---
title: "Power/light sub-circuits and cable for 50 kW, 60 kW"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:sub-circuits-cable-50kw-60kw"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sub-circuits-cable-50kw-60kw
site: BCT Engineering
---

# Power/light sub-circuits and cable for 50 kW, 60 kW

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

- Asked in: 2069 Bhadra, 8 marks
- Video: [SUB CIRCUITS FOR ELECTRICAL WIRING](https://www.youtube.com/watch?v=kIxuww6MNf4) — Ashfaq Qureshi
- 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/sub-circuits-cable-50kw-60kw
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

What are the power and light sub circuits in an electrification of building? Determine the size of cable used from MDB to SDB1 and SDB2, if the total power on SDB1 and SDB2 are 50 kW and 60KW, respectively. The distance between MDB to SDB1 and SDB2 are 60m and 100m respectively. The supply system is 3phase 4wire system with 400V, 50Hz. Current ratings of 3.5 core copper conductor cable for laid in ground are given below.

| 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

Light sub-circuits: ≤10 points/800 W, 1.5 mm² Cu, 6-10 A MCB; power sub-circuits: ≤2 sockets/3 kW, 2.5-4 mm² Cu, 16-20 A MCB. For SDB1 (50 kW, 90.2 A, 60 m): 25 mm² Cu (99 A, 1.33% drop). For SDB2 (60 kW, 108.3 A, 100 m): 35 mm² Cu (120 A, 1.91% drop).

## Answer

#### Light and power sub-circuits

A **sub-circuit** is a branch circuit taken from a distribution board (DB) through its own fuse/MCB to feed a group of points. In building wiring the points are split into two separate kinds of sub-circuit:

- **Light sub-circuit:** feeds lamps, fans and 6 A (5 A) socket points. As per IS 732 / NEA practice it carries at most **10 points or 800 W**, is wired in 1.5 mm² Cu and is protected by a 6 A (or 10 A) MCB.
- **Power sub-circuit:** feeds 16 A (15 A) power sockets for heaters, irons, AC, machines, etc. It carries at most **2 power sockets (about 3000 W)**, is wired in 2.5–4 mm² Cu and protected by a 16–20 A MCB.
- The two are kept on separate circuits (often separate DBs) so that a fault or overload on a power point does not plunge the area into darkness, and so that each can be sized for its own current.

#### Cable size from MDB to SDB1 and SDB2

Assumptions: power factor 0.8 lagging; Cu resistivity 0.0178 Ω·mm²/m; permissible voltage drop 2.5 % of 400 V (= 10 V) on a sub-main.

Current: I = P / (√3 × V_L × cos φ)

```text
SDB1: I1 = 50000 / (√3 × 400 × 0.8) = 90.21 A
SDB2: I2 = 60000 / (√3 × 400 × 0.8) = 108.25 A
```

From the table (3.5-core Cu, laid in ground):

- SDB1: 90.21 A → **25 mm²** (rating 99 A)
- SDB2: 108.25 A → **35 mm²** (rating 120 A)

**Voltage drop check:** ΔV = √3 × I × R × cos φ, with R = ρL/A (reactance neglected).

```text
SDB1: R  = 0.0178 × 60 / 25  = 0.0427 Ω
      ΔV = √3 × 90.21 × 0.0427 × 0.8 = 5.34 V
         = 5.34/400 = 1.33 %   < 2.5 %  OK
SDB2: R  = 0.0178 × 100 / 35 = 0.0509 Ω
      ΔV = √3 × 108.25 × 0.0509 × 0.8 = 7.63 V
         = 7.63/400 = 1.91 %   < 2.5 %  OK
```

**Answer: MDB → SDB1: 3.5-core 25 mm² Cu cable; MDB → SDB2: 3.5-core 35 mm² Cu cable.**

Note: if a 25 % margin for future load is added (1.25 × I = 112.76 A and 135.32 A), the sizes become 35 mm² and 50 mm² respectively.

## Check yourself

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

1. What is a light sub-circuit as described?
   A. It feeds 16 A power sockets with a 20 A MCB
   B. It feeds lamps, fans and 6 A sockets, with at most 10 points or 800 W on 1.5 mm2 Cu
   C. It feeds only motors above 5 kW
   D. It is the same as a power sub-circuit
   Answer: B. A light sub-circuit carries lamps, fans and 6 A sockets, limited to about 10 points/800 W, wired in 1.5 mm2 and protected by a 6-10 A MCB. (Key point: Light sub-circuit)

2. Why are light and power sub-circuits kept separate?
   A. A power-point fault should not darken the area, and each can be sized for its own current
   B. Because the regulations forbid copper in light circuits
   C. To make the cable cheaper by using the same breaker
   D. Because power circuits use DC
   Answer: A. Separation means an overload/fault on a power point does not cut off lighting, and each circuit is sized for its own load. (Key point: Why separate)

3. Which formula gives the line current for SDB1 (50 kW, 400 V, pf 0.8)?
   A. I = P / (V x cos phi) = 156 A
   B. I = P / (sqrt3 x V x cos phi) = 90.21 A
   C. I = P x sqrt3 / (V x cos phi)
   D. I = P / (3 x V) = 41.7 A
   Answer: B. For a 3-phase balanced load I = P/(sqrt3 VL cos phi) = 50000/(1.732 x 400 x 0.8) = 90.21 A. (Key point: Current calculation)

4. Using the table, which cables are selected for 90.21 A (SDB1) and 108.25 A (SDB2)?
   A. 16 mm2 and 25 mm2
   B. 25 mm2 (99 A) and 35 mm2 (120 A)
   C. 35 mm2 and 50 mm2
   D. 50 mm2 and 70 mm2
   Answer: B. Choose the smallest cable whose rating exceeds the current: 99 A > 90.21 A gives 25 mm2; 120 A > 108.25 A gives 35 mm2. A 25 % margin would raise these to 35 and 50 mm2. (Key point: Cable selection)

## 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×

## 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×