---
title: "Selecting MCCB and MCB for MDB and SDB"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:mccb-mcb-selection-mdb-sdb"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/mccb-mcb-selection-mdb-sdb
site: BCT Engineering
---

# Selecting MCCB and MCB for MDB and SDB

Industrial Power Distribution and Illumination · Chapter 5: Electrification of Industrial Building

- Asked in: 2071 Magh, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/mccb-mcb-selection-mdb-sdb
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

With suitable example, explain the detail procedure for selection of MCCB and MCB for MDB and SDB in industrial electrification.

## Short answer

Procedure: calculate each circuit load current, select MCB ≥1.25×Ib with appropriate curve (B/C/D), size SDB incomer with simultaneity factor, MDB feeders one size higher for discrimination, MDB incomer matching transformer LT current, verify breaking capacity against fault level, ensure Ib ≤ In ≤ Iz, add RCCB for earth leakage.

## Answer

MCCBs (moulded case circuit breakers, about 16–1600 A, adjustable trips, high breaking capacity) are used as MDB incomers and feeders; MCBs (up to 63–125 A, fixed trips, 6–10 kA) are used in SDBs for final circuits. Both are selected by rated current, breaking capacity, number of poles and tripping characteristic.

#### Procedure
1. **Load current of each final circuit** (Ib): I = P/(√3·V·η·pf) for 3-phase, P/(V·pf) for 1-phase.
2. **MCB for each outgoing circuit**: In ≥ 1.25 × Ib, next standard rating (6, 10, 16, 20, 25, 32, 40, 50, 63 A). Curve: B for lighting, C for mixed/socket, D for motors and transformers.
3. **SDB incomer**: total SDB current × simultaneity factor Ks, × 1.25 → MCB (up to 63 A) or MCCB.
4. **MDB outgoing feeders**: MCCB ≥ SDB incomer rating (one size higher for discrimination).
5. **MDB incomer**: sum of feeders × Ks of MDB × 1.25 → MCCB/ACB; it should match the transformer LT full-load current (I = kVA·1000/(√3·400)).
6. **Breaking capacity**: Icu ≥ prospective fault current at that point (e.g. near a 250 kVA, 4.5 % impedance transformer, Isc ≈ 361/0.045 ≈ 8 kA → 10 kA MCB/MCCB at SDB, 25–36 kA MCCB at MDB).
7. **Poles**: SP for single-phase, TP for motors, TPN/4P for 3-phase 4-wire incomers.
8. **Coordination**: Ib ≤ In ≤ Iz (cable rating); upstream larger than downstream for selectivity.
9. **Earth leakage**: RCCB (30 mA) for socket circuits, 100–300 mA at SDB incomer.

#### Example (400 V, 3-phase)
- SDB-1: three motors, 7.5 kW each, η = 0.88, pf = 0.85.
- SDB-2: 15 kW lighting and sockets, pf 0.9, balanced.
```text
Motor Ib = 7500 / (√3 × 400 × 0.88 × 0.85) = 14.47 A
Motor MCB: 1.25 × 14.47 = 18.09 → 20 A TP, D curve

SDB-1: 3 × 14.47 × 0.9 = 39.08 A
       1.25 × 39.08 = 48.84 → 50 A TP MCB incomer
SDB-2: 15000 / (√3 × 400 × 0.9) = 24.06 A
       1.25 × 24.06 = 30.07 → 32 A TPN MCB incomer

MDB feeders: 63 A MCCB (to SDB-1), 40 A MCCB (to SDB-2)
MDB demand  = (39.08 + 24.06) × 0.9 = 56.82 A
MDB incomer = 1.25 × 56.82 = 71.02 → 80 A TPN MCCB, 25 kA
```

| Location | Device | Rating |
|---|---|---|
| SDB-1 outgoings | TP MCB, D curve | 3 × 20 A |
| SDB-1 incomer | TP MCB | 50 A |
| SDB-2 incomer | TPN MCB + RCCB | 32 A |
| MDB feeders | MCCB | 63 A, 40 A |
| MDB incomer | TPN MCCB | 80 A, 25 kA |

## Check yourself

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

1. Where are MCCBs and MCBs typically used in the distribution scheme described?
   A. MCBs as MDB incomers and MCCBs for final circuits in SDBs
   B. MCCBs as MDB incomers and feeders; MCBs for final circuits in SDBs
   C. Both only at the SDB, since MDBs use fuses
   D. MCCBs only for lighting and MCBs only for motors
   Answer: B. MCCBs (up to about 1600 A, adjustable trips) suit MDB incomers and feeders; MCBs (up to 63-125 A, fixed trips) suit final circuits in SDBs. (Key point: MCCB vs MCB use)

2. A motor circuit has full-load current Ib = 14.47 A. How is its MCB chosen?
   A. In = 1.25 x Ib = 18.09 A, so the next standard rating, 20 A
   B. In = Ib = 14.47 A, so 16 A
   C. In = 2 x Ib = 28.9 A, so 32 A
   D. In = 1.25 x Ib, rounded down to 16 A
   Answer: A. The MCB rating is at least 1.25 x Ib, then rounded UP to the next standard size, giving 20 A (TP, D curve for motors). (Key point: MCB rating rule)

3. Which tripping curve is recommended for motors and transformers, and why?
   A. B curve, because it trips at the lowest multiple of rated current
   B. C curve, because it is meant for lighting
   C. D curve, because it tolerates high inrush/starting currents
   D. Any curve, since the curve does not affect nuisance tripping
   Answer: C. Motors and transformers draw large inrush currents; the D curve trips only at a high multiple of In, avoiding nuisance trips. B suits lighting, C mixed loads. (Key point: Curve selection)

4. What condition must the breaking capacity Icu of a breaker satisfy?
   A. Icu must be less than the prospective fault current
   B. Icu must equal the load current
   C. Icu must be at least the prospective fault current at that point
   D. Icu is unrelated to fault level and depends only on poles
   Answer: C. The breaker must safely interrupt the maximum short-circuit current at its location, so Icu >= prospective fault current (e.g. 10 kA at SDB, 25-36 kA at MDB near the transformer). (Key point: Breaking capacity)

## Related in Industrial Power Distribution and Illumination

- [Cable size MDB to SDB-1 (60 kW) and SDB-2 (75 kW)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-mdb-sdb-60kw-75kw) — Industrial Power Distribution and Illumination, chapter 5, asked 4×
- [MCB, MCCB and HRC fuse comparison](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amcb-mccb-hrc-comparison) — Industrial Power Distribution and Illumination, chapter 3, asked 3×
- [MCB principle, MCB/MCCB/RCCB, HRC cut-off](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amcb-mccb-rccb-hrc-cutoff) — Industrial Power Distribution and Illumination, chapter 3
- [Cable and MCB size for two motors (η = pf)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amotor-cable-mcb-efficiency) — Industrial Power Distribution and Illumination, chapter 5, asked 2×
- [Cable and MCB size for two motors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amotor-cable-mcb-pf) — Industrial Power Distribution and Illumination, chapter 5, asked 2×
- [Cable size MDB to SDB1 (80 kW) and SDB2 (10 kW)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-mdb-sdb-80kw-10kw) — Industrial Power Distribution and Illumination, chapter 5, asked 5×