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Old 12-02-2015, 11:38   #1
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Why do we fuse?

I understand that we fuse to protect the wire and that the fuse should be rated no higher than the ampacity of the wire. My quandary comes in on wire runs that have power sources at both ends such as the alternator to battery run, or the battery to bus bar run.

Most of what I've seen would say to fuse these runs at the battery end only. If we are fusing to protect the wire against an over current condition through an otherwise good wire then I can understand the single fuse. If we are trying to protect against a possibly compromised wire mid-run then it seems that maybe there should be fuses at both ends.

As I write this I realize that the compromised wire could likely start a fire at current levels that wouldn't blow a large fuse so we're probably just protecting an "intact" wire from overheating. So, that being said, why can't the fuse be located at either end instead of within 7" of the battery as I think is called for by ABYC?

I'm working on a fairly large rewire and would appreciate your comments. Thanks.
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Old 12-02-2015, 11:42   #2
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Re: Why do we fuse?

If a conductor has power at both ends (alternator, battery charger, etc.) then it should have overcurrent protection at both ends. Often, this requirement is ignored but it shouldn't be.
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Old 12-02-2015, 11:45   #3
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Re: Why do we fuse?

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Originally Posted by HoppySailor View Post
I understand that we fuse to protect the wire and that the fuse should be rated no higher than the ampacity of the wire. My quandary comes in on wire runs that have power sources at both ends such as the alternator to battery run, or the battery to bus bar run.

Most of what I've seen would say to fuse these runs at the battery end only. If we are fusing to protect the wire against an over current condition through an otherwise good wire then I can understand the single fuse. If we are trying to protect against a possibly compromised wire mid-run then it seems that maybe there should be fuses at both ends.

As I write this I realize that the compromised wire could likely start a fire at current levels that wouldn't blow a large fuse so we're probably just protecting an "intact" wire from overheating. So, that being said, why can't the fuse be located at either end instead of within 7" of the battery as I think is called for by ABYC?

I'm working on a fairly large rewire and would appreciate your comments. Thanks.
The alternator, solar is considered a current limited source and should not ever be able to ignite the wire, if properly sized. There is no failure mode of an alternator than can cause it to deliver more than its output. The wire should be comfortably sized to handle its full output for hours on end. Even if the alternator developed a dead short internally the battery bank is driving the dangerous current into that short, not the alternator, so the fuse goes as close to the battery as possible..

The battery bank, on the other hand, can provide in excess of 20,000A into a dead short. A single Odyssey Group 31 battery can deliver 5000A of short circuit current... This is why OCP is installed at the battery end or whenever a drop in wire gauge occurs. The battery bank is your dangerous current source that needs the OCP not the alternator, charger or solar system....
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Old 12-02-2015, 11:46   #4
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Re: Why do we fuse?

An alternator is a current source with a maximum current. If the wire is sized for the output of the alternator then even with a short to ground the wire will not overheat.
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Old 12-02-2015, 11:50   #5
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Re: Why do we fuse?

Thanks everyone for the comments so far.

In the case of multiple batteries wired to a bus, should fuses be installed at both ends, at the battery and the bus bar since there could potentially be more current available at the bar due to multiple batteries?
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Old 12-02-2015, 11:53   #6
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Re: Why do we fuse?

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If a conductor has power at both ends (alternator, battery charger, etc.) then it should have overcurrent protection at both ends. Often, this requirement is ignored but it shouldn't be.
This is not a requirement for an alternator......

"11.10.1.1.1 Overcurrent Protection Device Location

Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (178mm) of the point at which the conductor is connected to the source of power measured along the conductor. (See FIGURE 15.)


EXCEPTION: Self-limiting devices."
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Old 12-02-2015, 12:13   #7
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Re: Why do we fuse?

Thanks to all. That helped.
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Old 12-02-2015, 13:15   #8
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Re: Why do we fuse?

Mainesail, I've often wondered where the famous "seven inches" number comes from. Why not 8" or 12" or 4" ? It seems arbitrary and kinda odd! Any enlightenment?

Jim
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Old 12-02-2015, 21:20   #9
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Re: Why do we fuse?

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Thanks everyone for the comments so far.

In the case of multiple batteries wired to a bus, should fuses be installed at both ends, at the battery and the bus bar since there could potentially be more current available at the bar due to multiple batteries?
if you are talking about battery jumpers, making a single bank. they should all be short and protected with loom etc.

if you batteries are all far apart.. well.. find a better place.
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