How to determine the wire size

15-07-2018

Electrician

One of the important and main parts of electrical wiring are conductive cables. In order for everything to work as it should, without short circuits and molten wires, it is important to choose the wiring so that it can withstand the voltage. Make sure that the wire is reliable, safe and meets the requirements of operation, you can calculate the cross-section of the wire. If the wire is insufficient in cross section, it may cause a short circuit and a fire in the house. Let's figure out how to determine the cross section of the wire to avoid such consequences.

Serious question

If you have a plan for the replacement of electrical wiring, then first of all you need to decide what wire size is needed. When you calculate the planned load on the wire by current, it will be immediately clear what section is needed for normal operation. For this purpose, use the table below.

Suppose, having counted everything, you have determined the current strength of 27 A. It turns out that you need a two-core wire with a cross section of 2.5 mm². You went to the store, bought a good wire, with a 2.5 mm cross-section declared by the manufacturer² and made the installation of electrical wires. After the time when the system was up and running, the automat on the dashboard is turned off. This may indicate a damaged line.

After you inspect the wiring, it may become clear that there was a short circuit. The reason is simple - insulation on the wire melted due to strong heat. But how so? You did everything right, calculated, purchased the necessary wire. And the whole thing is not in the calculations and your connection error. The whole reason lies in deception. Yes, it is precisely in deception, since very often the manufacturers who produce wires deliver goods that do not correspond to the declared section. For example, you bought a cable with a wire area of 2.5 mm², but when you checked it yourself, it turned out that the cable section is only 2.1 mm²That is less than you need in power. So it turns out that the wire is heated, and increases the risk of a circuit.

But why should cable companies do this? It's all about greed, because, by lowering the cross-section of the wire, the manufacturing company saves a fairly significant amount of money. Judge for yourself: to make 1000 meters of wires with a cross section of 2.5 mm²It will take approximately 22.3 kg of copper. But if you make them with a smaller section, 2.1 mm², then their production will require 18.8 kg. It turns out that the savings in favor of the manufacturer is 3.5 kg of copper. And when large volumes of products are produced, this figure grows significantly.

So it turns out that the definition of the wire cross section is an important and serious step that needs to be done when installing the wiring. This way you can avoid melting the insulation, the circuit and, as a result, the fire.

How can I determine the cable cross section

So, you bought the wire, let's find out how you can measure its cross section. For this purpose you will need:

vernier caliper;
calculator;
the cable itself;
office knife or stripper for stripping.

Determined by the diameter of the wire. Let's look at how to do this in stages.

Take the wire and strip off the insulation with a stripper or a clerical knife.
Using a caliper, measure the diameter of the wire.
Now, you have to remember the lessons of geometry at school. In order to determine the area of a circle, there is a certain formula:

S_cr= ? r²,

where is the number = 3.14, and r is the radius of the wire.

But here we have a problem: using a caliper to measure the radius is impossible - only the diameter. Therefore, the formula needs to be slightly changed to the diameter. As we know, the radius is half the diameter. To make our data fit, the formula needs to be redone as follows:

S_cr= ( ? d²)/4,

where d is the diameter of the core.

In this case, the formula can be reduced by dividing the number? by four. As a result, we come to the standard formula for calculating the cross-section of the core by diameter:

Sqr = 0.785d²

It remains only to substitute the numbers to the formula and get the area of the circle, which will be its section. For example, the diameter of your copper wire is 1.68 mm. This squared number is 2.8224. Multiply 2.8224 by 0.785. If to round off, then as a result, the desired section turned out 2.2 mm².

That's all, as you can see, this procedure does not take much time and effort, but it is a very important part of the installation of electricity. After all, even a deviation of several tenths of a millimeter can play a cruel joke. But what if you are not an electrician and you do not have calipers at hand? Do not rush to run to the store, there is a simple method for calculating the diameter with the help of available tools.

A simple method for determining the diameter

Everything you need is on hand. This method will require only a pencil or pen and a ruler.

First of all, take the wire, the cross section of which you need to learn, and remove 300–400 mm of insulation along it.
Take a pencil, pen, felt-tip pen, or whatever will be at hand, and wind a wire on it. In order to achieve the most accurate result, the coils must be fitted close to each other.
The more turns you make, the more accurate the result will be. The recommended number is 10 or more turns.
Then count the number of turns that you wound on the pencil, and write down this number or remember.
Use the ruler to measure the total length of the counted turns. For example, you got 19 turns, which are 32 mm long.
To determine the diameter of the wire, it remains only to divide the length by the number of turns. It turns out like this: 32/19 = 1.68 mm.

That's all, now it remains only to substitute the diameter of the wire in the formula and you get a cross section of 2.2 mm wire². This result can be considered accurate under two conditions: the turns were wound closely, and their number exceeds 10 pieces - the more, the better. As a result, you do not have to spend extra money.

The only disadvantage of this method: if you lived with a large cross-section, you will not be able to wind it on a pencil.

How to measure the cross section, if the wire is stranded

Well, you say, if the wire is one, it’s easy to calculate the cross section. And what if it is stranded? There are a lot of such wires there, how to recognize it in such a case? Determining the cross-section of stranded wire is not as difficult as it seems. All you need to determine the cross section of one of the veins.

First, take the wire and remove the insulation from it.
Now all the wiring must be disconnected among themselves and recalculate their number.
Take one of the wires and measure its diameter.
Applying the formula that is presented above, determine the area of a single core.
Having a cross section of one core, you can find out the total cross section. To do this, the area of one wire multiplied by their total. For example, you have a wire consisting of 15 wires. You have learned that the cross section of a single wire is 0.2 mm². In order to find out their total cross section, 0.2 must be multiplied by 15. Get: 0.2? 15 = 3 mm².

But it should be noted that all the wires in the wire can not be connected closely. Due to the fact that they are round, an air gap is formed between them. It must be taken into account. To do this, multiply the resulting result by 0.91. Suppose if the cross section of the wire was 3 mm², then, multiplying it by a factor of 0.91, we obtain a cross section of 2.7 mm². That's all. There is nothing difficult, you just need to follow the instructions.

As you can see, determine the cross section of the wire under the force of anyone who knows how to use formulas. But, despite the simplicity of such work, it is one of the most important stages that must be done before wiring throughout the house.

Video

Learn more about the practical meaning of the wire section in the video: