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In theory, the diameter of the conductors should correspond to the declared parameters. For example, if it is indicated on the marking that the cable is 3 x 2,5, then the cross section of the conductors must be exactly 2,5 mm2. In fact, it turns out that the real size can differ by 20-30%, and sometimes more. What does it threaten? Overheating or melting of insulation with all the ensuing consequences. Therefore, before buying, it is advisable to find out the size of the wire in order to determine its cross section. How exactly to calculate the cross section of the wire in diameter and we will find out further.
How and with what to measure the diameter of the wire (wire)
To measure the diameter of the wire, a caliper or micrometer of any type (mechanical or electronic) is suitable. It is easier to work with electronic ones, but not everyone has them. It is necessary to measure the core itself without insulation, therefore first move it away or remove a small piece. This can be done if the seller allows. If not, buy a small piece for testing and take measurements on it. On a conductor stripped of insulation, measure the diameter, after which you can determine the actual cross-section of the wire according to the dimensions found.
Which measuring device is better in this case? If we talk about mechanical models, then a micrometer. It has higher measurement accuracy. If we talk about electronic options, then for our purposes they both give quite reliable results.
If you don’t have a caliper or micrometer, take a screwdriver and a ruler with you. You will have to strip a pretty decent piece of the conductor, so this time you are unlikely to do without buying a test sample. So, remove the insulation from a piece of wire 5-10 cm. Wind the wire around the cylindrical part of the screwdriver. The coils are laid close to each other, without a gap. All turns must be complete, that is, the “tails” of the wire must stick out in one direction – up or down, for example.
The number of turns is not important – about 10. It can be more or less, it’s just easier to divide by 10. You count the turns, then apply the resulting winding to the ruler, aligning the beginning of the first turn with the zero mark (as in the photo). Measure the length of the section occupied by the wire, then divide it by the number of turns. Get the wire diameter. It’s that simple.
For example, let’s calculate the size of the wire shown in the photo above. The number of turns in this case is 11, they occupy 7,5 mm. We divide 7,5 by 11, we get 0,68 mm. This will be the diameter of this wire. Next, you can search for the cross section of this conductor.
We are looking for a wire cross-section by diameter: formula
The wires in the cable are circular in cross section. Therefore, in the calculations we use the formula for the area of a circle. It can be found using either the radius (half of the measured diameter) or the diameter (see formula).
For example, let’s calculate the cross-sectional area of the conductor (wire) according to the size calculated earlier: 0,68 mm. Let’s use the radius formula first. First we find the radius: we divide the diameter by two. 0,68 mm / 2 = 0,34 mm. Next, we substitute this number into the formula
S = π * R2 =3,14*0,342 = 0,36 mm2
It is necessary to count as follows: first we square 0,34, then we multiply the resulting value by 3,14. We got the cross section of this wire 0,36 square millimeters. This is a very thin wire that is not used in power networks.
Let’s calculate the cable cross-section by diameter using the second part of the formula. It should be exactly the same value. The difference may be in thousandths due to different rounding.
S = π/4 * D2 = 3.14/4 * 0,682 = 0,785 * 0,4624 = 0,36 mm2
In this case, we divide the number 3,14 by four, then square the diameter, multiply the two resulting numbers. We get a similar value, as it should be. Now you know how to find out the cable cross-section by diameter. Whichever of these formulas is more convenient for you, use that one. No difference.
Correspondence table for wire diameters and their cross-sectional area
It is not always desirable or possible to carry out calculations in a store or in the market. In order not to waste time on calculations or not to make a mistake, you can use the table of correspondence of diameters and cross-sections of wires, which contains the most common (standard) sizes. You can write it down, print it out and take it with you.
| Conductor diameter | Conductor cross section |
|---|---|
| 0,8 mm | 0,5 mm2 |
| 0,98 mm | 0,75 mm2 |
| 1,13 mm | 1 mm2 |
| 1,38 mm | 1,5 mm2 |
| 1,6 mm | 2,0 mm2 |
| 1,78 mm | 2,5 mm2 |
| 2,26 mm | 4,0 mm2 |
| 2,76 mm | 6,0 mm2 |
| 3,57 mm | 10,0 mm2 |
| 4,51 mm | 16,0 mm2 |
| 5,64 mm | 25,0 mm2 |
How to work with this table? As a rule, the cables have a marking or a tag on which its parameters are indicated. It indicates the cable marking, the number of cores and their cross section. For example, VVNG 2×4. We are interested in the parameters of the core, and these are the numbers that come after the “x” sign. In this case, it is stated that there are two conductors having a cross section of 4 mm2. So we will check whether this information is true.
How to work with a table
To check, measure the diameter using any of the methods described, then refer to the table. It states that with such a cross section of four square millimeters, the wire size should be 2,26 mm. If your measurements are the same or very close (there is a measurement error, since the devices are not ideal), everything is fine, you can buy this cable.
But much more often, the actual diameter of the conductors is much less than the declared one. Then you have two ways: look for a wire from another manufacturer or take a larger section. Of course, you will have to overpay for it, but the first option will require a fairly long period of time, and it’s not a fact that you will be able to find a cable that meets GOST.
The second option will require more money, since the price significantly depends on the declared section. Although, not a fact – a good cable, made according to all standards, can cost even more. This is understandable – the cost of copper, and, often, for insulation, subject to technology and standards, is much higher. Therefore, manufacturers are cunning, reducing the diameter of the wires – in order to reduce the price. But such savings can turn into disaster. So be sure to take measurements before buying. Even trusted suppliers.
And one more thing: inspect and feel the insulation. It should be thick, solid, have the same thickness. If, in addition to changing the diameter, there are also problems with insulation, look for a cable from another manufacturer. In general, it is desirable to find products that meet the requirements of GOST, and not made according to TU. In this case, there is hope that the cable or wire will last a long time and without problems. This is not easy to do today, but if you are wiring a house or connecting electricity from a pole, quality is very important. That’s why it’s probably worth looking into.
How to determine the cross section of a stranded wire
Sometimes conductors are used stranded – consisting of many identical thin wires. How to calculate the wire cross-section by diameter in this case? Yes, exactly the same. Take measurements / calculations for one wire, count their number in a bundle, then multiply by this number. So you will find out the cross-sectional area of uXNUMXbuXNUMXba stranded wire.