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Voltage bands

We see 12V lighting everywhere, and it’s usually described as “Low Voltage” lighting, but in fact “Low Voltage” means at least 50V and up to as much as 1000V for AC, 1500V for DC. So the supply that comes into your house is a low voltage one. 12V lighting, pond pumps etc are officially classed as “Extra-Low Voltage”, which is anything under 50V AC, 120V DC.

The IEC definitions of voltage bands are:
* **Extra Low Voltage**: AC below 50V and DC below 120V
* **Low Voltage**: 50 - 1000V AC or 120 - 1500V DC
* **Medium voltage**: 1kV - 35kV
* **High voltage**: 35kV - 230kV
* **Extra-high voltage**: >230kV

Why does this matter? Surely when people say “low voltage lighting” it’s pretty clear that they mean “12V”, so what’s the problem?

This is not some pedantic technical point along the lines of “it’s not an earth, it’s a cpc” or “bulbs are what you plant in the ground, what you mean is a lamp”. If you start getting involved in doing your own electrical work, and start learning about regulations etc you’ll come across references to “Low voltage”, and it’ll be no good you thinking that that means 12V.

The Building Regulations, for example, which are law in England and Wales, contain a section on electrical safety, Part P, and the legislation says:

The requirements of this Part apply only to
electrical installations that are intended to
operate at low or extra-low voltage….

So if you don’t realise that “low voltage” includes your mains supply of 230V, you might think that the law doesn’t apply.

People are careless, and they get the terms wrong - even electricians will talk about “low voltage bulbs” when they don’t mean 500V daffodils, but as long as you know what you really mean when you say “low voltage” you’ll not go wrong. If you read the term somewhere, and it isn’t clear from the context what the writer means, then ask.

Older definitions


Before the IEC definitions of voltage bands listed above were adopted in the 1980’s, the Wiring Regs., following the pattern of the Electricity Supply Regulations, 1937, used rather different definitions:

* **Extra Low Voltage**: Normally not exceeding 50V between conductors, and not exceeding 50V AC or 30V DC between any conductor and earth
* **Low Voltage**: > ELV, but < = 250V
* **Medium voltage**: 250 - 650V
* **High voltage**: > 650V

For low, medium, and high voltage, the values were applicable to both AC and DC, and for the specified voltage existing either between conductors or between any conductor and earth.

These differences should be remembered when looking at older reference material, or seeing warning notices on some older equipment, since the current definition of low voltage includes all of the former medium-voltage band plus some of the older high-voltage band. For example, a standard 240/415V 3-phase system is classified as low voltage today, but used to be medium voltage.


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