---
title: "Breaking MVA and making kA (800 A, 33 kV)"
subject: "Switchgear and Protection"
chapter: 6
key: "semester_6/switchgear_and_protection:cb-breaking-making-800a"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-breaking-making-800a
site: BCT Engineering
---

# Breaking MVA and making kA (800 A, 33 kV)

Switchgear and Protection · Chapter 6: Circuit Breakers

- Asked in: 2078 Chaitra, 4 marks
- Video: [Making Capacity of Circuit Breaker| Breaking Capacity of Circuit Breaker| Fault Current](https://www.youtube.com/watch?v=daRNify1Hr4) — Electrical With Abhishek Joshi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-breaking-making-800a
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6

## Question

Calculate breaking capacity in MVA and making capacity in kA of a 3 phase circuit breaker with following parameters: Rated current = 800 A; Rated voltage = 33 kV; Ibsym = 25 kA.

## Short answer

Breaking capacity = √3 × 33 kV × 25 kA = 1429 MVA. Making capacity = 2.55 × 25 = 63.75 kA peak.

## Answer

**Breaking capacity** is the MVA corresponding to the symmetrical breaking current at rated voltage, and **making capacity** is the peak current the CB can close onto, taken as 2.55 times the symmetrical breaking current.

#### Given
Rated current = 800 A (normal current, not used for these values), *V* = 33 kV, *I_b(sym)* = 25 kA.

#### Breaking capacity
```text
Breaking MVA = √3 × V × I_b
             = √3 × 33 kV × 25 kA
             = 1428.9 MVA
```
#### Making capacity
```text
I_making = 1.8 × √2 × I_b = 2.55 × I_b
         = 2.55 × 25
         = 63.75 kA (peak)
```
**Answer: Breaking capacity ≈ 1429 MVA; making capacity = 63.75 kA (peak).**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In calculating breaking and making capacity here, the 800 A rated current is
   A. multiplied by 2.55 for the making current
   B. used as I_b
   C. not used, since it is the normal current only
   D. divided by sqrt(3)
   Answer: C. Breaking and making values depend on the symmetrical breaking current and voltage, not on the normal rated current. (Key point: Role of rated current)

2. Breaking capacity in MVA is calculated as
   A. V x I_b
   B. sqrt(3) x V(kV) x I_b(kA)
   C. 3 x V x I_b
   D. V x I_b / sqrt(3)
   Answer: B. For a 3-phase CB, MVA = sqrt(3) x line kV x I_b kA. (Key point: Breaking MVA formula)

3. With 33 kV and 25 kA, the breaking capacity is about
   A. 825 MVA
   B. 1429 MVA
   C. 2475 MVA
   D. 476 MVA
   Answer: B. 1.732 x 33 x 25 = 1428.9 MVA. (Key point: Breaking MVA value)

4. The making capacity for I_b = 25 kA is
   A. 25 kA peak
   B. 45 kA peak
   C. 63.75 kA peak
   D. 35.4 kA peak
   Answer: C. 2.55 x 25 = 63.75 kA peak. (Key point: Making current value)

## Related in Switchgear and Protection

- [Breaking, making, short-time ratings (500 MVA)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-500a-500mva) — Switchgear and Protection, chapter 6
- [CB ratings: 2500 A, 1500 MVA, 33 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-2500a-1500mva) — Switchgear and Protection, chapter 6
- [CB ratings: 33 kV, 1500 A, 3000 MVA](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-1500a-3000mva) — Switchgear and Protection, chapter 6
- [CB terms: making, breaking, operating duty](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-terms-making-breaking-duty) — Switchgear and Protection, chapter 6
- [Specifying breaker on 132/66 kV transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abreaker-x-specification) — Switchgear and Protection, chapter 6
- [Selecting CB for 6.6 kV, 650 MVA](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aselect-cb-6-6kv-650mva) — Switchgear and Protection, chapter 6

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- [Compensation capacity for 15 MW, 33 kV load](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acompensation-capacity-15mw-33kv) — Power System Analysis I, chapter 6
- [Current chopping overvoltage in 132 kV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Achopping-overvoltage-10a) — High Voltage Engineering, chapter 3
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