The question of choosing the cable section for wiring in a house or apartment is very serious. If this indicator does not correspond to the load in the circuit, then the wire insulation will simply start to overheat, then melt and burn. The end result is a short circuit. The thing is that the load creates a certain current density. And if the cable cross section is small, then the current density in it will be large. Therefore, before buying, it is necessary to calculate the cable cross-section according to the load.
Of course, you should not just randomly choose a wire with a larger cross section. This will hit your budget first. With a smaller cross section, the cable may not withstand the load and will quickly fail. Therefore, it is best to start with the question, how to calculate the load on the cable? And only then, according to this indicator, select the electric wire itself.
Power calculation
The easiest way is to calculate the total power that a house or apartment will consume. This calculation will be used to select the wire section from the power line pole to the introductory machine to the cottage or from the access panel to the apartment to the first junction box. In the same way, wires are calculated for loops or rooms. It is clear that the input cable will have the largest section. And the farther from the first junction box, the more this indicator will decrease.
But back to the calculations. So, first of all, it is necessary to determine the total power of consumers. For each of them (household appliances and lighting lamps) this indicator is indicated on the case. If not found, look in the passport or in the instructions.
After that, all the powers must be added. This is the total power of the house or apartment. Exactly the same calculation must be done along the contours. But there is one point of contention here. Some experts recommend multiplying the total by a reduction factor of 0.8, adhering to the rule that not all devices will be connected to the circuit at the same time. Others, on the contrary, suggest multiplying by a multiplying factor of 1.2, thereby creating a certain margin for the future, due to the fact that there is a high probability of additional household appliances appearing in a house or apartment. In our opinion, the second option is the best one.
Cable selection
Now, knowing the total power indicator, you can select the wiring section. The PUE has tables that make it easy to make this choice. Here are a few examples for an electric line energized with 220 volts.
- If the total power is 4 kW, then the wire cross section will be 1.5 mm².
- Power 6 kW, cross section 2.5 mm².
- Power 10 kW - cross section 6 mm².
There is exactly the same table for a 380 volt electrical network.
Calculation of the current load
This is the most accurate value of the calculation carried out on the current load. For this, the formula is used:
I=P/U cos φ, where
- I is the current strength;
- P is the total power;
- U - voltage in the network (in this case, 220 V);
- cos φ is the power factor.
There is a formula for a three-phase electrical network:
I=P/(U cos φ)*√3.
It is by the indicator of current strength that the cable cross section is determined according to the same tables in the PUE. Again, here are a few examples.
- Current strength 19 A - cable cross section 1.5 mm².
- 27 A - 2.5 mm².
- 46 A - 6 mm².
As in the case of determining the cross section by power, here it is also best to multiply the current strength indicator by a multiplying factor of 1.5.
Odds
There are certain conditions under which the current inside the wiring can increase or decrease. For example, in open electrical wiring, when the wires are laid along the walls or ceiling, the current strength will be increased than in a closed circuit. This is directly related to the ambient temperature. The larger it is, the more current this cable can pass.
Attention! All of the above tables of PUE are calculated under the condition that the wires are operated at a temperature of + 25 ° C with the temperature of the cables themselves not more than + 65 ° C.
That is, it turns out that if several wires are laid in one tray, corrugation or pipe at once, then the temperature inside the wiring will be increased due to the heating of the cables themselves. This leads to the fact that the permissible current load is reduced by 10-30 percent. The same applies to open wiring inside heated rooms. Therefore, we can conclude: when calculating the cable cross-section, depending on the current load at elevated operating temperatures, you can choose wires of a smaller area. This, of course, is a good savings. By the way, there are also tables of reducing coefficients in the PUE.
There is one more point that concerns the length of the electrical cable used. The longer the wiring, the greater the voltage loss in the sections. Losses equal to 5% are used in any calculations. That is, this is the maximum. If the losses are greater than this value, then the cable section will have to be increased. By the way, it is not difficult to independently calculate the current losses if you know the wiring resistance and current load. Although the best option is to use the PUE table, in which the dependence of the load moment and losses is established. In this case, the load moment is the product of the power consumption in kilowatts and the length of the cable itself in meters.
Let's look at an example in which an installed cable 30 mm long in a 220-volt AC mains can withstand a load of 3 kW. In this case, the load moment will be equal to 3 * 30 \u003d 90. We look at the PUE table, which shows that this moment corresponds to a loss of 3%. That is, it is less than the face value of 5%. What is allowed. As mentioned above, if the calculated losses would exceed the five percent barrier, then a larger cable would have to be purchased and installed.
Attention! These losses greatly affect lighting with low-voltage lamps. Because at 220 volts 1-2 V is not strongly reflected, but at 12 V it is immediately visible.
Currently, aluminum wires are rarely used in wiring. But you need to know that their resistance is 1.7 times greater than that of copper ones. And, therefore, their losses are as many times greater.
As for three-phase networks, here the load moment is six times greater. It depends on the fact that the load itself is distributed over three phases, and this is, accordingly, a throne increase in torque. Plus a double increase due to the symmetrical distribution of power consumption by phases. In this case, the current in the zero circuit should be equal to zero. If the phase distribution is asymmetric, and this leads to an increase in losses, then you will have to calculate the cable cross-section for the loads in each wire separately and select it according to the maximum calculated size.
Conclusion on the topic
As you can see, in order to calculate the cable cross-section by load, you have to take into account various coefficients (reducing and increasing). On your own, if you are an electrician at the level of an amateur or a novice master, this is not easy to do. Therefore, advice - invite a highly qualified specialist, let him do all the calculations himself and draw up a competent wiring diagram. But the installation can be done with your own hands.
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