Fuse colours · time-current curve

Blade fuse ratings, colours and blow times

A fuse does not blow the moment the current passes its rating. It heats up, and the more it is overloaded the faster it goes. Here is how long that takes, what the colours mean and how to choose one.

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How fast a blade fuse blows

Current, × rating	Time to blow (typical)
up to 1.1×	holds
1.35×	about 2 minutes
1.6×	about 10 seconds
2×	about 1 second
3.5×	about 0.1 second
short circuit	milliseconds

These follow the blade-fuse time-current characteristic of ISO 8820-3, which the WireForge simulator uses; exact times vary between makers.

Colour codes (ATO / mini)

Rating	Colour
2 A	grey
3 A	violet
4 A	pink
5 A	tan
7.5 A	brown
10 A	red
15 A	blue
20 A	yellow
25 A	clear / white
30 A	green

Choosing the rating

Take the load current plus about 25 %, and never more than the wire can carry. The fuse exists to protect the wire: if it is rated above the wire, the wire overheats first.

Watch a fuse heat up

Draw a battery, a 10 A fuse and a load that draws 13–14 A.
Simulate: the fuse is overloaded and the panel shows roughly how many seconds it has left.
Increase the load: the time shrinks; make a short and it blows at once.

Questions

Why did my fuse not blow straight away?

Because a moderate overload takes time: at 135 % of its rating a fuse can hold for minutes. Only a heavy overload or a short blows it at once.

Is a 25 A fuse clear or white?

Both are used for 25 A blade fuses; some makers use natural/clear plastic, others white.

Can I replace a 10 A fuse with 15 A?

Only if the wire and the load are made for 15 A. Otherwise the wire is no longer protected.

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