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Fuse sizes.. why so high?


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im going to make a long story short (i can provide full story if needed) and get down to the question..

when installing dual agm batteries and amps 2000rms and higher running on STOCK alt.. why is it necessary to use fuses that are 250amp in size when the stock alt will prob only put out and dedicate 50amps or so to your car audio system?

i was told this by a shop when i asked for a 200-250amp fuse and my response was that the alt may not put out that much amps but the 2 agm batteries , especially when fully charged, may be able to put out high amount of amps at least for a short period of time..thus the requirement for 200-250amp fuses.

am i right or is there another reason or , should i just go for a 100amp max fuse as he suggested?

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I take the more realistic and safer approach. Fusing according to wires limit is bad if your not pulling that much current through the wire. All your doing is giving that fuse a longer time to blow in the result of a short happening.

Say if you have 1/0 wire thats rated to handle 300 amps of current at 17 feet, most people would fuse it for 300 amps but if your not pulling 300 amps of current through that wire and only pulling 150 amps then you should fuse it at 150 amps.

If something ever goes wrong with that wire the 150 amp fuse will blow long before a 300 amp fuse will and that difference can be drastic such as maybe some burnt carpeting vs a burnt to the ground vehicle.

Putting a smaller fuse on a wire will not limit the voltage, just like putting a crazy high amp fuse will not increase the voltage being sent through (yet some people think this).
 

Also another thing people never consider is that a fuse will not blow as soon as the amperage draw exceeds its rating. Meaning a 150 amp fuse will not blow once it sees 151 amps, nor will a 300 amp fuse blow once it sees 301 amps.

Typically it will take a fuse an increased amount of current larger than its rating for a short period of time to blow, for example a 150 amp fuse will not blow until it see 200 amps of current for about a minute and a 300 amp fuse may not blow until it sees 400 amps of current for a minute straight.
Now these numbers are just estimates and no two fuses are created equal and often 2 of the same fuses rated the same from the same company maybe completely different.

Now in your case it depends on where these AGM batteries are installed. Are these 2 batteries in the rear? Are you bypassing your front battery? Is the wire coming straight off the alternator in the front going to the rear two agm batteries? Are you fusing the wire from your alternator to the front battery, and then wiring  the front battery to the 2 rear agm batteries. What is your stock alternators amperage rating?
There are so many different wiring possibilities to give a definite answer on without knowing how your wiring.

So if you where fusing the wire that goes from the alternator to the front battery only and it was say a 80 amp alternator, fusing that wire for 100 amps would be ideal.
However say that you are running the wire from your alternator to the front stock battery and then to the rear 2 agm batteries you would probably want to fuse it for whatever your amps fuse rating is (maybe slightly higher), because with that front battery now in the mix the amperage draw from your amplifier will pull from your 2 agms and the front battery as well.

So it comes down to how everything is being wired.

 

 

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ok,, i think its a good idea to explain the entire situation for best response.

I have stock alt, maybe 110-120amps, 2 agm batteries one under hood, one in trunk. 0g running in between them.

im running bc2000 amp and a 4chan that is currently pulling maybe 200-300rms..

the 0g wire coming from the front of car is connected to the rear + post of battery. I was originally going to get 0g terminals , fuse that wire with a 250amp fuse BUT the shop i went to did not have any 0g terminals left so instead he sold me this dual battery terminal post that has a 4g and 0g input and a 100amp fuse on top. he said this would do the trick.

so i installed it. stuck the 4g wire into it and into my 4chan amp.. stuck the 0g wire from front battery into it, and called it a day

some time later i took the plastic holder cap thingy off and noticed the 100amp fuse was corroded and melted.. but somehow the damn thing was still working and music was playing.

i then concluded , and not sure if im right , but my theory is this 100amp fuse melted because there was just too much current demand/draw from both amps (esp the bc2000) and that dinky 100amp fuse couldnt handle it and started melting.

note: that dual terminal and 100amp fuse was a shared fuse for both the 0g / bc2000 amp and the 4g 4chan amp..

i am adding a pic for reference..

i went back to shop today and he gave me a 150amp mini anl fuse and told me to use that instead.. i asked him for a 250 and he said i dont need it.. i still think i need it.. should i go and get a 250amp mini anl fuse and install it?

thanks for the help!

 

33awe1t.jpg

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Class is back in session!

Simply put, if that 100 amp fuse didnt blow, your not passing 100 amps or more of current through that wire.

The fuse got hot and melted because of a bad/loose connection which causes resistance and in return heat.

You need 2 fuses on that wire that runs between the front battery and rear battery. 1 fuse does nothing in protecting the wire. If a short would happen and that 1 fuse blew you still have power coming from the battery on the other side which is still dumping energy into the shorted wire which can result in a fire.
I personally cant stand those mini anl style fuses, and prefer the full size ANL fuse.

