---
title: "Cable size MDB to SDB-1 (60 kW) and SDB-2 (75 kW)"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:cable-mdb-sdb-60kw-75kw"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/cable-mdb-sdb-60kw-75kw
site: BCT Engineering
---

# Cable size MDB to SDB-1 (60 kW) and SDB-2 (75 kW)

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

- Asked in: 2075 Baisakh, 4 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/cable-mdb-sdb-60kw-75kw
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

Determine the size of cable used from MDB to SDB-1 and SDB-2 for the following conditions. The total power on the SDB-1 and SDB-2 are 60 kW and 75 kW respectively. The distance between MDB to SDB-1 and SDB-2 are 80m and 125m respectively. The supply system is 3-phase, 4 wire with 400V, 50 Hz. The current ratings of Cu conductor laid in ground are given as follow.

| Conductor area (sq.mm) | 1.5 | 2.5 | 4 | 6 | 10 | 16 | 25 | 35 | 50 | 70 | 95 | 120 | 150 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3.5 core cable current rating (A) | 21 | 27 | 31 | 45 | 60 | 77 | 99 | 120 | 145 | 175 | 210 | 240 | 270 |

## Short answer

SDB-1 (60 kW, 80 m): 3½ core 35 mm² Cu (drop 6.10 V). SDB-2 (75 kW, 125 m): 3½ core 50 mm² Cu (drop8.34 V). Both within2.5% limit.

## Answer

Each feeder is sized by current rating from the given table and checked for voltage drop.

Assumptions: pf = 0.8 (not given), allowed drop 2.5 % of 400 V = 10 V, ρ (Cu) = 0.0178 Ω·mm²/m.

```text
I  = P / (√3 × V_L × cosφ)
ΔV = √3 × I × ρ × L × cosφ / A
```

#### MDB to SDB-1 (60 kW, 80 m)
```text
I = 60000 / (√3 × 400 × 0.8) = 108.25 A
By current: 35 mm² (120 A)
ΔV = √3 × 108.25 × 0.0178 × 80 × 0.8 / 35
   = 6.10 V (1.53 %)  OK
```

#### MDB to SDB-2 (75 kW, 125 m)
```text
I = 75000 / (√3 × 400 × 0.8) = 135.32 A
By current: 50 mm² (145 A)
ΔV = √3 × 135.32 × 0.0178 × 125 × 0.8 / 50
   = 8.34 V (2.09 %)  OK (< 2.5 %)
```

| Feeder | I (A) | Cable | Rating (A) | ΔV |
|---|---|---|---|---|
| MDB–SDB-1 | 108.25 | 35 mm² | 120 | 6.10 V (1.53 %) |
| MDB–SDB-2 | 135.32 | 50 mm² | 145 | 8.34 V (2.09 %) |

**Answer: MDB–SDB-1: 3½ core 35 mm² Cu; MDB–SDB-2: 3½ core 50 mm² Cu.** If a stricter 2 % limit is used, SDB-2 needs 70 mm² (drop 5.96 V = 1.49 %).

## Check yourself

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

1. What cable carries SDB-1 (60 kW, pf 0.8, 400 V)?
   A. 35 mm² (120 A) for 108.25 A
   B. 25 mm² (99 A)
   C. 50 mm² (145 A)
   D. 16 mm² (77 A)
   Answer: A. I = 60000/(√3 x 400 x 0.8) = 108.25 A, so 35 mm² (120 A). (Key point: SDB-1 current)

2. SDB-2 (135.32 A, 125 m, 50 mm²) has a drop of 8.34 V. What is the verdict at the 2.5 % limit?
   A. Not OK, 10 V exceeded
   B. OK, since 2.09 % is below 2.5 %
   C. OK only if pf is 1
   D. Not OK, as 50 mm² is too small by current
   Answer: B. The limit is 10 V (2.5 % of 400 V) and 8.34 V is within it. (Key point: SDB-2 drop)

3. If a stricter 2 % limit (8 V) were used, what happens to SDB-2?
   A. 50 mm² still passes
   B. It needs 70 mm² (drop 5.96 V)
   C. It needs 35 mm²
   D. It needs 150 mm²
   Answer: B. 8.34 V exceeds 8 V, so the next size, 70 mm², is used. (Key point: Stricter limit)

## Close variants set in other exams

- [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×
- [Cable size MDB to SDB1 (70 kW) and SDB2 (90 kW)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-mdb-sdb-70kw-90kw) — Industrial Power Distribution and Illumination, chapter 5, asked 5×

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