WHY is the alt--->batt wire in the Big 3 fused based on wire size?

[phenryiv1 10+ year member]

Alright, I have an argument that I need you guys to help me settle.

After reading ALL 13 pages of the Big 3 thread, and after reading 4 pages of search results, I STILL cannot figure this out. I think that if I had a better understanding of basic 12V principles, this would be easy for me to answer, but I am really stumped.

EVERYTHING that I have on here says (numerous times) to fuse the alt--->batt run based on the wire, not on the rating of the alternator. Fair enough- I did that. I posted my pics and my writeup on a forum specific to my vehicle. In one of the pics, you can clearly see my 200A fuse for the 1/0 alternator--->battery wire. That sent people into a tizzy. Their argument:

Is that a 200A fuse there? WTH would you need a 200A fuse. As good as no fuse. I use a 100A fuse w/ no problems whatsoever...

I responded that I had read on here and on sounddomain to fuse to the wire and not to the alternator. His response:

That makes no sense. If your alternator goes to hell and spikes 150A say good buy to the electical componets of your car.

Then, a second guy chimed in:

Just want to say that he (First Sentra Guy) is right here. In my recent accident I had a wire going from alt+ to bat+ and something shorted out in my electrical system and set the engine on fire (right on the alternator). I can't help but think this might have been because of this extra wire (which was not fused) and a 200A fuse is gonna do about the same.

So it got me thinking- WHY do we fuse to the wire and not to a rating just above the rated output capacity of the alternator? Since in theory, anything ABOVE the rated output from the Alt SHOULD blow your factory fuse, you have effectively sidestepped that safety device (the factory fuse) and now we have exposed the OEM electrical components to a spike or surge that it is not designed to take.

I want to think that the advice that I get on a car audio forum is more accurate than that on a general automotive forums, as it applies to 12V matters, but I wanted to see if anyone here could give me some supporting info on WHY we fuse to the wire, regardless of the factory wiring and electronics capacities.

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[iceteebone 5,000+ posts]

your electrical system is already fused by the car's manufactor

 

[phyphoestilic 5,000+ posts]

I fuse the max current draw, not the wire. Some people claim to fuse by the wire with their lives

 

[phenryiv1 10+ year member]

your electrical system is already fused by the car's manufactor

I am not trying to be argumentative or dense, but...WHERE? If you mean in the factory fuse box, doesn't the high-flow path sidestep the OEM alternator fuse?

This may shed some light on what it is that I DON'T understand, and hence help me figure out why we fuse to the wire. It may also help me silence the critics who say that I am nuts.

 

[hemiscott 10+ year member]

You have to take both into consideration, Max current draw and wire gauge. You don't want 10 gauge from an alt. to battery, if you are running a h.o. alt.. But, thats just what i think.

 

[hemiscott 10+ year member]

As far as the fuse goes, Its just like putting a fuse inline with an amp. If you know what you are doing and are careful, you really don't need one but, to be on the safe side, it will not hurt and is highly recommended. No matter, how careful you are, accidents do happen though.

 

[ultimate157 5,000+ posts]

your electrical system is already fused by the car's manufactor

I am not trying to be argumentative or dense, but...WHERE? If you mean in the factory fuse box, doesn't the high-flow path sidestep the OEM alternator fuse?
This may shed some light on what it is that I DON'T understand, and hence help me figure out why we fuse to the wire. It may also help me silence the critics who say that I am nuts.

Nearly every single system in your vehicle is run off some type of fuse. Just because there is no fuse between the alt and battery doesn't mean there should be one.

When an alternator fails, it doesn't send magic 500a of current, it dies, sending 0 current. If the voltage regulator fails it will either send a very low voltage or a high voltage (15-20volts.) In any of these three cases, a fuse in the wire between your alt and battery will NOT blow, no matter how low of an amperage you fused it for.

The only reason I see to fuse the wire is if you feel you there is a possibility you can mess it up, and someone have the wire fall off on either end; Therefore saving yourself from an electrical fire.

 

[05AztecRed 10+ year member]

It's common electrical knowledge across the board: car/home/industrial...

You always fuse according to the wire size. A 1/0 wire can handle up to 300 amps of power which is 3600Watts (V x A = Watts)(12V x 300Amps = 3600Watts)

In this case , installing a 1/0 wire IS overkill, and most people do it because the larger wire size allows more current to flow thru the wire than the stock 8ga.(stock on most cars) wire, NOT because the car actually NEEDS to pass 300 amps worth of power.

You are really only allowing more current to pass. You could just run 1 or 2 additional lines of 4ga. or 8ga., but if you do that, you might just as well do 1 large wire run...

