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Length of fusible links?

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knobbler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote knobbler Quote  Post ReplyReply Direct Link To This Post Topic: Length of fusible links?
    Posted: Nov/02/2018 at 5:47pm
I'm in the process of verifying the wiring harness I've built for my '78, and I'm fairly satisfied with most of it. I've still got one detail that's been bugging the heck out of me, though: fusible link lengths.

I understand that fusible links work by being a smaller gauge conductor than the wire they're in series with, resulting in their having a higher resistance/lower ampacity than the rest of the circuit, so the service manual diagrams are helpful in determining what gauge to use. The problem, though, is that the actual length of the links is never defined. Since resistance of a conductor is going to be determined by it's total conductive area, the length is a pretty important detail to know if you're trying to install links that will perform as originally designed (or, in my case, replace them with fuses of a type that function in the same general manner as fusible link does).

If, like me, you don't have an OE harness to reference, trying to pin down the lengths that you need can be a little difficult. I haven't found any resources that provide this information, so I'm wondering if anyone here might be able to point me to one that does. I've already chosen my MEGA and MIDI fuses to replace the links, based off an estimated ampacity of the links in the diagrams and expected draw of the circuits they're supposed to protect, and I'm pretty confident that my choices are about where they should be. All the same, I have a compulsion to check my work until I'm as close to certain that it's good as I can be. That said, finding a link length reference would help settle that nagging feeling that the safety of my configuration still needs further review.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Nov/02/2018 at 9:20pm
The rule of thumb is the fusable link is 4 gauge wire size smaller then the original conductor and the length is 6" to 12". Longer length for more amps and #8 link being 8" for automotive.
The minimum length is important as the fusible link conductor has many more strands - conducting more current and has fire retardant insulation. 
The conductor should burn out before the insulation and being a fixed length, burn out in the center.
There is a previous post on this but I have not found it.
Fuses blow at a set current but a fusable link can take over current longer. They protect the circuit but allow longer periods of over current in circuits, usually placed before the fuses.
Replacing a fusable link with a breaker is better then replacing it with a fuse. Instantaneous current rush like turning on headlamps and cranking a stereo at the same time may blow a fuse. A fusable link will heat and then cool. Breakers tend to have a time delay and will reset, but cycling them will shorten their lifespan.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Nov/02/2018 at 9:46pm
Since you are choosing fuses, my advice is this. Find the max ampacity of the wire in the circuit and protect for that. It may be tricky with old wire and unknown spec. But new wire will be rated. The ampacity will be based on gauge, insulation temp rating, length of circuit Including the ground wire side. Then de rate that for bundling in a hot engine bay. And slightly more for a fused circuit. Fuses vary with how they take overload, just like breakers.

Likely de rate by 20-25%

I can help with specifics if you need or
want. Pitch a few runs my way and I can figure it.

This will protect mains.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Nov/03/2018 at 8:01am
I'm not really sure why you are looking for a fusible link length if you aren't going to use them but NAPA publishes the length of the wire style fusible links they sell.  This is just one example.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Nov/03/2018 at 2:49pm
I think the idea is to figure out the amp ratings that are appropriate for the fuses that are replacing the links.
A reasonable thing to do, considering the nature of what these circuits power while underway.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Nov/03/2018 at 3:04pm
I have an original alternator to battery link, which is not standard wire. It is a flame retardant silicone link, which also is more flexible than most auto wires. You can get a similar replacement, by feel, through hobby stores, on RC Car battery wiring kits.

I can post a picture of the power link I removed and replaced for my modded wiring for a Ford Alt setup.

If adding or using existing A/C, a link just like it, is used to power the blower relay.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Nov/04/2018 at 7:48am
Originally posted by Jmerican Jmerican wrote:

I think the idea is to figure out the amp ratings that are appropriate for the fuses that are replacing the links.
A reasonable thing to do, considering the nature of what these circuits power while underway.


