Fuse Rating Calculator
Calculate the correct fuse rating for any electrical circuit.
Find the right fuse size based on load current, voltage, and safety margin.
A fuse is the simplest and most fundamental form of electrical circuit protection. It contains a thin wire that melts and breaks the circuit when too much current flows — protecting the wiring, components, and preventing fires. Choosing the wrong fuse rating is a common and potentially dangerous mistake.
Too Small a Fuse
If the fuse rating is too low, it will blow during normal operation — a nuisance that is often “fixed” by replacing with a larger fuse or bypassing the fuse entirely, which is dangerous.
Too Large a Fuse
If the fuse rating is too high, the fuse may never blow even when there’s a real fault — allowing wires to overheat and potentially catch fire.
The Correct Approach
- Calculate the normal operating current of the load
- Apply a safety margin (typically 125–150% of the operating current)
- Round UP to the next standard fuse size
Fuse Rating Formula
Operating current (A) = Power (W) ÷ Voltage (V) Recommended fuse rating = Operating current × 1.25 to 1.5
Standard Fuse Sizes
| Series | Common Ratings |
|---|---|
| Fast-blow (F) | 0.1, 0.2, 0.25, 0.315, 0.4, 0.5, 0.63, 0.8, 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8, 10, 12.5, 16, 20 A |
| Slow-blow (T) | 0.1, 0.2, 0.315, 0.5, 0.8, 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8, 10 A |
Fast vs Slow Blow
| Type | Symbol | Use When |
|---|---|---|
| Fast-blow | F | Sensitive electronics, semiconductors, precision circuits |
| Slow-blow / Time-delay | T | Motors, transformers, capacitive loads with high startup current |
Slow-blow fuses tolerate brief current surges (like a motor starting) without blowing, but still protect against sustained overloads.
Temperature Derating
Fuses are rated at 25°C. In hot environments (above 25°C), the fuse blows at lower current due to the ambient heat — apply a derating factor:
- 50°C: use 80% of rated current
- 75°C: use 70% of rated current
- 100°C: use 50% of rated current
Voltage Rating
Always ensure the fuse voltage rating equals or exceeds the circuit voltage. Fuses in high-voltage circuits must have sufficient voltage rating to safely interrupt the arc when the fuse element melts.