Information About Electricity
How To Determine The Cable Size For 3 Phases
28 December 2021

In our previous article, we discussed: 'How to select the correct cable size for 1 phase.' In this article, we will discuss on how to pick the correct cable size for 3 phases. 3-phase voltages are usually used for multiple story buildings such as hotels, apartments, malls, factories, etc. The standard voltages used in a 3-phase is 380 volts. How did we get the number 380 volts? To explain this further, let’s go over the following review:


Here's How You Can Determine The Cable Size For 3 Phases!


3-Phase Voltage


A 3-phase voltage is represented by three lines with one end meeting one another, causing every line forming an angle of 120º. That angle is what differentiates between one phase and another of 120º. For more details, you may view the following figure.


3 phase voltage


To find out where the number 3 comes from, let’s use a simple mathematical equation with the help of the following figures:


 how to calculate 3 phase voltage


You can see an explanation of the figures above in the following description:


Figure 1: Representation of the 3-phase and neutral voltage, where the phase-to-neutral voltage is 220 V (VRN=220 V, VSN=220V, VTN=220V).

Figure 2: A sample of two phases, where between the two different phases is an angle of 120º.

Figure 3: Drawing a line from R to S, which represents the inter-phase voltage or VRS.

Figure 4: The area of the RSN triangle is divided in half by drawing the line NA, creating two identical triangles, with the angle RNA = angle SNA, of 60°.

Figure 5: A triangular piece of RNA from Figure 4. From this figure is where the calculation will begin.


Remember this equation to calculate the feet of the triangle. Referring to Figure 5, we can obtain the following formula:


 formula for calculating voltage


From the calculation above, we may refer back to Figure 4, RS = RA + AS, and RA = AS making RS = RA + RA or RS = 2RA, which equates to:


formula for calculating voltage 1


VRS = 380 Volt


The following also applies to VST and VTR.


How To Calculate Power?


Before calculating power, make sure you know the capacity of the electric appliances beforehand.


For example, PT. Wilson Surya Unggul has a clean water pump of 7.5 kW, how much is the amperage?


formula for calculating power 1



I = Ampere

P = Power (Watt)

V = Voltage (220 V)


Cos Ø = 0.8

 = 1.73


formula for calculating power 3


From the following ampere, we are required to use cables with a greater current-carrying capability of about 125% as a safety factor.


 formula for calculating power 2


Deciding The Cable Size


After knowing the amperage with the safety factor, the size of the cable can then be determined according to the current-carrying table below:


NO       Cross Section mm²                 Amperage      

1          0.75 mm²                                 12 A                 

2          1 mm²                                      15 A                 

3          1.5 mm²                                   18 A                 

4          2.5 mm²                                   26 A                 

5          4 mm²                                      34 A                 

6          6 mm²                                      44 A                 

7          10 mm²                                                61 A                 

8          16 mm²                                                82 A                 

9          25 mm²                                                108 A              

10        35 mm²                                                135 A              

11        50 mm²                                                168 A              

12        70 mm²                                                207 A              

13        95 mm²                                                250 A              

14        120 mm²                                  292 A 


Then we can choose the cable that has the current-carrying capability of 17.82 A, which is the cable with the cross-section of 1.5 mm2.


Below is a table with examples of calculating 3 phase with the known kW:


NO         KW                        Voltage                  Ampere                 Safety Factor 125 %                          Cross Section Cable

1             4KW                      380V                      7.61A                     9.51A                                    0.75 mm²

2             5.5KW                   380V                      10.46A                   13.07A                                  1 mm²               

3             7.5KW                   380V                      14.26A                   17.83A                                  1.5 mm²

4             15KW                    380V                      28.52A                   35.65A                                  6 mm²  

5             18.5KW                 380V                      35.18A                   43.97A                                  6 mm²

6             022KW                  380V                      41.83A                   52.29A                                  10 mm²             

7             30KW                    380V                      57.04A                   71.30A                                  16 mm² 


With the steps above, you can now decide the size of the cables to use and also how to apply them in daily life. Don’t forget to also use the right cables based on your needs to match with its capabilities as well.


For cable products, choose Wilson Cables with its array of benefits and capabilities such as 3x faster to extinguish, made of 99% pure copper, easy application and labels printed with digital marking.


Other Interesting Information:

1. How To Determine The Right Cable Size For 1-Phase!
2. The Differences Between Phase, Neutral and Grounding Power Cables
3. Understanding the Difference Between Medium Voltage and Low Voltage
4. Why Are High-voltage Cables Installed a Little Loose?
5. Get to know 5 Kinds of Electric Voltage Measuring Instruments

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