There is NO absolute requirement that you size at 300 amps, but if you are going to follow the basic electrical theories, than you size per wire.

 

[BoomBoomBoy 10+ year member]

I am not trying to be argumentative or dense, but...WHERE? If you mean in the factory fuse box, doesn't the high-flow path sidestep the OEM alternator fuse?

It's the fusible link inside the fuse box in the engine compartment. That's like the main circuit breaker on an automobile.

 

[helotaxi 5,000+ posts]

Fuses serve two purposes. One is to protect a component from itself. In the event of some sort of internal fault in the component or user error that causes an overcurrent situation, the fuse prevents or minimizes further damage. The second purpose is to protect the wire that it is on from overehating a catching finr in the event of a short circuit. If you do fuse the batt->alt wire, you want to fuse it near the batt not the alt. It is the huge current potential of the battery that you are worried about not the comparatively wimpy output of the alt. You are fusing in this case to protect the wire (and your car) and the fuse should be sized to the wire allowing minimum voltage drop while still offering protection.

I've noticed a lot of stereo illiterate people on car specific boards. It seem that the less they know the more willing they are to prove it, too. In most cases you can pretty much ignore what they say when it comes to the proper way to do anything related to your system.

 

[phenryiv1 10+ year member]

I can pretty much respond to this "set" of responses all at once.

First off, thank you to all for NOT acting like I was an idiot. All of you presented information that helps me understand the reason and logic behind why we fuse to the wire and not to the components. While some of the info presented was not really related to what I asked, it was still great general information that gave me some good background.

What I am taking away is that while the fuse is not necessary, it is never a bad idea. The reason that it is not necessary is because a failure of the alternator or regulator will cause either a 0 current transmission, or a VOLTAGE jump- not an ampreage jump. While this might be able to cause damage to the BATTERY, anything else that might be affected are already protected by the OEM fast-blow fuses, therefor placing them in a position of safety, in the unlikely event that the alt--->batt line has a problem.

Thank you again for all of your help. If anyone has additional information that might be of assistance, please feel free to add to this thread.

 

[QtrHorse 10+ year member]

Listen to Helo, he knows his way around the 12V system.

Basically it comes down to this. You fuse that connection to protect yourself from your vehicle potentionaly catching fire if there is a short or your battery does something crazy. Anything a human does has the potential for failure, it's just added insurance. Just like hopefully, I will never need my added car insurance in case I kill someone and their family decides to sue me but it's there just in case.

 

[Radioflyer97 10+ year member]

I see no reason to fuse that particular wire. I'm betting my optima red top can take any power spike that might occur....save those **** random lighting bolts, but since i drive a plastic car, i dont' attract lightning as much as i used too...it seems to ignore me now:naughty:

 

[PhatTonyDeMarco 5,000+ posts]

I see no reason to fuse that particular wire. I'm betting my optima red top can take any power spike that might occur....save those **** random lighting bolts, but since i drive a plastic car, i dont' attract lightning as much as i used too...it seems to ignore me now:naughty:

I have no fuse, but there is clearly a reason to fuse it. Most of use use zip ties to secure that wire, or any wire for that matter, in place. I remember a member on here who was stuck in traffic for a while, and the ties melted, wire fell and the protective outer coating melted exposing bare wire. This causes a short, which caused a fire, which burned his car to the ground. Had there been a fuse in place it simply would have popped and the entire thing would have been avoided.

I am confident in my placement of this wire, having a small inline six in my Jeep, I have plenty of empty engine compartment space.

I have helped a few friends do this upgrade as well, and two of them had to run them very close to the hot components of the engine; for this I had them fuse it.

Depends on the install circumstances really, but it should be fused for peace of mind.

 

[Thnking 10+ year member]

phenryiv1:

The alternator - battery fuse is definitely a necessity.

Fuses are voltage/current rated. You can have either an over current at V voltage or over voltage at C current situation causing the fuse to blow. Both cases can occur, voltages can rise in some cases to 100V from the alternator, and current can exceed the rated output by 50%.

Basically your thinking on wire size ampacity vs fuse size is correct. The wire, first off, has to be sized to handle the current from the alternator (really it's the insulation of the wire which is damaged first). In most cases, it makes more sense to base the wire size on a very low voltage drop % of maximum expected ampacity, as opposed to thermal insulation properties. Once this is taken into account, the fuse size should be based on max alternator output or below. This fuse energy rating, is going to be much lower than ampacity rating of the wire, when done correctly.

Also, just to clear things up, an alternator "dying", doesn't necessarily mean current/voltage is reduced to 0 from its nominal number. It's very typical for an event to occur which damages an internal component(s) of the alternator/regulator caused by spiking of current/voltage.