---
title: "Power vs lighting system and cable sizing factors"
subject: "Industrial Power Distribution and Illumination"
chapter: 5
key: "semester_6/industrial_power_distribution_and_illumination:power-vs-lighting-cable-factors"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/power-vs-lighting-cable-factors
site: BCT Engineering
---

# Power vs lighting system and cable sizing factors

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

- Asked in: 2074 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/power-vs-lighting-cable-factors
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5

## Question

What is the main difference between power and lighting system? Describe different factors to be considered while determining the sizes of cables.

## Short answer

Power system uses 3-phase 400 V, high inductive loads, larger cables, D-curve breakers; lighting uses 1-phase 230 V, steady loads, smaller cables, B/C-curve MCBs. Cable sizing factors: load current, current-carrying capacity, derating, voltage drop, short-circuit rating, protective device coordination, conductor material, and future expansion.

## Answer

#### Power system vs lighting system
| Point | Power system | Lighting system |
|---|---|---|
| Loads | Motors, heaters, ACs, machines, power sockets | Lamps, fans, small 5 A sockets |
| Supply | Mostly 3-phase 400 V | Mostly single-phase 230 V |
| Current per circuit | High (16 A and above) | Low (6–10 A per circuit) |
| Cable size | 2.5 mm² and above | 1.5 mm² Cu typical |
| Protection | MCCB/MCB (C or D curve), overload relays, starters | MCB (B/C curve), RCCB |
| Load behaviour | Inductive, starting surges | Mostly steady, fairly constant |
| Wiring | Separate power DB and circuits | Separate lighting DB and circuits |

The two are kept on separate circuits (often separate DBs) so that a fault on a machine does not black out the lighting, and so that energy can be metered separately.

#### Factors in determining cable size
1. Load (design) current and type of load.
2. Current-carrying capacity of the cable from tables.
3. Derating for ambient temperature, grouping and laying method.
4. Permissible voltage drop and length of run.
5. Short-circuit current and fault duration.
6. Rating of the protective device (Ib ≤ In ≤ Iz).
7. Conductor material and insulation (Cu/Al, PVC/XLPE).
8. Future expansion and economy (energy loss cost).

## Check yourself

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

1. What is the main difference between power and lighting systems?
   A. Power supplies lamps and fans; lighting supplies motors
   B. Both are identical
   C. Power feeds motors and machines, mostly 3-phase with high current; lighting feeds lamps, mostly single-phase with low current
   D. Lighting uses 3-phase 400 V and 2.5 mm² and above
   Answer: C. Power circuits carry heavy, inductive loads; lighting circuits are light and steady. (Key point: Main difference)

2. Why are power and lighting kept on separate circuits?
   A. A machine fault will not black out lighting, and energy can be metered separately
   B. To use thicker cables
   C. To raise voltage
   D. It is required by MCB colour code
   Answer: A. Separation gives continuity of lighting and separate energy metering. (Key point: Why separate)

3. What does Ib ≤ In ≤ Iz mean in cable sizing?
   A. Cable rating ≤ load
   B. Design current ≤ protective device rating ≤ cable current capacity
   C. Device rating ≤ design current
   D. Voltage drop ≤ short circuit
   Answer: B. The protective device must carry the load but trip before the cable overheats. (Key point: Coordination)

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