Since the school curriculum does not include the study of the topic “how to calculate the cross-section of a wire,” even in adulthood many people don’t even know how to do this. If the future specialty is related to electrical engineering, then the teacher in the educational institution will talk about the relationship of electric current and the cross section of the conductor. Otherwise, the person remains in ignorance. Although, at first glance, it may seem that in everyday life there is no need to figure out how to calculate the wire cross-section, this is far from the case. Each person at least once in his life starts a major overhaul, including the replacement of home electrical wiring. Of course, you can use the services of repair crews. At the same time, not knowing how to calculate the cross-section of the wire and simply believing the workers, you can either overpay for a completely unnecessary cross-section of the cable conductors (some electricians mount with a margin - for fidelity), or subsequently conduct a repeated replacement due to the wrong cross-section. It is not necessary to study everything thoroughly. It is enough to have only a general idea of how to calculate the cross-section of a wire to understand why workers choose just such a cross-section, and not some other.
Let's see why the choice of section is generally necessary? As you know, the transmission of electrical energy is carried out through metal conductors made of steel (power lines), copper, aluminum. The more electrons pass per unit of time per unit length of the wire (the so-called current density), the higher its heating. Thus, if the current value is too high and the cross section is small, an increase in the temperature of the conductor will be observed, which can provoke a fire hazard situation. There is always heating, but if the current cross-section of the wire is correctly made, then the temperature remains within acceptable limits. In the form of formulas, all of the above can be expressed as follows.
The cross section of the conductive material:
S = 2 * 3.14 * (R * R)
The linear expansion for a solid:
L = L1 * (1 + k * T),
where L1 is the length at zero degrees Celsius;
T is the heating temperature;
k is the expansion coefficient (tabular value).
The value of the cross section when heated:
S = 3.14 * ((L1 * (1 + k * T) * (L1 * (1 + k * T))
Current Density, as already indicated:
J = I / S
If we now substitute the value of S in the formula during heating, it becomes obvious that all three quantities are interrelated, and temperature plays a significant role.
One of the simplest questions that newcomers often “stumble upon" is “how to determine the cross section of a wire.” First, with the help of a caliper or a ruler (accuracy below), we determine the diameter. Then we square it and multiply by 0.78 (the area of the circle). So simple.
But in practice, the relationship between the cross section and the electric current is more important. That is, you need to know, for example, which wire is suitable for home wiring. The current value is determined by knowing the power consumption of the device or their group. So, if a kettle with a capacity of 2 kilowatts is connected to the outlet, then its current will be 2000 W / 220 V = 9 A. Now, using the special table provided in the Electrical Installation Rules, you can find the required section. The total number of cores in the cable is taken into account; conductor material (aluminum, copper); laying method (openly, in a pipe, gates); voltage and current rating. For example, a copper wire with a cross section of 1 mm square, being walled up in a wall, is capable of transmitting a current of 14 amperes for a long time, which corresponds to 3 kW (for an electric network with a voltage of 220 volts).
The dependence of current, power and voltage is derived from a simple formula:
P = U * I,
where P is the electrical power in watts;
U is the line voltage in volts;
I is the current in amperes.
Pay attention to the units! The result will be incorrect if, for example, instead of the value in watts, substitute the number in kilowatts in the formula.