The neutral wire connects the circuit back to the power source. The neutral wire, in particular, connects the circuit to a ground or busbar, which is normally attached at the electrical panel. Although they may not necessarily carry an electrical current, they should be handled with the same care as hot wire. If you attach any other wiring to the neutral slot, you might get a nasty surprise if it gets damaged.
Neutral wires are usually black or white, but they can also be green or grey. They usually measure between 14 and 16 gauge wire. Neutral wires cannot be used as a conductive path unless they are part of a metal junction box or conduit system. Even then, they should never be used as a conductive path from one fixture to another because this creates a risk of electric shock. Neutral wires are always connected to the neutral terminal on the breaker or fuse panel; they do not have terminals of their own. If you're working with old wiring and find that some circuits are wired with two black wires while others have three or four, this probably means that some houses were built before black was used as a ground conductor, so they used red instead. Either way, it's best to treat all these wires as if they weren't there, which means using a non-conductive connector on one end and a conductive connector (such as a screw) on the other.
A neutral wire is a type of wire that returns current to its source of power and/or normal voltage. In essence, it permits electricity to be returned to its original source. We'll go into the neutral wire's function a little later. For now, just know that it does not carry a live load.
In the United States, electrical systems were originally designed so that each circuit would include a hot wire and a neutral wire. The hot wire carries current so that appliances can use electricity for their intended purpose. The neutral wire is always white or grey, while the hot wire can be any other color except red. If you were to connect your black and white wires together, you would create a short circuit which could start a fire or damage other components on the line. This is why they are called "hot" and "neutral", respectively.
In newer homes, circuits may have both hot and neutral wires inside one conductor. This reduces the amount of material needed in construction and is less likely to cause problems with connections between circuits. However, these types of cables cannot be used with older wiring methods such as joint boxes or panelboards, so if you move into an old house, you might need to replace some of the cable to use with new equipment.
There are two types of neutral wires: continuous and 3-wire.
A neutral is a circuit conductor that, in normal operation, completes the circuit back to the source. Neutral is often linked to ground (earth) at the main electrical panel, street drop, or meter, as well as at the supply's ultimate step-down transformer. The connection to earth helps prevent any voltage on the neutral conductor from causing current to flow in Earth. If this connection were not made, every time lightning strikes the pole it would create a short circuit between the two metals in the ground, which could cause damage to other properties near these lines.
The National Electrical Code requires that all circuits not located in a load center must have a ground fault circuit breaker (GFCI). These are built into many types of outlets now, but they can also be found as separate devices. They use the same technology as the classic "electricity detector" used by astronauts when they walk on the moon - a resistor is always present to detect any abnormal voltage, so that people can be warned of an electrified surface before they contact it. A GFCI can also warn people if someone has been tampering with the outlet wiring; if this occurs, the GFCI will see this change and immediately isolate the wronged outlet(s).
In addition to the protective features of a GFCI, outlets installed in accordance with national code requirements include an RCA connector designed to protect against electric shock from live wires.