- How to choose the right cross section
- Selection of wire cross section for rated current
- Additional aspects of the choice of wire cross-section
- Conclusion
A simplified table for selecting the conductor cross section for rated power
A table of power versus wire cross-section was developed specifically for beginners in matters of electrical engineering. In general, the choice of wire cross-section depends not only on the power of the connected loads, but also on the mass of other parameters.
In one of the main books of any electrician - PUE, the whole point is devoted to the correct choice of wire cross-section. And it is on the basis of it that our instruction was written, which should help you in the difficult task of choosing a wire section.
How to choose the right cross section
Why you can not use power tables
First of all, you should know that any table of the dependence of the wire cross section on power cannot contradict the PUE. Indeed, it is on the basis of this document that not only professionals, but also design bureaus, make their choice.
Therefore, all those tables and videos that you can find on the Internet in large numbers, offering to make a choice precisely by power, are a kind of averaged option.
So:
- Almost any table of wire cross-sections by power offers you to choose a wire based on the active power of the device or devices. But, those who studied well at school should remember that active power is only an integral part of the total power, which also contains reactive power.
What is cosα
- These components differ by cosα. For most electrical appliances, this indicator is very close to unity, but for devices such as transformers, stabilizers, a variety of microprocessor technology and the like, it can reach 0.7 or less.
- But any power section table is not accurate, not only because it does not take full power into account. There are other important factors. So, according to the PUE, the choice of conductors with voltage up to 1000V should be carried out only by heating. According to clause 1.4.2 of the EIC, the selection of short-circuit currents for such wires is not mandatory.
- In order to select a wire cross-section for heating, the following parameters should be taken into account: rated current flowing through the wire, type of wire - one-, two- or four-wire, method of laying the wire, ambient temperature, the number of laid wires in the bundle, insulation material of the wire and, of course, wire material. Not one table of wire loading capacity is not able to combine so many parameters.
Selection of wire cross section for rated current
Of course, it is difficult to combine all these parameters in one table, but you need to choose somehow. Therefore, so that you can make a choice with your own hands and head, we offer you the basic aspects of choice in an abridged version.
We discarded all section selection parameters for high-voltage cables, underused wires and left only the most important.
So:
- Since the PUE uses a table for selecting the section of the wire by current, we need to find out what current will flow in the wire at certain power values. This can be done according to the formula I = P / U × cosα, where I is our rated current, P is the active power, cosα is the apparent power factor and U is the nominal voltage of our power supply network (for a single-phase network it is 220V, for a three-phase network it is 380V).
The photo shows a table for selecting the cross section of the wire from PUE for aluminum conductors
- A logical question arises, where to get cosα readings? Usually it is indicated on all electrical appliances or it can be displayed if the total and active power are indicated. If the calculation is carried out for several electrical appliances, then the average is usually accepted or the rated current for each of them is calculated.
Note! If you can’t find out cosα for some devices, then for them it can be taken equal to unity. This, of course, will affect the final result, but the additional margin of safety for our wiring will not hurt.
- Knowing the loads for each of the planned groups of our power grid, the table of the dependence of the wire cross-section on current given in the PUE can be used by us. Only for proper use should dwell on some more points.
- First of all, you should decide on the wire that we plan to use. Rather, we should determine the number of cores. In addition, you should determine the method of laying the wire. After all, with the open method of laying the wire, the intensity of heat removal from it is much higher than when laying in pipes or corrugation. This is taken into account in the PUE tables.
Wire cross section selection table for copper conductors
Note! When choosing the number of wire cores, zero and protective cores are not taken into account.
- In addition, the table of the current cross section of the wire will help you determine the choice of material for the wiring. After all, based on the results, you can evaluate what material you better accept.
Note! When choosing a wire cross section, always select the nearest larger cross section value. In addition, if you intend to mount the new wiring to the old one, then consider that, according to clause 3.239 of SNiP 3.05.06 - 85, the old terminal blocks will not allow the use of a wire with a cross section of more than 4 mm2.
Additional aspects of the choice of wire cross-section
But when the table of the dependence of current on the cross section of the wire is considered, we must not forget about the conditions in which the wire is laid. Therefore, if you have a place to be conditions that are not favorable for the conditions of heating the wire, then it is worth paying attention to additional aspects.
Table of correction temperature coefficients
- First of all, it is the ambient temperature. If it differs from the average + 15⁰С, based on which the calculation is made in the tables of the PUE, then you should make correction factors. You will find a summary table of these coefficients below.
- Also, the table of load and wire cross-section according to Clause 1.3.10 of the PUE requires the introduction of correction factors for the joint laying of loaded wires in pipes, trays or simply bundles. So, for 5-6 wires laid together, this ratio is 0.68. For 7-9 it will be 0.63, and for a larger number it is 0.6.
Conclusion
We hope that our copper and aluminum wire load table will help you make your choice. And the technique proposed by us will allow even a non-professional to make the right choice.
After all, the price of an error can be very high. What is only the statistics of fires that occurred due to a short circuit. And the reason in most cases is not meeting the standards for heating wiring.