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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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