If that is the goal, then this link should be of help.

http://www.minimopar.net/electrical-links.html
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Nov/05/2018 at 6:09pm
Originally posted by Trader Trader wrote:

The rule of thumb is the fusable link is 4 gauge wire size smaller then the original conductor and the length is 6" to 12". Longer length for more amps and #8 link being 8" for automotive.
The minimum length is important as the fusible link conductor has many more strands - conducting more current and has fire retardant insulation. 
The conductor should burn out before the insulation and being a fixed length, burn out in the center.
There is a previous post on this but I have not found it.
Fuses blow at a set current but a fusable link can take over current longer. They protect the circuit but allow longer periods of over current in circuits, usually placed before the fuses.
Replacing a fusable link with a breaker is better then replacing it with a fuse. Instantaneous current rush like turning on headlamps and cranking a stereo at the same time may blow a fuse. A fusable link will heat and then cool. Breakers tend to have a time delay and will reset, but cycling them will shorten their lifespan.


I would only replace links with breakers. Ever.
And I've done that. 
Note where the factory used links and where they used fuses and think about it.
I used breakers for my Jeep's Alpine system years ago - to handle the surges without blowing.
They (AMC) used breakers for power accessories like power windows and so on, and not fuses.. 
Trader gave the reasons pretty well.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote knobbler Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2018 at 3:42am
First off, sorry for posting this and then disappearing! Been hectic times trying to meet my timetable for the Gremlin, so I had spaced on this until earlier today.

Secondly, thanks to you all for your very helpful responses on this! It's good to get some feedback from people who are more knowledgeable on this stuff, since I know enough to be dangerous (and with wiring, dangerous gets kicked up to real dangerous). I want to come back to this and address a few of these posts directly, but in the meantime I'll try to clarify a couple things that got touched on:

1. Yep, my motivation for verifying the length was to double-check the expected equivalent amp rating.

2. I'm working to replace the fusible links with fuses, true, but not blade fuses or anything like that. The replacements I've been planning are MEGA and MIDI fuses, which are designed to function in a manner similar to fusible link -- depending on which you use and the amp rating, they'll continue to conduct indefinitely at (I believe) 10% over rating. As the rating is exceeded, the time that they'll withstand the current decreases from 5-7 minutes, down to a matter of seconds. They're pretty much a slow-blow, similar to Maxi fuses (but with a different form factor).

GM (and Chrysler, if I recall) used MEGA fuses with some option packages for a number of year models. For instance, the type of CS130 I'm using was used in '88 Suburbans, and if they had the appropriate option code, the main battery link was substituted with a MEGA fuse. That said, I'm sure it's possible that I'm misunderstanding their function or totally ignorant of some important drawbacks of their use, so I'd much rather be told that this is a dumb design than have my Gremlin burn down after spending the last few years trying to get it going. If anyone wants to hit me with some sense on this, by all means, please do! :D

3. I didn't spend much time considering breakers as a replacement because I immediately had dramatic thoughts of doom, gloom, and fused contacts. In hindsight, I should've given more consideration to the option -- they're used often enough that I'm open to the idea, but I'll need to do more research. I think I'll look to see if I can find anything that compares using them to the type of fuses I've been planning on using.

That's about all I have time to cover for now. Gotta get back in the shop so I can finish with today's tasks before it's time to start on tomorrow's. Thanks again!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jmerican Quote  Post ReplyReply Direct Link To This Post Posted: Nov/15/2018 at 11:16am
You are very much on the right track with your assessment of the fuses you are considering.
Regarding the opinions stated on this thread, I will add this. Breakers, particularly the type of breaker that is common in automotive use, are thermal type, self resettting. This can be a hazard, and the trip amperage and curve is affected by underhoood temp. Usually they are located in a fuse box, but when they are not, lookout. Fuses are excellent when sized for ampacity and overload curve. Breakers can work, but more advanced non thermal breakers are less common in cars. Fuses have and are used throughout a modern system, and protection is orders of magnitude better, but the systems are also orders more busy.

If you are building a higher performing charging system, lets just say there is low hanging fruit to be picked.
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