2 x 200A fuses = 1 x 400A fuse?

[Tday01]

I have just upgraded my inverter charger and it requires a 400A time delay fuse to replace the existing 250A fuse.

The 400A fuse is huge and expensive. Is there any reason not to use two 200A fuses wired in parallel? (Both have same time delay)

Best

Tony
SV On Delay
ispaf.com

[S/V Illusion]

That's a hell of a big charger load to require a 400a fuse butt in answer to your question, both 200a fuses in parallel is equivalent. I would first check the tolerance specs from the mfg.

[rebel heart]

It's a little "goofy" and if you got right down to it I doubt the current traveling along each is dead on 50.0%. But it should work. Definitely something the next owner of the boat will be shaking his head at going "wtf was that guy thinking."

Depending on the cost, I might do the same.

[Tday01]

Cost (in Cartegena) is $150 vs $60. The bigger issue is that the larger fuse is too big to fit in the space without major rejigging of boxes.

The 400A load is actually for the inverter side at full 10 min over power.

Thanks forbthebreplies

Tony

[Boulter]

Hi:

I wouldn't do this unless I was also running two wires. I think it will be more by luck than design that this idea will work well. There are so many ways for the resistance paths to vary, and not just right now either, how will things age. If there is 5 or 10 feet of wire in the paths, it will go a long way to keeping things equal.

Boulter

[Paul Elliott]

I agree with Boulter, if you *have* to do this, run seperate long wires to each of the fuses. Paralleled fuses do not "load-share" very well, and the more vulnerable of the two will fail first. Your 2 x 200A paralleled fuses will actually be good for (just a guess) 350 A at best. Anyway, it would certainly be best to use the correct fuse.

[The Garbone]

Boulter is on the right track.

You should definitely make sure you have the proper gauge of wire feeding the fuses and the inverter. 400amps pulled through a too small a gauge will heat it up real quick and I would worry about fire.

[senormechanico]

If you're going to go with two in parallel, you must make sure each leg to the fuses is equal in conductivity, i.e. equal length wires, identical connections etc.
Small differences will make for different currents in the legs.

[delmarrey]

The big question, is the holder rated for 400 amps? The problem with two legs, if one fuse blows, so will the other right away. The weak link syndrome.

Pay the toll and pass easily!

[Gordan]

2 x 200A fuses are still only going to give you 200A. Fuses work on the basis of low resistance. As the load goes up they heat up with the purpose of being the first component to "burn" up. As the temperature of the metal goes up the resistance will go up. Electricity likes the path of least resistance and now the load shifts to the other fuse which then heats up even faster because more current is trying to be passed through it and away from the other fuse. The heat takes time to go away, so very quickly you will get into a current switch cycle between both fuses and one of the fuses will blow and if you are over 200A the other will soon follow.

You will find instances where fuses in parallel are used in order to handle very very short power spikes. In the case of your vessel continuous service does not qualify as a voltage spike.

If you need 400A then you are better off 'jiggering' things to make it fit.

[senormechanico]

I've gotta agree with Gordan.
In a perfect world, they'd add capacity in parallel, but he's right.

[noelex 77]

400A !!
You are going to need 6/0 or 7/0 wire just to carry the current. Once you calculate voltage drop it will probably need to be thicker again.

[Andina Marie]

I beg to disagree with Gordon. If one of the fuses is taking more than its share, the rise in temperature willl INCREASE its resistance and cause more current to be diverted to the other fuse so the system becomes self balancing.

In particular, using 2 separate supply cables is worse than 2 closely paralled fuses. Now the wiring resistance has less chance of being "matched" so one fuse will be carrying more than its share turning your 400 amp fuse into perhaps a 350 amp fuse.

Having stated that the fuses will inherently share the load equally there is a good reason NOT to use 2 fuses in parallel. When you do get an overload, no 2 fuses are going to be EXACTLY the same so one fuse will open before the other. For a moment the second fuse is now given the task of breaking a 400 amp circuit with a fuse designed for 200 amps. This may cause more heat than a 200 amp fuse was designed to dissipate and cause it to explode. On a 12 volt circuit this is probably unlikely but still a good reason for using the correct fuse.

[Tday01]

It is being wired with 4/0 per the installation manual.

Given that it is more likely that the fuses will blow too soon than too late, and they are already bought and paid for, I think I'll take my chances. They are both time delay, and it will be rare event that I am putting more than 250 through the cables.

Of course, if they start to blow, I'll have to reconsider.

On the other hand, if they start to b

[Florida Mike]

The only thing I would add is that you bring the power into one fuse and out of the other. And then of course make sure that your jumper method is exactly the same on both ends. Just my two cents, Mike.