Household Wiring

The standard U.S. household wiring design has two 120 volt "hot" wires and a neutral which is at ground potential. The two 120 volt wires are obtained by grounding the centertap of the transformer supplying the house so that when one hot wire is swinging positive with respect to ground, the other is swinging negative. This versatile design allows the use of either hot wire to supply the standard 120 volt household circuits. For higher power applications like clothes dryers, electric ranges, air conditioners, etc. , both hot wires can be used to produce a 240 volt circuit.

Polarized Receptacles

The high voltage (about 120 volts effective, 60 Hz AC) is supplied to the smaller prong of the standard polarized U.S. receptacle. It is commonly called the "hot wire". If an appliance is plugged into the receptacle, then electric current will flow through the appliance and then back to the wider prong, the neutral. The neutral wire carries the current back to the electrical panel and from there to the earth (ground). The ground wire is not a part of the electrical circuit, but is desirable for prevention of electric shock.

The two receptacles in a common "duplex" receptacle receive power from the same circuit leading from the main electrical supply panel. They are wired in parallel so that two appliances which are plugged into the receptacle receive the same voltage, but can draw different amounts of electric current. Parallel wiring is the standard for 120 volt circuits in the entire house, making possible the independent use of all appliances, supplied by the same voltage.

Polarized Receptacles

The wider prong on the polarized plug will permit it to be plugged in only with the correct polarity. The narrower prong is the "hot" lead and the switch to the appliance is placed in that lead, gauranteeing that no voltage will reach the appliance when it is switched off. A non-polarized plug may have the switch in the neutral leg and thus be a shock hazard even when it is switched off.

The Electrical Code and Variations

U.S. electrical wiring is governed by a general electrical code. For example, current code dictates three-prong polarized receptacles and dictates the use of ground fault interrupters in locations where an electrical appliance may be dropped in water.

No attempt to summarize the code will be made here, but a few examples may be instructive. One recent variation which is in force in some locations is the requirement that the neutral tie block and ground wire tie block be separate. The neutral tie block is grounded at the center tap of the transformer which supplies the house, and the ground tie block is tied directly to ground via a ground stake or other grounding mechanism. This is different from the circuit panel illustration which reflects the long-standing practice of having a common neutral and ground tie block which was grounded at the house as well as at the transformer.