---
title: "Cable size MDB to SDB1 (70 kW) and SDB2 (90 kW)"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:cable-mdb-sdb-70kw-90kw"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/cable-mdb-sdb-70kw-90kw
site: BCT Engineering
---

# Cable size MDB to SDB1 (70 kW) and SDB2 (90 kW)

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

- Asked in: 2075 Bhadra, 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-70kw-90kw
- 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 SDB1 and SDB2 for following conditions. The total power on the SDB1 and SDB2 are 70 kW and 90 kW respectively. The distance between MDB to SDB1 and SDB2 are 30 m and 120 m respectively. The supply system is 3 phase 4 wire system with 400 volts. Current rating of Copper conductor for laid in ground is given:

| 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 | 36 | 45 | 60 | 77 | 99 | 120 | 145 | 175 | 210 | 240 | 270 |

## Short answer

For SDB1 (70 kW, 30 m), select 3½ core 50 mm² Cu (drop 1.87 V). For SDB2 (90 kW, 120 m), select 3½ core 70 mm² Cu (drop 6.87 V). Both within 2.5% limit.

## Answer

Each feeder is sized by current rating from the 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 × 400 × 0.8)
ΔV = √3 × I × ρ × L × cosφ / A
```

#### MDB to SDB1 (70 kW, 30 m)
```text
I = 70000 / (√3 × 400 × 0.8) = 126.30 A
By current: 50 mm² (145 A)
ΔV = √3 × 126.30 × 0.0178 × 30 × 0.8 / 50
   = 1.87 V (0.47 %)  OK
```

#### MDB to SDB2 (90 kW, 120 m)
```text
I = 90000 / (√3 × 400 × 0.8) = 162.38 A
By current: 70 mm² (175 A)
ΔV = √3 × 162.38 × 0.0178 × 120 × 0.8 / 70
   = 6.87 V (1.72 %)  OK
```

| Feeder | I (A) | Cable | Rating (A) | ΔV |
|---|---|---|---|---|
| MDB–SDB1 | 126.30 | 50 mm² | 145 | 1.87 V (0.47 %) |
| MDB–SDB2 | 162.38 | 70 mm² | 175 | 6.87 V (1.72 %) |

**Answer: MDB–SDB1: 3½ core 50 mm² Cu; MDB–SDB2: 3½ core 70 mm² Cu.**

## Check yourself

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

1. For SDB1 (70 kW, 0.8 pf, 400 V) what current and cable result?
   A. 126.30 A, 50 mm² (145 A)
   B. 126.30 A, 35 mm² (120 A)
   C. 162.38 A, 70 mm²
   D. 101 A, 25 mm²
   Answer: A. I = 70000/(√3 x 400 x 0.8) = 126.30 A, so 50 mm² (145 A). (Key point: SDB1 size)

2. For SDB2 (90 kW, 120 m) what current and cable are selected?
   A. 126.30 A, 50 mm²
   B. 162.38 A, 50 mm² (145 A)
   C. 162.38 A, 70 mm² (175 A)
   D. 162.38 A, 95 mm²
   Answer: C. I = 162.38 A needs at least 175 A rating, so 70 mm². (Key point: SDB2 size)

3. What does the voltage drop check on SDB2 show?
   A. 6.87 V (1.72 %), within the limit
   B. 20 V, exceeds the limit
   C. 0.47 V
   D. 12 V, exceeds the limit
   Answer: A. ΔV = √3 x 162.38 x 0.0178 x 120 x 0.8 / 70 = 6.87 V, below 10 V. (Key point: Voltage drop check)

## 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 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×
- [Power/light sub-circuits and cable for 50 kW, 60 kW](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asub-circuits-cable-50kw-60kw) — Industrial Power Distribution and Illumination, chapter 5, asked 4×