Electric Cable Selection of LT Side / Low-Voltage Line

Suppose your boss gave you a task of electrical cable selection (measuring cable size) of the LT side or low voltage side of the substation. What will you do? We can do the cable selection for our home but it is difficult for the substation. Today we will discuss cable selection of LT side.

We have to keep two things in mind:

  1. Electrical cable selection is based on the circuit breaker.
  2. The rate of electrical cable selection will be slightly higher than the circuit breaker rating.

We can remember a sequence for circuit breaker, cable, and busbar selection.

Line Current>Circuit Breaker>Cable>Busbar

It means circuit breaker rating will be greater than line current, cable rating will be greater than circuit breaker rating, and busbar rating will be greater than busbar rating.

We know that 400 volts 3 phase and 220-volt single line is called low voltage line. We will only discuss 400 volts 3 phase cable selection. Here the calculation is based on industry load. Because most of the industry uses 400 volt 3 phase.

Let us see the low voltage line diagram from a single line diagram. 33/11 kV line is called a high voltage line and 11/0.4 kV line is a low voltage line.

measuring cable size

Low Voltage Line Cable Selection/ Cable Selection of LT Side (Measuring Electrical Cable Size)

First, we have to know the line current and then the circuit breaker rating.

Line Current Calculation

measuring cable size

We will do 2000 KVA (step-down) 11/0.4 kV low voltage line cable line selection.

Notice the picture. power is shown in 4 parts in the picture: 600 KW, 500 KW, 300 KW, and 200 KW. We can use power from these loads. We can’t use more load than these powers.

Now we will do circuit breaker selection from the current rating and cable selection from the circuit breaker.

measuring cable size.

Suppose we have to do a 500 KW load current calculation.

As our load is 500 KW, so we will first measure the load current.

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 load line current is, I = 902 A. Now we will do circuit breaker selection based on the current value.

Circuit Breaker Selection

A circuit breaker is always greater than the current rating and for circuit breaker selection for load, we have to multiply 1.25 safety factor by the load current. This means we have to multiply 1.25 by the current value for circuit breaker selection.

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

                 I= 1127 A

We have to do circuit breaker selection higher than the current rating. As 1250 is greater than 1127 ampere, circuit breaker of 1250 ampere rating is MCCB- 1250 A.

cable selection of LT side

Cable Selection

Cable selection is done from a slightly higher than circuit breaker rating. We have got 1250 ampere in circuit breaker rating. We have to look at the chart to take higher ratings.

By looking at the cables rm, cable selection is done. we can know the size by looking at the rm.

Here is a cable selection chart. It has two types of cable ratings.

  1. Copper Cable Rating
  2. Aluminum Cable Rating

Copper cable is mostly used. Here is the copper and aluminum cable rating.

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

The highest 500 rm is used on the low voltage side. In the table, 500 rm is 900 ampere. It means the highest 900-ampere current can flow on the low voltage side.

Our load circuit breaker rating is 1250 ampere and 900 ampere is the highest current flow. So now we will do the cable selection? We will use multiple cable rather than one cable.

We can easily do cable selection by using one or more cable. If we connect one or more cable in one phase point, same amount of current will flow in each cable.

Notice the cable rating below and you will understand how multiple cable is used.

Circuit breaker rating: 1250

So we have to do cable selection greater than 1250 rating.

From the table:

Suppose, a 1 * 185 rm cable is selected.

1 * 185 rm= 460 A rating, calculate how many cables do we need???

460 * 3 = 1380 A

We are getting 1380 for 3 cables, which is greater than the circuit breaker rating 1250 A.

measuring cable size

So we selected 3 ( 1 * 185 ) rm size cables for the 1250 ampere rating circuit breaker.

Now you can ask how many cables do we need for 3 phase and 1 neutral? The answer is for each phase and neutral we need ( 1 * 185 ) rm size cable. So we need 4*3=12 cables.

That was all for today’s article cable selection of LT side.

Hope now you understand the cable selection of LT side.

If you have any questions on cable selection of LT side, you can ask in the comment section.

You can also read: Why is DC Supply Used in Substations?