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Old 27-10-2012, 14:56   #16
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Re: 2 x 200A fuses = 1 x 400A fuse?

I would not run two fuses in parallel. The reason is, that one fuse (and the associated fuse holder and wiring split) will always have a little more or less resistance than the other, is they will carry the load unequally, and that could result in one blowing prematurely. As a practical matter that imbalance may be negligible, but I wouldn't do it that way without advice from a professional electrician or a fuse vendor saying it absolutely could not be a problem.

Now, Andina (who has been at this for a long time) says you can get away with it--with the caveat that the second fuse may literally explode when it does go. Exploding? In the power system? Eh, no, generally not a good thing. Tends to really hurt the top of your head after you've jumped up at the BANG! and slammed into the overhead. Kludge jobs are for getting home before dark and storms. Rob a 7-11, buy the right fuseholder, save the fireworks for the 4th of July.
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Old 27-10-2012, 15:07   #17
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Re: 2 x 200A fuses = 1 x 400A fuse?

I found a high quality fuse and fuse holder for $38.03. What prices are you seeing for 400 amp fuses and their compatible holders? I don't know what might be considered too expensive.

BLUE SEA SYSTEMS AFB400 - 400 Amp Fuse And Holder

The wire is what is going to be expensive. Twice the length of wire rated for 200 amps just might be close to the same price as one run of wire rated for 400 amps, because it ends up being the same amount of copper that you need to buy. If for whatever reason, (maybe economies of scale because a lot more wire for 200 amp circuits is sold) wire for 400 amps ends up being more expensive than twice the length of wire for 200 amps then run the two 200 amp lengths in parallel to each other the same 400 amp fuse. This way it will not be necessary to have two 200 amp fuses in parallel with each other. It might look goofy or unconventional, but it will work.

For the same cross sectional area two wires each at half the cross sectional area will have more than twice the ampacity of a single wire at double the cross sectional area. Why is this? Because the sum of the surface area of the two wires has a greater surface area to volume ratio than the single larger wire. The amount of surface area has to do with its ability to shed heat. Wires ampacity is partially rated according to its ability to shed heat. It won't matter though because you are going to gauge wire in the the circuit for maybe a 3% voltage drop or less?

Here Blue Seas is saying you want a fast response fuse for most inverter applications. You will obviously want to go with what the manufacturer of the inverter recommends.
http://bluesea.com/products/5502
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Old 27-10-2012, 16:23   #18
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Re: 2 x 200A fuses = 1 x 400A fuse?

This is from Cooper-Bussmann's (possibly the largest fuse manufacturer in the world?) Application Guide:

Quote:
Fuse-Links in Parallel
There are many applications where fuse-links are used in parallel. As the surface area of two smaller fuse-links is often greater than an equivalent rated larger fuse-links, the cooling effects will also be larger. The result may provide a lower I2t solution, giving closer protection to the devices or a lower power loss solution.

Only fuse-links of the same type or part numer should be used in parallel, excepting that only one may be required to provide indication. All the fuse-links would be mounted to allow equal current and heat flow to the connections. In larger installations it is best to parallel fuse-links of close cold resistance values.

The I2t value of parallel fuse-links are given by:

i2t x N2

where N is the number of parallel fuse-links connected together. Mountings should ensure at least 5 mm between the adjacent parallel fuse-links.
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Old 27-10-2012, 16:52   #19
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Re: 2 x 200A fuses = 1 x 400A fuse?

Quote:
Originally Posted by Dsanduril View Post
This is from Cooper-Bussmann's (possibly the largest fuse manufacturer in the world?) Application Guide:
Thank you for your contribution -- I learned something today!

My summary:
  • Fuses that are closely-coupled in parallel will do a fairly good job of sharing current.
  • Additional wire in series with the individual fuses will not improve things, and probably make things worse.
  • The paralleled fuses will need to be wired so as to have a balanced current flow (much as you should wire paralleled batteries).
  • It's probably still best to use one properly-rated fuse.
  • Be aware of the fuse ratings for current interruption.
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Old 27-10-2012, 17:00   #20
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Re: 2 x 200A fuses = 1 x 400A fuse?

Tony, if you don't have room for a new 400A breaker and holder, how do you find the room for parallel fusing with equal branches?

I think you're on the right track when it comes to overall cost.
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Old 27-10-2012, 21:32   #21
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Re: 2 x 200A fuses = 1 x 400A fuse?

Quote:
Originally Posted by Paul Elliott View Post
Thank you for your contribution -- I learned something today!

My summary:
  • Fuses that are closely-coupled in parallel will do a fairly good job of sharing current.
  • Additional wire in series with the individual fuses will not improve things, and probably make things worse.
  • The paralleled fuses will need to be wired so as to have a balanced current flow (much as you should wire paralleled batteries).
  • It's probably still best to use one properly-rated fuse.
  • Be aware of the fuse ratings for current interruption.
And don't forget the 5 mm spacing (so they can cool properly).
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Old 27-10-2012, 21:53   #22
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Re: 2 x 200A fuses = 1 x 400A fuse?

To answer the price question. I am in Cartagena Colombia and thats what they cost here. Maybe the 400 is imported and subject to 50% duty and the 200s are local.

On size, the 400 is a huge cube and the 200s are knife type. As it turns out the 200s still had to be mounted in a seperate box.

Tony
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Old 28-10-2012, 06:09   #23
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Re: 2 x 200A fuses = 1 x 400A fuse?

Quote:
Originally Posted by noelex 77 View Post
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.
400A fuse and 4/0 wire is common size for 3,000W inverter/chargers, which are common size inverter/chargers on boats. It is what Victron, Outback and Magnum recommend for their units of this size.

