Fuse Rating Calculator

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 and 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. That is a nuisance, and the usual “fix” is a larger fuse or a bit of foil across the holder, which is how a minor annoyance turns into a fire.

Too Large a Fuse

If the fuse rating is too high, the fuse may never blow even when there is a real fault, and the wiring becomes the thing that melts instead. The fuse is meant to be the weakest link in the circuit. Make it stronger than the wire and you have simply moved the failure somewhere more expensive.

The Correct Approach

1. Calculate the normal operating current of the load
2. Apply a safety margin (typically 125–150% of the operating current)
3. 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

The two lists are different lengths and that is not an oversight. The 5x20 mm glass slow-blow series stops around 10 A, because a time-delay element that survives a motor inrush at higher currents will not fit in a small glass tube. Above 10 A you are into ceramic cartridges, blade fuses or bolt-down types, which have their own size ladders.

Fast vs Slow Blow

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 the ambient heat does part of the melting for you, so the fuse blows at a lower current. 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.