What does an electrical service look like?
There are logically four wires involved with supplying the main panel with power. Three of them will come from the utility pole, and a fourth (bare) wire comes from elsewhere.
The bare wire is connected to one or more long metal bars pounded into the ground, or to a wire buried in the foundation, or sometimes to the water supply pipe (has to be metal, continuous to where the main water pipe entering the house. Watch out for galvanic action conductivity “breaks” (often between copper and iron pipe). This is the “grounding conductor”. It is there to make sure that the third prong on your outlets is connected to ground. This wire normally carries no current.
One of the other wires will be white (or black with white or yellow stripes, or sometimes simply black). It is the neutral wire. It is connected to the “centre tap” (CEC; “center tap” in the NEC) of the distribution transformer supplying the power. It is connected to the grounding conductor in only one place (often inside the panel). The neutral and ground should not be connectedanywhere else. Otherwise, weird and/or dangerous things may happen.
Furthermore, there should only be one grounding system in a home. Some codes require more than one grounding electrode. These will be connected together, or connected to the neutral at a common point – still one grounding system. Adding additional grounding electrodes connected to other portions of the house wiring is unsafe and contrary to code.
If you add a subpanel, the ground and neutral are usually brought as separate conductors from the main panel, and are not connected together in the subpanel (ie: still only one neutral-ground connection). However, in some situations (certain categories of separate buildings) you actually do have to provide a second grounding electrode – consult your inspector.
The other two wires will usually be black, and are the “hot” wires. They are attached to the distribution transformer as well.
The two black wires are 180 degrees out of phase with each other. This means if you connect something to both hot wires, the voltage will be 220 volts. If you connect something to the white and either of the two blacks you will get 110V.
Some panels seem to only have three wires coming into them. This is either because the neutral and ground are connected together at a different point (eg: the meter or pole) and one wire is doing dual-duty as both neutral and ground, or in some rare occasions, the service has only one hot wire (110V only service).
Learn more about What is a circuit? here >>