Is it possible to combine two fuses together (with different current rating) to make a bigger fuse?

My Pronautic 1260p battery charger has a a pair of paralleled 40 amp ATC fuses to protect a 60 amp charge circuit. The fuses are blown. If the fuses are not well matched for resistance, more current will flow through one of the fuses. After the first one blows the second will blow right away, so the protection scheme is good, but there is some risk of nuisance failures. Always replace in pairs from the same lot to minimize the mismatch.

having two fuses of the same kind, size and rating is often common. Say you need a 40A, you can compliment it with 2 x 20A, same fuses. Current splitting. even if one of the fuse blows a bit slow, still it will work. paralleling different rating fuses does not looks like a nice idea.

The question is how will the currents split. That depends on the resistance of the fuses.

If the 5A fuse had 8 times the resistance of the 40A fuse then all would be good. the two fuses would move into overload at the same time.

So the question is will that be the case in practice. I strongly suspect the answer is probablly not but there are two opposing affects in play. Lets assume the two fuses are of the same size and type.

On the one hand the fuse wire itself in the 5A fuse will probably have more than 8 times the resistance of that in the 40A fuse due to the way current carrying capacity scales with wire size.

On the other hand there will likely be contact resistances. These will likely be similar for the two fuses (and therefore make up a smaller proportion of total resistance on the 5A fuse) but contact resistances can also be quite unpredictable.

In summary the overall result is likely to be unpredictable. The combination may carry a bit more than the 40A fuse alone but probablly not the full 12.5% more (note that the current ratings of fuses are the normal operating current not the current at which they are guaranteed to blow).

Don't Do This.

Let's say you put two fuses in parallel. You would have some really unpredictable results. The reason is we don't know the resistance of the fuses (which determines how the currents would be balanced between each one) and even if we knew what they are, it most likely won't work. Here's why:

Your aim is to make the circuit blow at 45A so let's say your circuit's current is 40A. One fuse is rated at 40A and the other is at 5A. Assuming they both have roughly the same resistance thus current splits evenly between them. Thus you'll have 20A for each fuse. That means the 20A going through the 5A fuse will blow. Then with that 20A having nowhere to go, it then goes through the 40A fuse thus having the full 40A going through the 40A fuse. That fuse then blows. So by putting these 2 fuses in parallel, congrats you just have a 40A fuse with a slower reaction time.

Overall just buy a 45A fuse. Don't mess around by combining fuses to get a different rating.

It's done all the time with large medium voltage fuses, but they are matched sets of equal size (1/2 the rating) and tested together as a set. You cannot do this on your own.

The other comment is wrong. The current won't divide evenly. Yes, they should break when the current reaches 45a but some fuses are fast blowing, and some slow, AND you need to blow 2 fuses (create enough heat to do so), so I would say that the fuse reaction time would rise quite a bit compared to a normal 45a

I still don't recommend to parallel fuses, unless they're the same, and even then, it's still better to get one big one.

As for the comment saying "the current will divide evenly and blow the smaller fuse", think of a thick 0gauge wire running from point A to point B, running a high amperage, for example 80 amps. Then get the thinnest wire you can find, for example a 30 gauge wire, and run it from point A to point B in parallel to the 0 gauge wire. Will the thin wire blow? No, most of the current would go through the thicker wire, as it has the least resistance.

Fuses are safety devices and they are specified and characterized to be used "standalone".

Although in theory you could characterize a fuse so that you could determine what happens when you put two in parallel, no vendor will do that, because it is something no sane designer would want to do.

Therefore, putting two fuses in parallel will go against the very purpose of a fuse, i.e. having predictable behavior under the specified conditions.

This means that, by doing that, you put at risk the equipment (and possibly its user) the fuse is intended to protect.

Bottom line: DON'T DO THAT!

BTW: a fuse is not simply characterized by its current rating, but by lots of other parameters, which vary among manufacturers. Think only of the "reaction speed". What do you expect if putting a 5A(T) (slow) fuse in parallel with a 40A(FF) (ultrafast) fuse? Which will blow first? Very messy and unreliable behavior is ensured!