Let's say I need a 45A fuse. However, I have only a 40A and a 5A fuse in my pocket. If I put them together in parallel, will it act as single 45A fuse ? (so it will blow up if current is superior to 45A).
My guess is the 5A fuse might blow up before reaching 45A, because the resistance ratio of each fuse might not be the same as the ratio of their current rating (5/40). Because of that, there might be more than 5A going trought the 5A fuse before reaching 45A.
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!
The currently existing answers to this question aren't very good. I think there are two reasons for this:
First I will explain the problems with the question, then provide an answer comprehensive enough that one can figure out the answer for their particular case.
The equivalent circuit here is two resistors in parallel, with some load current flowing through the combination. A fuse opens when its rated current has been exceeded, for some duration of time.
If we know the resistances, we can know the current division, and thus how close two (well-defined) fuses are to opening, under some applied current.
Without resistance being known, the only thing that can be concluded is this:
Thus, without knowing anything else about them, two 10A fuses in parallel might have a combined rating somewhere between 10 and 20A. We need more information to determine where exactly.
I think this arises from an overestimated confidence in what a fuse actually does, how it works.
It's a common misconception, I think -- and not at all an unreasonable one. Fuses seem quite precise. Consider the range of ratings offered: do we really need 6.3, 8, 9, 10, 12A, etc. for some particular piece of equipment? With so many to choose from, they must be pretty precise (~10%), right?!
Well, as it turns out, they aren't.
A fuse is rated for a nominal current, for which it will never open, within ratings (ambient temperature, etc.).
A fuse is NOT rated to open at some tiny increment above that current. In fact, fuses are rated to open at several times the rated current -- and, this takes some time to happen. A typical fuse might take some minutes to open at 50% overload, and some seconds at 100-200% overload. The fuse only opens rapidly when the overload is extreme (fault conditions: short circuit current flow), 10s or 100s of times nominal. (A typical residential circuit might be rated say 10A, but might deliver several thousands of amperes into a fault.)
Of course, exact ratings may vary -- consult the datasheet. They will detail exactly when the fuse is, and isn't, supposed to open, and other conditions which may affect it. For example, it may open faster / at lower currents, at higher temperature -- indeed, thermal fuses are optimized to encourage this effect; their current ratings do indeed depend on temperature.
The current-time trade off is given by the time curve of the fuse. This can also be found in the datasheet. Mind that it is only a typical curve -- real parts will have min/max some distance around the ideal curve (the datasheet might show this, too).
Example: Bussmann ATC blade fuses datasheet
Note that the curves slope straight up on the left side, an asymptote as current approaches the rating (or, somewhat above it). (An ambient temperature derating curve also appears on page 3.)
Or if not found in the datasheet as such, supporting information may be present in specific standards. For example, breakers are usually given lettered (A / B / C) designations, relating to how fast they open for certain loads. (Well, the breakers I've seen, at least, still provide their curves; just to say, maybe some manufacturers defer strongly to a standard. This does happen with certain ratings on other components, where a standard is given -- or even assumed -- rather than detailed right there for you.)
So, given the loose tolerances of fuses in practice, the question as asked isn't very useful: when would you ever need a 45A fuse, when you have 40A and 5A fuses, where the 40A fuse itself wouldn't be close enough (it might not open until 60A or more!)?
But that's a matter of value, not underlying merit: it's still an interesting question because we might use, say, a pair of identical fuses, to roughly double the capacity. Here, while the tolerance of each fuse might be pretty gross (-0/+50%?), we're also making a pretty sizable change -- potentially doubling the rating -- so the matter of tolerance goes away, and the question is useful to ask.
Let me emphasize the context in which this advice applies:
If you can't afford to, basically put them in and try it out, and that the worst that can happen is the equipment simply doesn't function and there are no extenuating circumstances beyond that -- who cares? At worst, the equipment is still out of use. If it's wired properly, it's not starting fires, it's not blowing up, it's fine.
Also, if you are in one of these contexts, I hope and pray you know better than to get your advice from The Internet. Please consult a professional. Preferably a PE (literal: Professional Engineer) who carries the legal responsibility to weigh on matters of mortal importance.
So, with that out of the way...
This is indeed used occasionally in commercial product design. It's discouraged I think, but can be done responsibly.
A typical use-case would be where it is desirable to use a cheaper or more common type, that isn't available in higher ratings, and just a little more range is needed.
For example, I've seen car audio amplifiers with two or three blade style fuses in parallel, as needed for power ratings up to 1kW at 12V (or a bit more, as audio typically has a high crest factor; and again, fuses take some time to open: more current is permissible, momentarily, if it averages out in the end). Blade style fuses aren't available above 30 or 40A, and this avoids both the inconvenience and expense of a much larger single fuse. The application is fail-safe (you just don't have extra audio anymore..) and approved type fuses shall be used.
I've also seen parallel fuses in industrial equipment, for example two semiconductor type fuses bolted together, whether for the lower cost (fuses of this size -- say 600V 200A -- are quite pricey in general, and that price may vary widely between ratings or makes), space constraints, or perhaps the same (single) rating is used elsewhere in the equipment, so the reused part allows for a better quantity discount -- lower unit cost. The service manual may recommend matching fuses for this service (I'm an engineer, not a tech, I haven't done this myself; but I have seen fuses in stock rooms labeled by resistance.)
What do manufacturers have to say about this? Excellent question. If in doubt, ask the manufacturer!
I'm just going to follow a single example here:
High speed fuse application guide - Eaton Bussmann
This guide contains several references to parallel fuses:
Not exactly a definitive treatise, but shows it's a standard approach, not a shocker.
From a design standpoint:
What not to do:
Ultimately, the only thing the fuse is there to do, is protect the wiring -- to open the circuit if current goes too high (by -- remember, a fairly generous margin), and clear it safely, i.e. without starting a fire, erupting in arc flash, etc. Or occasionally, something else can be protected as well (semiconductor fuses are so-named because they operate fast enough to protect the most robust of semiconductors: diodes and thyristors). But that's it.
They are not precision limiting elements, nor will they (in general) save equipment from gross overload or other failure. (But using a wrong value fuse may well [further/excessively] damage equipment when that failure does come; always use manufacturer recommended replacement parts.)
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 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).
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.
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.
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.
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.
Guys Its a great question dont dismiss. The answer is really about risk cost / and circumstance . Perhaps resources are limited maybe in a war zone? Just got to get it working ?
Here example of appropriate risk application . Your fluke etc multimeter: How often have you blown that expensive internal 11Amp fuse measuring current in line: So Put a cheap protective fuse in line with our meter: Whats in the box? ( Not a convenient quick blow 10A fuse ? oh got a lot of two 5A ? Ok Put two 5A in parallel Same characteristic eg quick blow . Put meter in line Meter reads 8A. Score! Even if one fuse blows (unlikely) your precious 11A internal is still protected . Lets try again oooh 10 A coming down one fuse blows then other micro seconds after Still your precious 11A internal fuse is ok YOu have been warned. The question is risk No I have not put my home at risk but putting any old fuse combination in . BUT If I was eg in a war zone and I only has two 50A fuses to replace a 100A fuse to keep essential services going . Of course it is a question of risk analysis. ciao Ms Otis
