This month we will discuss where electricity comes from. Most public utility electricity in the United States is generated at coal power plants (56%); the remainder is nuclear (22%), natural gas (10%), hydroelectric (10%), and the remainder biomass/wind/solar and other. These various fuels are used to turn generators that create high-voltage (pressure) electricity. This electricity is transmitted at voltages of 60kV or more through conductors to local substations that contain transformers which reduce the voltage for distribution, usually at 601 to 60kV, to residential and commercial neighborhoods. Modern power supply systems are configured in a grid system that allows the utility to manage changes in demand and system failures. Transmission lines do not contain a grounded (neutral) conductor. Distribution lines sometimes do have a grounded (neutral) conductor, depending on the preference of the utility. Additional transformers at the neighborhoods further reduce the voltage to the standard 120/240 volt alternating current used in houses.

The 120/240 volt configuration is accomplished by the grounded neutral center tap of the last transformer; creating a potential of 120 volts from either hot to the grounded (earth referenced) conductor. When one “hot” is swinging positive, the other is swinging negative, allowing both simultaneously available for operating smaller, 120 volt devices. Larger, 240 volt appliances will use both of the ungrounded conductors and not the grounded conductor.

All electrical circuits must consist of a loop (two conductors must be present for the electrons to flow). The utilization device limits the amount of current (amperage) by resistance (think small pipe) which limits flow. In 120 volt circuits the electricity flows to/from the transformer through the ungrounded (hot) and grounded (neutral) conductors. With 240 volt circuits the electricity flows to/from the transformer through the ungrounded conductors; the neutral is not needed.

The equipment grounding conductor (usually the bare or green insulated conductor) is not a part of the intended electric circuit. The grounding conductor is a safety path for electricity back to the transformer in the event of a malfunction such as a fault to ground (electricity “leaking” to metal components not intended as part of the circuit). This additional path is low resistance allowing current to flow and hopefully tripping the breaker to open the circuit and protect people.

We'll talk next month,

Mike Casey
Kaplan Professional Schools
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