Contents of the article:
wire The current cross-section and power are parameters that indicate the purpose of a cable. In other words, where the wire can be used and where it is impossible.
Data collection
The section is selected according to the power or current of the instruments that will be connected later. Such a method is called “according to the load”, because the devices are the load on the cable. If the equipment requires high energy costs, then, respectively, and the cable to it will have to connect powerful. If it does not, then a wire with a small cross section will be quite enough. How to choose the cable itself and what to follow?
First of all, you need to collect data on those devices to which the wires will go. Such data are called passport data, they are necessarily written in the technical passport of the device. It contains data such as:
- device model;
- voltage;
- power consumption;
- certificate mark;
- manufacturing country;
- production date;
- recycling mark;
- protection class and so on.
In addition, if the registration certificate, for example, you have lost, then special signs are put on the devices or stickers are glued. They display basic data. Including the power consumption that we need. You can select the wire size for power and without it.
If there are no signs with a sticker, but you remember the model( it can be written on the case), then it does not matter. Try searching for information about the device on the Internet. Absolutely, as a last resort, use the data of average statistics. There is a special table of the estimated power consumption of various appliances, such as: drill, toaster, refrigerator, washing machine, air conditioning and so on. 
Only here there is one important nuance. See the power range in the table? It is difficult to guess: what to choose.
Always take the maximum!
When you begin to make a calculation of the cable cross-section for power, you will end up with an overpowering instrument. This is very good, as a result you will need a cable with a large cross section. Such cables are little heated and, accordingly, work longer.
If the device requires more power, then a wire with a small cross section will simply burn out.
Load method
As already mentioned, the load is the device. He may be one or maybe several. No matter how many of them, always add up all the power of the devices to which you connect the conductor. All these powers must be expressed in only one unit of measurement! In watts or kilowatts, otherwise you will get confused in calculations.
“Kilo” is multiplication by a thousand.1 kW = 1000 watts.
If the power values of the devices are different, then we make them the same - translate. Suppose we have one device consumes 100 watts and the other 3.5 kW.Leaving the value of the first intact, and the value of the last translate, we get 3500 watts. If you want to convert watts to kilowatts, then divide by a thousand.
Power calculated. Now choose the cable section. The table of cable power by section is presented below. There is nothing difficult in it, as it is necessary to simply select the columns where the phases are indicated. If you have one phase in the network, then we take the voltage of 220 volts. If three - 380 volts. 
Then we find the number that is slightly more power that you counted. Found? On the left is the corresponding conductor cross section and its diameter. This is the cable you need. If at hand there is a table of cable cross section for power, then no difficulties will arise.
In this table, the values for copper and aluminum wires are different. What lived you need - in such columns and see.
Sometimes there are difficulties with the choice of the material from which the cable cores are made. Copper is used as wiring houses and apartments. It is believed that copper wires are flexible, practical and reliable. True, they are more expensive than aluminum cables. Of course, if the copper conductor has a large cross section( when there is a high load in the house), then you can’t call it flexible. And the price will be higher. Therefore, in such cases, feel free to take aluminum wires - a good savings.
For power and length
The choice of cable section for power and length is slightly different. It happens when a conductor has a length of several tens or even hundreds of meters. Losses in the cables themselves will have to be taken into account, otherwise the energy may not be enough for the equipment. There is another table that tells you what to do next, taking into account all the losses.
You need to know the power that is allocated to a house or building. The allocated power is the power of all the equipment that works in the house. And the distance from the post to the building where the cable comes from. This distance is easy to measure yourself.
Be sure to take a small margin of wire before laying the wiring.
With a larger cross-section, the wire heats up less and insulation, along with it. This means that the likelihood of a fire or a circuit is reduced. Also, very often it happens that the number of appliances in the house can increase. Let's say you put a fridge, an air conditioner and an electric stove. A year later, they decided to buy a computer, a toaster, two televisions and something else that runs on electricity. Wiring is simply not enough power to withstand such a number of equipment. You will have to make sure that the powerful equipment is not turned on at the same time, or to completely change the wiring. And you can simply lay the wiring in advance with a margin section. So it is more rational: then it is not necessary to suffer.
Current calculation
Current cable selection is also possible. To do this, it is necessary to carry out the same data collection on stickers, plates or technical passport. Only now we need not power in watts, but current in amperes. The characteristics indicate the current that is maximally consumed by the device.
We again collect data from all devices and summarize it. And we also translate everything into one unit, similarly: 1 mA( milliampere) = 0.001 A and 1A = 1000 mA.For example, 2.3A is 2300 mA.Just sometimes for some reason they specify in milliamperes.
The very first table shown above can determine the cross section not only by the number of watts. It is also a table for determining the cross-section of wires for power and current simultaneously. That is, will have to work again with her. Note: the numbers are not all. For example, you have a current consumption of 25 amperes, and you need a copper wire. In the table of this number is not. Choose a larger value. It is equal to twenty-seven amperes - therefore, be guided. It turns out that the required cable cross-section for current is 4 square millimeters.
Never choose a lower value to save! At best, the circuit breaker will work, stopping the supply of electricity. If there is no such automaton, and this is the worst case, then there is a high probability of equipment failure or even a fire. Do not save on the safety of your home and yourself.
Laying wires
Nevertheless, when current passes through the wire, the conductor heats up. A lot of current - a lot of heat. What are we talking about: the laying of the wire can be closed or open. Closed is when the wire is under a special pipe. Open - when it is not covered with anything, that is, a bare wire attached to the wall.
Here you can cheat. The temperature will be different, with different sections of the conductor, even if the current value remains unchanged. So, if the cable is open, a smaller section is quite acceptable. Heat will go into the air, and the wire, respectively, cool.
Wires with a small cross-section, in pipes, cable channels or the wall can not cool down - the heat has nowhere to go. Therefore, when the wire strip is closed, only a larger section is necessary, otherwise the insulation will deteriorate. There is also a table that will help you choose a conductor, taking into account its laying. The principle remains the same: copper or aluminum conductors, current and power.
Cable routing table: 
But you can get confused. For example, we need a copper conductor, with a power of 7.3 kW( 7300 W).The network is single phase, we will put it closed. We look at the plate. We remember that everything is taken at the maximum values. Find the number of 7.4 kW.And we see that the desired section will be 6 square millimeters.
Alternatively, we want to lay aluminum conductor openly. We know that the dispensing current is 40 amperes. The table has the number 39. You can not! We take more - sixty amps. We see that the conductor we buy with a cross section of ten square millimeters. And if it’s closed we will lay out, then 16. And they were not mistaken, and there is a reserve. Before you buy a wire, take a caliper and the first plate with you. Just in case, check: does it have a diameter? If, in fact, it turns out to be less than declared, then do not take this wire!
