Cable Size and Circuit Breaker Selection for High Tension and Low Tension

There are some calculations for selecting circuit breakers for LT and HT sides. Today we will discuss cable size and circuit breaker selection and also the circuit breaker rating chart.

There is no need for any calculation for the HT section circuit breaker and it is very easy to calculate. But LT side circuit breaker needs calculation.

  • VCB is used for HT Side
  • ACB, MCB, MCCB etc is used for LT side

Breakers we can find in the market

Circuit Breaker Rating Chart

VCB

SerialRated Current (A)Breaking Capacity (KA)
163025
2125040

ACB

SerialRated Current (A)Breaking Capacity (KA)
163085
280085
3100085
4125085
5160085
6200085
72500100
83200100
94000100
105000100
116300150

MCCB: Adjustable Type

SerialRated Current (A)Breaking Capacity (KA)
140, 43, 100 & 12525
2160, 200 & 25025
325025
4320 & 40065
550065
663085
780085
81000100
91250100
10160065

Circuit breaker calculation in high tension (Cable size and circuit breaker selection )

1250A rating VCB is selected for 33 kV to 11 kV transformer and 630A rating VCB is selected for 11KV to 0.4 kV. The last 0.4 kV is connected with the low tension side, and VCB connected with low tension means we have to choose 630A rating VCB.

  • 1250 A VCB in the HT section
  • 630 A VCB is selected for the above LT section connected with the HT section.

Look at the picture:

সার্কিট ব্রেকার হিসাব

In the above picture, a step-down transformer with a 5 MVA rating is used. 33 kV line has entered in its primary and got out as 11 kV from secondary. This is a high tension section, where 1250 A rating VCB is used.

circuit breaker calculation

In the above picture, a 2500 KVA rating transformer is used. 11 kV has entered into its primary and 0.4 kV got out from its secondary. It means, its primary section is HT, and the second section is LT. HT section to LT section is connected with transformer. In this case, the VCB of the HT section will be 630A.

circuit breaker calculation

For the selection of the circuit breaker, the value of the current should be almost the same as the line current.

Breaker 1 selection

We have to calculate using the above transformer value of breaker 1.

P = 2500 KVA

V2 (Secondary) = 0.4 kV

I =?

From the picture notice that there is no resistive, inductive, or capacitive load above 2500 kV. In this case, do you remember the condition?

Apparent Power, P = √3 V I (KVA)

Active Power, P = √3 V I Cosθ (KW)

Re-Active Power, P = √3 V I Sinθ (KVAR)

Which formula will be used?

Definitely, P = √3 V I because there is no active and reactive power above 2500 KVA. In this case, we will get whatever is sent from the grid.

So, I = P / √3 V

I = (2500 * 10^3) / √3 * 0.4 * 10^3

I = 3608.43 A

So we have to select a circuit breaker around 3608 A or slightly more. There is no 3608 A circuit breaker on the market. Now, look at the above table.

So we can use a 4000 A rating ACB circuit breaker for 3608 A.

Breaker 2 and 3 selection

Notice the above picture, 3608 amperes from transformers output is divided in 2,3,4,5,6 through busbar. So, we have to calculate the current for each breaker separately. To calculate the current, we have to know the power. We have already found that LT voltage is 0.4 kV.

The capacitor bank is connected to breakers 2 and 3. This part gives reactive power. To calculate the power,

Re-Active Power, P = √3 V I Sinθ (KVAR) should be applied.

We know that, for power factor,

Cosθ = 0.8, Sinθ = 0.6

We have already found that, V = 0.4 kV, I = 3608 A

P = √3 V I Sinθ

P = √3 * 0.4 * 10^3 * 3608 * 0.6

P = 1499817 ≅ 15,00000

P = 1500 KVA

Breakers 2 and 3 have two capacitor banks, where 750+750=1500 KVAR can be used.

Power for breaker 2, P = 750 KVAR

I = P / √3 V

I = 750 * 10^3 / √3 * 0.4 * 10^3

I = 1082 A

We can’t find a circuit breaker with this value in the market. So we can use ACB / MCCB 1250 A from the chart. The price of the ACB 1250 A is much higher than MCCB 1250 A. So we can choose the MCCB 1250A Rating breaker.

So MCCB 1250A breaker was selected for breakers 2 and 3.

Total load power calculation of breaker 4,5,6

Here we have to calculate active power, where V = 0.4 KV, I = 3608A, Cos θ = 0.8

Active Power, P = √3 V I Cosθ (KW)

P = 1999756 ≅ 2000000 W

P = 2000 KW

So we will calculate the load power for breaker 4,5,6 = 1000 + 500 + 500. If you want you can calculate with 100+750+250.

Breaker 4 calculation

P = 1000 KW, V = 0.4 kV, I = ?

I = P / √3 V Cosθ

I = 1804 ≅ 1800 A

As breaker 4,5,6 is connected with load, based on the international standard 20% extra breaker has to be selected.

so, 1800 * 20% = 360

I = 1800 + 360

I = 2160 A

This ampere rating breaker is around 2500 A ACB. So for breaker 4, 2500 A rating breaker selected.

We have to select breakers 5,6 in the same way.

That was all for today’s article on cable size and circuit breaker selection.

Hope now you understand the circuit breaker rating chart.

If you have any queries on the circuit breaker rating chart, feel free to ask in the comment.

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