short circuits, electrocution and fires. A ground wire casing is green. 

A key difference between a ground wire and a common wire is that common wires don’t redirect the electrical current outside the electrical system.

Every home must have proper ground wiring. It is highly advised to get your home checked by an electrician if you are unsure of the status of your electrical wiring.


Simply explained: How a home electrical system works

Electricity is an inseparable part of any modern-day household. In this section, we provide a general overview of a basic home electrical system and how it works to supply power from the main power line going to your house to the outlets in your home.

A local utility company supplies electricity to your home with the help of a power line or a conduit located under the ground. Typically, homes are equipped with three-wire service: one neutral and two hot wires.

The hot wires supply power to 120-volt appliances, and hot wires and neutral wires make up a 240-volt circuit for large appliances, like your A/C.

The utility company monitors the consumption of electricity through an electric meter fitted at the exact location where electricity enters your home.

Next to the electric meter is the main electrical panel. It distributes electricity to electrical circuits to power the lights and appliances installed within the house.

A circuit is a circular journey that starts and ends at the same place. If there are any interruptions in this journey, the circuit is deemed dead.

In a circuit, you will find a hot wire (typically with a black casing) that goes from the main panel and powers lightbulbs, appliances and other receptacles. Then comes a common wire, which returns the electricity to the main panel. Apart from these, there is a ground wire that goes back to the main panel and the ground.

In various locations of the house, you will also find subpanels connected to the main panel. Subpanels transfer power to places that have separate branch circuits — for example, large appliances.

Transformers are also a part of electrical systems, typically installed to power low-voltage appliances such as doorbells, sprinkler timers, and certain types of outdoor or indoor lighting.

A transformer is a device that transmits electric energy from one AC (alternating current) circuit to one or more other circuits. It either increases or reduces the voltage when necessary such as taking high-voltage electricity down to low voltage. 

Note: National Electrical Code (NEC), or NFPA 70, is a regionally accepted standard for the secure installation of electrical wiring and equipment across the United States. Read more about the codes and standards here.

Home electrical system terms to understand

When electricians visit our homes to repair our appliances or electrical system, they use specific terms that may seem confusing. We have listed important terms to help you better understand your home’s electrical system.

  • Electrical panel. An electrical panel (or a breaker panel) is a metal box that connects the main power supply to the home. Its key role is to distribute electrical current to different circuits inside a home.
  • GFCI outlet. A ground fault circuit interrupter (GFCI) outlet is installed in moisture-prone areas, like bathrooms and kitchens. It acts as a mini circuit breaker. Let's say you are using an electrical item that accidentally comes in contact with water. In that case, there is some probability of electrical shock. A GFCI unit cuts off the flow of electricity to prevent such mishaps.
  • Arcing. Electrical arcing happens when an electrical current jumps from one connection to another. Arcing happens when a circuit gets overheated or overloaded.
  • Connectors. A connector is an electromechanical device that creates an electrical connection between different aspects of an electrical circuit. A connector can also connect two different electrical circuits to make them part of one big circuit. 
  • Alternating current. AC is an electric current that reverses its direction sporadically. It is the exact opposite of direct current (DC), which only flows in a single direction.
  • OHM. This is the SI unit used to measure an electrical device’s electrical resistance.
  • Amp. An amp or amperage is the unit that measures constant electrical current.
  • Busbar. Metal bars that carry electric current.


Costly electrical system repairs are covered under a Cinch warranty

As a homeowner, you understand the prominent role electrical systems play in your household. Like all appliances and systems in the home, electrical systems are prone to problems and sudden breakdowns such as leakage in current flow. If left undiagnosed, such problems can have dangerous repercussions. 

However, your electrical system is in safe hands with the Cinch Home Services Built-in Systems plan. It covers several components and parts, including direct current wiring, light fixtures, and built-in exhaust, vent and attic fans. 

Bid goodbye to overbearing repair costs and avoid back-and-forth dealings with inexperienced electricians. With Cinch, you can have a vetted electrician sent to your home to repair your electrical systems. 

Get a quote today!

Learn about common wires and how a home's electrical system works.

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