Cable Selection of LT Side/Low-Voltage Line Easy Methods-2020

Suppose the boss of the office gave you a task that you have to make an electrical cable selection on the LT side or the low voltage side of the substation, what would you do? Many of us live in homes

Cable selection can be done very easily but not in the case of substations or clear concepts of many. In this article, we will discuss in detail how to select a low voltage side cable.

We need to remember two things

  1. Electrical cable selection is based on the circuit breaker.
  2. The circuit breaker rating is a bit higher than the rating or electrical cable selection at the above rating.

We can remember a rule when it comes to the circuit breaker, cable, bus selection.

Line Current> Circuit Breaker> Cable> Bus Bar

That is, the circuit breaker rating will be larger than the line current, the rating of the cable will be larger than the circuit breaker rating, the busbar rating will be larger than the rating of the cable.

We know that 400 Volts 3 Phase and 220 Voltage is the single line of the line  (LT)  Called the low-voltage line. We’re just here 400 Volts  3 I will discuss how cable selection is done in phase line. Here are calculations based on industry loads. Because 400 Volts 3 The phase is mostly used in the industry.

No home here or 220 Cable selection of the volt single line was not discussed. If you would like to read about cable selection of home single phase, please visit the link below.

So we have not seen the diagram of a low-voltage line from a single line diagram. 33/11 The kV line is called the high voltage line and  11/04 T Cove Line 2 Called T High Voltage Line End

cable selection of low voltage line

Cable Selection of Low Voltage Line (Determination of Size of Electrical Cable)

First, we need to find out the line current and then the circuit breaker rating. Only then can we make cable selection.

Line Current Calculation

cable selection of low voltage line

We are  2000 KVA Transformer (step-down)  11 / 0.4 KV I will make cable selection of low-voltage lines.

First, let’s look at the image above. The above figure shows the power in four parts, respectively   600 KW, 500 KW, 300 KW, 200 KW.

It is from these powers that we can use the load. We cannot use more loads of this power.

This time we will select the circuit breaker from the current rating and cable selection from the circuit breaker.

cable selection of low voltage line

Assume from the image above 500 KW  Current calculation of the load must be done.

Because of our load 500 KW, So first we will find out the current of this load.

Load (P) = 500 KW

Voltage (V) = 400 V

P = √3 * V * I * cos θ

I = P / √3 * V * cos θ

I = 902 A

500 KW I got load current I = 902 A. Our first job is done. The second task is to select a circuit breaker based on the current value.

Circuit Breaker Selection

The circuit breaker is always larger than the current rating, and in case of load, the load current for the circuit breaker selection 1.25 Safety factor is to be calculated. That means the current value will be 1.25  The circuit breaker has to be selected.

Line current I = 902 A * 1.25 (s.f-safety factor)

I= 1127 A

The circuit breaker must be selected above or below this current rating. Then 1127 Rating higher than or above the ampere 1250  Ampere. Then 1250 Being the circuit breaker of the ampere rating MCCB – 1250 A.

cable selection of low voltage line

If you have trouble understanding the circuit breaker selection, please visit the following text. The article describes the circuit breaker in detail.

Cable selection

Cable selection is done above the circuit breaker rating or above the rating. We got the rating of the circuit breaker 11250 Ampere. If you want to get the above rating then you should look at the cable chart.

Of cable  RM The size of the cable is selected. On the cable RM, You will find out what the size of this cable is.

Below is a chart for cable selection (there are two types of cable ratings).

  1. Copper cable rating and
  2. Aluminum cable rating.

Copper cables are mostly used. Below are the copper and aluminum cable ratings.

SerialPhysical Data ( Cross-Sectional Area )
Core * mm^2
Electrical Data
(Current Rating at 35 degrees in Air)
Electrical Data
(Current Rating at 35 degrees in Air)
11 * 1.522
21 * 2.530
31 * 2.530
41 * 44931
51 * 65041
61 * 106953
71 * 169473
81 * 2512597
91 * 3516097
101 * 50195151
111 * 70245190
121 * 95300232
131 * 120350272
141 * 150405314
151 *185460357
161 * 240555430
171 * 300640448
181 * 400770540
191 * 500900630
201 * 6301030721
211 * 8001160812
221 * 10001310917


Maximum on low-voltage side 500 rm Cable is used. In the table above  500rm Being 900 Ampere. That is, a cable is the highest on the low voltage side 900 The current of the ampere will be able to flow.

Our load circuit breaker is rated 1250 Since the ampere and our cable 900 Ampere is the highest flowing current. So how do we make a cable selection? We will use a single cable without using a single cable.

We can easily select the cable using one or one cable. If one or more cable connections are given at one phase point, the same amount of current will flow in one cable.

If you look at the cable rating below, you will understand how to use a single cable.

Circuit Breaker Rating 1250 

That is 1250 It has to be a little higher rating of cable selection (to determine the size of the cable).

From the table above:

Well, 1 * 185 rm The cable is selected.

1 * 185 rm= 460 ratings, then calculate how many cables may cost ???

460 * 3 = 1380 A

That is 3 Take the cable  1380  What I am getting is the circuit breaker rating 1250A More than that

cable selection of low voltage line

Than 1250 For Ampere Rating Circuit Breaker 3 Ta (1 * 185) rm Size cable selection.

The question may come here at 3 o’clock 1 How many cables will be needed for neutrals !!! The answer is for each phase and neutrals 3 Ta  ( 1 * 185 ) rm Size cable is required. Then you need a total cable 4*3=12, Ta.



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