---
title: "Fusing current and current carrying capacity"
subject: "Switchgear and Protection"
chapter: 3
key: "semester_6/switchgear_and_protection:fusing-current-carrying-capacity"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fusing-current-carrying-capacity
site: BCT Engineering
---

# Fusing current and current carrying capacity

Switchgear and Protection · Chapter 3: Fuses

- Asked in: 2077 Chaitra, 6 marks
- Video: [What is fusing current](https://www.youtube.com/watch?v=oXJRcormx5A) — Techno Raj Gyan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fusing-current-carrying-capacity
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3

## Question

Define fusing current and current carrying capacity of a fuse with an example.

## Short answer

Current carrying capacity is the continuous RMS current a fuse can carry without melting (e.g., 16A). Fusing current is the minimum current that melts the element within a specified time, equal to rating times fusing factor (e.g., 23.2A for 16A fuse with factor 1.45).

## Answer

#### Current carrying capacity (current rating)
The **current carrying capacity** of a fuse is the r.m.s. current that the fuse element can carry **continuously without melting** and without its temperature exceeding the permitted limit. It is marked on the fuse as the rated current.
- It depends on element material, cross-section, length, contacts, enclosure and ambient temperature.
- The fuse rating is chosen slightly above the normal full-load current of the circuit.

#### Fusing current
The **fusing current** (minimum fusing current) is the minimum current at which the fuse element melts and opens the circuit within a specified (long) time, e.g. 1 hour for small fuses or 4 hours for large ones.
- It is always greater than the current rating.
- It depends on the material, length, diameter, contact condition and surroundings of the element.
- **Preece's formula** for a round wire: I = k·d^(3/2), where d = diameter and k a material constant (e.g. k ≈ 80 for copper with d in mm, I in A).

#### Fusing factor
```text
 Fusing factor = Min. fusing current / Rating  (> 1)
```
Typical: 1.1–1.5 for HRC fuses, about 2 for rewirable fuses.

#### Example
- A lighting circuit draws 12 A. A fuse with current rating **16 A** is chosen; it carries 16 A indefinitely.
- If its fusing factor is 1.45, the minimum fusing current = 1.45 × 16 = **23.2 A**.
- Thus currents up to 16 A are carried continuously; at about 23 A or more (overload) the fuse melts after a long time, and at a short-circuit current of, say, 1000 A it blows in milliseconds.

| Point | Current carrying capacity | Fusing current |
|---|---|---|
| Meaning | Carried continuously | Causes element to melt |
| Value | Rated value (lower) | Higher (rating × fusing factor) |
| Effect | No operation | Fuse operates |
| Example | 16 A | 23.2 A |

## Check yourself

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

1. What does the current carrying capacity (rating) of a fuse mean?
   A. The minimum current at which the element melts
   B. The r.m.s. current the element can carry continuously without melting
   C. The maximum short-circuit current it can interrupt
   D. The current at which it blows in milliseconds
   Answer: B. The rating is the continuous current the element carries without melting or exceeding its temperature limit. (Key point: Current carrying capacity)

2. The fusing current of a fuse is best described as:
   A. The current it can carry indefinitely
   B. Always equal to the rated current
   C. The minimum current at which the element melts within a specified long time
   D. The current at which it blows instantly in a short circuit only
   Answer: C. Minimum fusing current melts the element within a specified long time (e.g. 1 h or 4 h) and is always above the rating. (Key point: Fusing current)

3. A 16 A fuse has a fusing factor of 1.45. What is its minimum fusing current?
   A. 11.0 A
   B. 16.0 A
   C. 17.5 A
   D. 23.2 A
   Answer: D. Fusing current = fusing factor x rating = 1.45 x 16 = 23.2 A. (Key point: Fusing factor)

4. According to Preece's formula, the fusing current of a round wire varies with diameter d as:
   A. I = k d^(3/2)
   B. I = k d
   C. I = k d^2
   D. I = k / d
   Answer: A. Preece's formula is I = k d^(3/2), with k a material constant. (Key point: Preece's formula)

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## Related in other subjects

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