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Old 28-10-2012, 07:31   #24
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Re: 2 x 200A fuses = 1 x 400A fuse?

The ABYC maximum amperage for wire in conduit or bundled 4/0 wire is:
for 75C rated wire
189A inside the engine space
252A outside the engine space

For 105C wire
264A inside the engine space
311A outside the engine space.

Generally voltage drop considerations are going to be the factor that dictates a larger wire size, but even this maximum current carring capacity is in danger of being exceeded.
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Old 28-10-2012, 07:32   #25
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Re: 2 x 200A fuses = 1 x 400A fuse?

MArk is right. It's a Magnum 3012 3000W inverter charger. The charger portion is 160A max current. Their recommended wire for 5ft run is 4/0. This will still cause a slight voltage drop. To deal with this, there are sense wires. The maximum continuous power for a 5 minute surge is 3950W (hence the fuse).

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Old 28-10-2012, 07:42   #26
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Re: 2 x 200A fuses = 1 x 400A fuse?

Quote:
Originally Posted by Tday01 View Post
MArk is right. It's a Magnum 3012 3000W inverter charger. The charger portion is 160A max current. Their recommended wire for 5ft run is 4/0. This will still cause a slight voltage drop. To deal with this, there are sense wires. The maximum continuous power for a 5 minute surge is 3950W (hence the fuse).

Tony
The charger performance is fine with smaller gauge cable. The inverter performance is more of a concern.

At the maximum 5 min output at your quoted 3950w I would expect the current draw to be
3950/0.92x11.5=373A
( 0.92 is a typical efficiency factor and 11.5v is a realistic voltage at the inverter after some voltage sag in the battery and voltage drop in the wires)

You can see this exceeds the ABYC recommendations considerably.
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Old 28-10-2012, 07:58   #27
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Re: 2 x 200A fuses = 1 x 400A fuse?

I am assuming that the ABYC recommendations are due to concerns about overheating. This is something that would happen over a longer time than 5 mniutes (at 3950W). Continuous rating for the inverter is 3000W. This gives 283 A, which is somewhere between the recommendation for 75 and 105 rated wire. (I believe mine is 90C).

So I think 4/0 is close.

Currently the wire is not bundled, but alone, if that makes a difference.
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Old 28-10-2012, 08:25   #28
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Re: 2 x 200A fuses = 1 x 400A fuse?

Quote:
Originally Posted by Tday01 View Post
Currently the wire is not bundled, but alone, if that makes a difference.
Yes unbundled the maximum amperage the wire can cope with is higher, but wire in conduit is considered bundled. With these sort of wires conduit is a good idea.

90C wire is considered by the standard and the rating for bundled wire is 269A and 211A for inside and outside the engine space respectively. Sorry I do not have the table for unbundled cable on this computer.
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Old 28-10-2012, 12:00   #29
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Exclamation Re: 2 x 200A fuses = 1 x 400A fuse?

While I'm still fairly new at the marine electrical world, I never saw a neutral pole on a breaker before my boat, I would want you to also check one thing no one else has stated here make sure the new fuses are of the same interrupting rating as the 400A specified. If the distance between the end caps of the 200A are smaller than the 400A you are asking for trouble as the larger the gap the more fault rated current the fuse will quench. A shorter distance is a lower rating and will not quench the same amount of arc current. From Cooper industries .pdf that you can google : Fuse Interrupting Rating
"A protective device must be able to withstand the destructive energy of shortcircuit
currents. If a fault current exceeds a level beyond the capability of the
protective device, the device may actually rupture, causing additional damage.
Thus, it is important when applying a fuse or circuit breaker to use one which
can sustain the largest potential short-circuit currents. The rating which defines
the capacity of a protective device to maintain its integrity when reacting to
fault currents is termed its “interrupting rating”. The interrupting rating of most
branch-circuit, molded case, circuit breakers typically used in residential
service entrance panels is 10,000A. (Please note that a molded case circuit
breaker’s interrupting capacity will typically be lower than its interrupting
rating.) Larger, more expensive circuit breakers may have interrupting ratings
of 14,000A or higher. In contrast, most modern, current-limiting fuses have an
interrupting rating of 200,000 or 300,000A and are commonly used to protect
the lower rated circuit breakers. The National Electrical Code® 110.9, requires
equipment intended to break current at fault levels to have an interrupting
rating sufficient for the current that must be interrupted. The subjects of
interrupting rating and interrupting capacity are treated later in more detail"

There are some good pictures from the lab showing how that works and doesn't when under rated devices are used. also if you look at Fluke's web site they have a great section on multimeter safety with a video that shows them being tested I suppose a harbor freight meter will be fine in most cases but a Cat IV rated meter will last a long time I know I have had the same Fluke 27/FM DMM since the early 90's, before Cat ratings, and have never had to change a fuse in it. I have now been using the new wireless meters from fluke and have been beta testing the 2nd gen wireless system, CNX3000. Wireless in that you hookup the meter then detach the display and can read the meter from a safe distance in the case of arc flash protection or for a boat hook up at the receptacle then walk over to the fuse or breaker panel and energize the circuit while seeing what is going on at the remote location.
Best of Luck
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Old 28-10-2012, 12:08   #30
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Re: 2 x 200A fuses = 1 x 400A fuse?

my guess is due to differences in mfg, wire ends and other resistances, that one 200 amp fuse will blow prior to the other... meaning you really have a 200 amp fuse system. In a perfect world, if they both loaded up uniformly it would work....
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