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Question: "I am rewiring for a dryer plug from a circuit box. My old dryer had only three wires, a red, black and white. My new wire also has a ground. Is the ground needed or does the white wire act as the ground?"

Answer: "The black and red leads each supply 120 volts of power and the white wire or "neutral" is simply there as a return feed for unused or excess power. The bare or green wire is the ground wire, and is there to reduce the chance of an electric shock when a short circuit occurs (...) It is hoped that the ground wire will discharge excess electricity."

Physicist's comments: While it's quite common to hear a phrase "120 volts of power", it's not a power that 120 volts refer to, but rather a voltage (electric potential, or energy per unit of electric charge). A unit for electric power is 1 W (1 watt.)

This confusion of units makes unclear what is it that the authors refer to in the next part of the sentence - a power (watts), a voltage (volts), or perhaps a current (amperes, known as amps), when they claim that unused or excess fraction of it returns to the outlet. In reality, while the dryer uses up the electric power and the electric potential, the electric current flowing into the dryer is flowing out unchanged.

Let's talk about the DC current first. DC current is a stream of electrons that flow in one direction at a constant rate. Imagine a water current in a creek, turning the waterwheel; the water does not get stuck or used up while passing over the wheel. But as the water falls down, its energy gets transferred to the waterwheel. Same for the electric current; a constant stream of electrons keeps passing through a device delivering energy to it, but none of electrons get stuck or used up.

Now, almost all electrical power in the United States is generated with a sinusoidal voltage that oscillates from the peak voltage of +170 volts to - 170 volts and back to + 170 volts at the rate of 60 times each second. The effective voltage important for power condiderations is then 120 volt (rms or root-mean-square voltage). The resulting AC current also has a sinusoidal variation with time, i.e. the electrons change the direction of flow 120 times each second. (In other countries, rms voltage of 240 volts and frequency of 50 Hz is used.) But again, no matter in what direction the electric current happens to flow, at any instant of time the number of electrons entering a device is equal to the number of electrons exiting that device.

In the United States nearly all residences and many small commercial facilities are supplied power by a three wire connection to the power system. One wire (red) supplies AC sinusoidal voltage with the effective voltage of 120 volt, the second wire (black) also supplies AC sinusoidal voltage with the effective voltage of 120 volt, and the third (white) wire is the return wire, or so called neutral wire. The key point is that the sinusoidal voltage in the black wire is half a cycle shifted with respect to the voltage in the red wire, i.e. at the instant when the voltage in the red wire happens to be at + 170 volt, the voltage in the black wire is at - 170 volt, and vice versa. One has then three possibilities to connect a single device:

    a) A low power device can be connected between the red and neutral wire, at an effective voltage of 120 volt. Clearly, the entering current in the neutral wire is then identical to the exiting current in the red wire, and half a cycle later the role of each wire reverses, i.e. the exiting current in the red wire is equal to the entering current in the neutral wire.

    b) Another low power device can be connected between black and neutral wire, again at an effective voltage of 120 volt. The exiting (entering) current in the neutral wire is equal to the entering (exiting) current in the black wire.

Note that if two devices are connected at the same time, one as in (a), the second as in (b), and both are electrically balanced, at any instant of time the return current from the first device is exactly opposite to the return current from the second device, so these currents cancel out and the combined current in the neutral wire is zero. Of course, such a perfect balancing happens rarely, but the combined neutral current is usually less than each line current, and this results in reduced power losses.

    c) A large power device can be connected between red and black wires, at an effective voltage of 240 volt. The neutral wire is not needed in this situation, since the entering current in the red wire is exactly matched by the exiting current in the black wire, and v.v.

Note, that while power is connected to the load by three wires, this is not considered to be a three phase system, but a single-phase three-wire system.

A large appliance, such as a dryer, contains several individual devices, connected to the power as in (a), (b) or (c). If the individual loads are reasonably well balanced, the resulting current in the neutral wire will be small compared to those in red or black wire.

Finally, let us address the issue of the ground (green) wire. It's goal is not to "discharge excess electricity". If insulation of any of the three wires becomes frayed, the naked wire may make a contact with a metal part or with the case of an appliance. When you touch the case, your body will close the circuit and the current will start to flow from the hot wire through the case and your body to the ground, resulting in electric shock and possibly death. The goal of the ground wire is to provide a permanent path from the case to the ground. Since the ground wire has much smaller resistance than your body, the instant the naked hot wire gets in contact with the case and thus with the ground, the current in the frayed hot wire will increase enormously, causing the fuse to blow immediately.

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