If that shop cant stock common items that a guy such as myself stocks working out of my home garage then find a new shop or order product online. There is NO EXCUSE for that imo.

Refer to awesome paint skills drawing below.

vcfghb.jpg

If this was my install, or an install I was doing for someone this is what It would essentially look like.

The yellow boxes are fuses. The front battery would need a fuse within 12 inches going to the rear battery. The rear battery would need a fuse within 12 inches coming into the battery. The reason this needs 2 fuses is due to if a short ever does happen someplace on that wire then both fuses will blow and stop the transfer of power between each battery. Having only 1 fuse on that wire will not cut off power to both batteries if an issue or short ever did happen which is not safe!

IIRC Crescendo recommends a 250 amp fuse on your amplifier, so between the rear battery and the amp install a 250 amp fuse. Keep in mind fusing is done to protect the wire and the wire only, if an amp is going to blow it will blow regardless of having a fuse or not having a fuse.
 

Now for the 2 fuses that need to be on your 1/0 power wire going between the front and rear battery.. Those can be 100 to 150 amps each and here is why.

Voltage and Amperage follow the path with least resistance. You can have the best power wire in your vehicle but it still will have resistance in the cable from one end to the other. This is why people do multiple runs of wire, which in return lowers the resistance and gives you less voltage drop.

Resistance is also why if you check your vehicles voltage with a Digital Multi-meter when its running and at the front battery and say it says 14 volts but your rear battery only shows 13.6 volts, meaning you are losing 0.4 volts through your power wire (the same applies to your radios preout voltage going through the RCA cables, but not as bad since that is AC and not DC).
The shorter your power wires are the less resistance and voltage drop you will have from one end to the other. So a 18 foot power wire will have more resistance and more voltage drop compared to a 4 foot power wire.

That being said your amp is in the rear of the vehicle only a few feet away from that rear battery, which results in minimal resistance and voltage drop. So your amp is essentially going to post the mass majority of power from that rear battery because it is closer. 

So to make things easier lets use Crescendos fuse rating for the example.

Your amp draws about 250 amps total when turned up all the way (which it don't since music is dynamic, but lets just say). 175 amps of that power is being supplied by your rear battery, while that other 75 amps it needs to make that 250 amp draw is coming from your front battery and alternator (which are a lot farther away.

So in the real world if that 100 amp fuse that you put in there did not blow that means your current draw from the front to the back does not exceed the limitations of that fuse, but if the current draw did exceed that fuse it would be blown!

Now here is the kicker, if you ever upgrade to a nice high output alternator that makes say 300 amps you will most likely have to change those 2 fuses on the power wire that goes from the front battery to the rear to a larger fuse around the alternators charging capabilities.

 

In my truck for example I have 18 runs of 1/0 power wire, 8 are positive 8 are negative wires.

I have three 390amp alternators which make 1,170 amps of current. I also have 2 agm batteries in the front that are connected to the 5 batteries in the rear with those 18 runs of 1/0 wire. I have four 5,000watt rms amps in the rear as well.
Each of my positive wires is fused at 150 amps on both ends (18 total anl fuses and fuse holders). So with that 1200 amps of fusing being fused as close to my alternator rating as possible is the safest way to protect those wires. Sure I could fuse each wire for 300 amps which is the wires rating, but why? If Im not blowing the 150amp fuses then switching to larger 300 amp fuses will only increase the time for the fuses to blow if a short ever happened.

Now sure I could say screw all those 18 runs of power wire and do it with 4 positive 1/0 wires and 4 negative 1/0 wires and fuse the positive at 300 amps each (giving me the same 1200 amp rating), but then that means those 8 runs of power wire will have more resistance than the 18 runs of power wire which results in more voltage drop.

 

 

 

 

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I didnt understand that method before too. That you should use smaller fuses if you dont have that amount of power being ran. If u actually think about it, it makes sense. Fusing for wire is only going to protect the wire after the fuse if its an output run(Out from alt) or protect the wire before the fuse if its an input run (into batt). But what about all ur other electrical equipment thats using the alt and batt? You want the fuse to tell u something is wrong  ASAP. Not just for the wire but for safety of everything in the vehicle. Any abnormalities should pop ur fuse so u catch the issue early. If ur fuse is double the amp size of ur normal functions......you'll never know something is wrong till its way too late. 

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Also that batt terminal looks cheaply made? Does the flat end thats melted have full contact to metal on that dual terminal? Looks like the melted end is only resting on the grey plastic. The only point of contact for the fuse on the melted side is the bolt? If so, that could be causing the resistance/heat. 

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