Voltage of wires and cables

The Voltage of Wire and cable is a measure of the strength of an electrical supply. 

A voltage may exist even when no current is flowing. In older textbooks, you will find terms like 'electrical potential' or 'electro-motive force, which gives a better feel for what voltage means. 

Strictly, a voltage is only defined between two points. 

When only one point is specified, we tacitly assume that the other point is the earth (which means exactly what it says: the ground beneath our feet). 

The earth is not a very good conductor of electricity, but there's an awful lot of it, which makes up for this to a certain extent. 

So when I say 'there are 230 volts at this point, what I really mean is that the voltage difference between this point and earth is 230 volts (it's a bit more complicated than this in practice, as we shall see).

Voltage is measured in volts, which is abbreviated to 'V'. So '230V' means '230 volts'. 

The mathematical symbol for voltage is also 'V'. 

Incidentally, although you'll hear electricians talking about '240 volt' mains, in fact, our mains supply voltage has been 230 volts for about ten years, to make our electrical equipment compatible with that of the rest of Europe.

To get an alternating current, we need an alternating voltage. 

So the electrical mains voltage will cycle from about 325 volts to zero, to -325 volts, then back to zero, and so on, 50 times per second.

The variation of the voltage waveform over time.

One complete cycle of this variation lasts one-fiftieth of a second

Why is the maximum voltage of the cable 325 volts and not 230 volts as we normally say? 

It turns out that this waveform (which varies between high and low voltages) carries the same amount of energy as a constant voltage about 70% of the size. 

So when we talk about a 230V AC supply, we mean a supply that would carry the same energy as a constant voltage of 230 V. 

This actually means an AC voltage that reaches 325 volts at certain points and is zero at others. 

Electrical engineers refer to the '230 volt' figure as the 'root mean square' voltage, for reasons that you'll find in an engineering textbook. 

This is abbreviated to 'arms', so you'll sometimes see the domestic mains voltage written as '230 Vrms'. 

Unless indicated otherwise, you can expect voltages and currents described in electrical manuals and manufacturers' catalogs as 'RMS' figures, and then ignore this fact completely. 

The reason you can ignore it is that — in domestic work — so long as all measurements of voltage, current, and power are RMS measurements, all the calculations still give correct answers.

230 volts is quite enough to give you a nasty shock, and sometimes these shocks can be fatal. 

In some parts of the world lower voltages are used, for increased safety. 

For reasons that will be explained later, it is more efficient (i.e., less wasteful of energy) to distribute electricity at a higher voltage, but increased efficiency is gained at the expense of safety.