---
title: "Process to determine wire size"
subject: "Basic Electrical and Electronics Engineering"
chapter: 4
key: "semester_1/basic_electrical_and_electronics_engineering:wire-size-determination"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/wire-size-determination
site: BCT Engineering
---

# Process to determine wire size

Basic Electrical and Electronics Engineering · Chapter 4: Electrical Installations

- Asked in: 2081 Kartik (new course), 3 marks
- Video: [Cable size Circuit breaker amp size How to calculate What cable](https://www.youtube.com/watch?v=KBvyX80Pdec) — How2D2
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/wire-size-determination
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-4

## Question

Explain the process to determine the size of the wire.

## Short answer

Add all load powers, compute load current using I=P/(V cosφ) for single phase or I=P/(1.732 VL cosφ) for three phase, apply a 20–25% safety margin, then select a standard conductor size whose ampacity meets this current. Finally check voltage drop (≤3–5%) and coordinate with the protective device; e.g. 2 kW heater at 230 V needs 2.5 mm² copper with 16 A MCB.

## Answer

The size (cross-sectional area) of a conductor must allow it to carry the load current safely, without overheating and without an excessive voltage drop. The steps are:

**1. Find the total load.** Add the power ratings of all appliances on the circuit:

$$
P_{total}=\sum P
$$

**2. Calculate the load current.**

Single phase:
$$
I=\frac{P}{V\cos\phi}
$$

Three phase:
$$
I=\frac{P}{\sqrt3\,V_L\cos\phi}
$$

**3. Apply a safety margin and diversity factor.** Increase the current by 20-25% for future expansion (or use a diversity factor if not all loads operate at once).

**4. Select the wire size for current-carrying capacity.** From the manufacturer's or a standard table (such as IS/IEC), choose a conductor whose rated ampacity is at least the design current, considering the ambient temperature, the method of installation (conduit, open) and the number of cables grouped together. Typical copper sizes: 1.5 mm² (lighting, up to 10 A), 2.5 mm² (power sockets, 16 A), 4 mm² (geysers, ACs, 25 A), 6 mm² (32 A).

**5. Check the voltage drop.** The drop in the cable should not exceed about 3-5% of the supply voltage:

$$
\Delta V=I\cdot R=I\cdot\frac{\rho L}{A}
$$

Here $L$ is the total conductor length (go and return for a single-phase circuit). If the drop exceeds the limit, choose a larger size.

**6. Check the short-circuit withstand and coordinate with the protective device** (MCB rating at or above the load current and not above the cable rating).

Example: a 2 kW heater at 230 V, pf 1, draws $I=8.7$ A; with a 25% margin this is 10.9 A, so a 2.5 mm² copper wire with a 16 A MCB is selected.

## Check yourself

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

1. How is the load current of a single-phase circuit found?
   A. I = P x V x cos(phi)
   B. I = P/(V cos(phi))
   C. I = V/(P cos(phi))
   D. I = P/(sqrt(3) V cos(phi))
   Answer: B. For single phase I = P/(V cos phi); the sqrt(3) form is for three phase. (Key point: Load current)

2. Why is the calculated current increased by 20-25%?
   A. To reduce the wire size
   B. To allow for future expansion or non-simultaneous loads
   C. To lower the voltage drop to zero
   D. Because cos(phi) is always 1.25
   Answer: B. A margin (or diversity factor) covers future load growth before choosing the cable. (Key point: Safety margin)

3. What is checked after choosing a wire size for current capacity, and what if the limit is exceeded?
   A. Voltage drop (about 3-5%); choose a larger size
   B. Voltage drop; choose a smaller size
   C. Insulation colour; change the cable colour
   D. Frequency; change the supply
   Answer: A. Drop = I rho L/A should stay within about 3-5% of supply voltage; a larger area lowers it. (Key point: Voltage drop check)

4. In the example of a 2 kW, 230 V heater, which selection results?
   A. 1.5 mm^2 wire with 6 A MCB
   B. 2.5 mm^2 copper wire with 16 A MCB
   C. 6 mm^2 wire with 63 A MCB
   D. 4 mm^2 wire with 3 A MCB
   Answer: B. I = 8.7 A, with 25% margin 10.9 A, so 2.5 mm^2 copper and a 16 A MCB (above load current, below cable rating) are chosen. (Key point: Protection coordination)

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