# why fuse within 18'' of battery?

Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=56387
Printed Date: December 05, 2022 at 8:31 AM

Topic: why fuse within 18'' of battery?

Subject: why fuse within 18'' of battery?
Date Posted: May 25, 2005 at 3:19 AM

I'm new to this site and see me on here alot due to the wealth of information available. So I decided to ask an odd ball question some of you pros will only know...

My ? is when hooking up an amp in a car, the possitive wire from the battery has (is supposed to anyway) have a fuse on it for obvious reasons. I have heard and read many times  that the fuse should be located no more than 18" from the battery. I have always done this because I figured there must be a reason for it. But what exactly is the reason for having the fuse no more than 18" from the battery? Also, is that the actual physical distance or the wire distance that this applies to. I have always tried to keep it physically within 18" but the wire might have been 20" for all I know. Anyways,. just curiously wanting to expand a little knowledge!!

Replies:

Posted By: vseth
Date Posted: May 25, 2005 at 9:12 AM

Here's my take on it ( e-guess ):

- current flows from positive to negative, so the closer you have the fuse at the positive end, the larger the part of the circuit that's protected by the fuse. The unprotected part is only between the positive terminal and the fuse and as such it is recommended to keep it as short as possible.

- so if down stream of one individual circuit, a short circuit was born ( let's say due to a bare wire ), Ohm's law would dictate that  I = V / R, and considering that the only resistance is the minimal resistance of the good conducting wire, that's a huge current. The huge current draw would initiate upstream from the positive end, but since the fuse is of a good value for this individual circuit, the current would pop the fuse and not let the damaging current draw to get downstream of the fuse.

- ofcourse the fuse protection is not only for short circuits, but protecting the circuit components and the wire that makes up the circuit from current overload, as these have their maximum current capacities before they get fried. Frying components pose fire hazard, that not good for car, so fuse it.

Hey, there's always the problem with e-knowledge, you can never be sure about it.

Posted By: auex
Date Posted: May 25, 2005 at 10:48 AM
It is to limit the chance of the wire shorting out between the fuse and the battery where there is no protection. It is just a general rule of thumb.

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Date Posted: May 25, 2005 at 1:28 PM
Cool. That's the only logical reason I could think of but wasn't sure. Could of been for some other reason for all I know. The reason I thought it could be for a different reason ios because the distance of the fuse box to the battery. Why don't auto manufacturers put the fuse box closer to the battery? Some do I but alot don't. Is this simply due to the limited amount of space engineers have to play with sometimes? Anyways, thanks for you input.

Posted By: homer1
Date Posted: May 25, 2005 at 2:09 PM

auex said:

"It is to limit the chance of the wire shorting out between the fuse and the battery where there is no protection. It is just a general rule of thumb."

Great answer - of course the best protection would be to fuse it right off the battery, but that is not always an option.

The 18 inch length came about thru rules made for car audio competitions - USAC, IASCA, etc... to figure points for installation quality, neatness, and safety. If I remember right, the judges used a tape measure to check the distance, and due to the size of some power cables - they defined the measurement  length to be thru the centerline of the cable! (Late 80's, early 90's some guys were so anal about it. they were talking about removing the wire between the battery & fuse at each competition to get an exact measurement. - kinda extreme?!?!?)

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Experience gained is directly proportional to the amount of equipment destroyed - break it & learn!

Date Posted: May 27, 2005 at 9:15 AM
Like said above, closer the fuse or circuit breaker to the battery, more wire is protected from shorts.

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Penn State electrical engineerin' major

Date Posted: May 27, 2005 at 11:02 PM

Interesting. Thanks to everyone who replied.

Posted By: saterdaynight
Date Posted: May 31, 2005 at 8:48 PM
hello everyone.

not to rain on anyones responces here, but i noticed that it has been said that current flows from + to -.

i was taught that current flows from - to + so i went back to my electronics text book (electricity and electronics, by Howard H Gerrish and Willian E Dugger) and from it i will read (or rather type) what it has to say

"There are two theories about the direction of current flow. the ELECTRON THEORY OF CURRENT FLOW defines the direction of electron flow as moving from NEGATIVE TO POSITIVE in the external circuit. Anouther theory, the CONVENTIONAL THEORY OF CURRENT FLOW states that current flows from POSITIVE TO NEGATIVE in the external circuit. in both cases, the current always flows in the opposite direction of the current in the voltage source."

then it goes on to say that it will use the electron theory (- to+) and that is how every electronic circuit i have worked on has been.

so.... it stands to logic that IF by that theory current flows - to + then everyone elses theory about wire shorting wouldnt make much sence, aditonaly, if you have say a current or voltage or both spike (what blows the fuse) remember that current flows at the speed of 186,000 miles per second (the speed of light) anyways, so the idea is to put the fuse at the source of the circuit, so that it will blow BEFORE it (current) reaches anything else... so if the standard, is for current to flow from negitive to positive, why would you put the fuse at the end of the circuit (the + end)???????

so i feel that the question goes unanswered, and i would realy like to find someone that knows. now, on the outher hand, the regulations for competition makes sence i suppose, but not putting the fuse at the wrong end of the circuit, and i will bet, that if you contact any manufacture of amplifier (that you own) and ask what direction current flows in THERE design, they will tell you - to +.....

nateraly, all you beleivers that it flows + to - will tell me that (duhh, they put the fuse in the right place) but thats not what i was taught.

anyway, just my 2 cents...

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speed is my drug of choice, let me drive...

Posted By: auex
Date Posted: May 31, 2005 at 9:23 PM
saterdaynight wrote:

hello everyone.

not to rain on anyones responces here, but i noticed that it has been said that current flows from + to -.

i was taught that current flows from - to + so i went back to my electronics text book (electricity and electronics, by Howard H Gerrish and Willian E Dugger) and from it i will read (or rather type) what it has to say

While that is correct (especially from an EE's perspective), - in the car is common and all metal/chassis is ground/earth. In most vehicles the + is fused and protected from shorting out(there are oddball + grounding vehicles out there but are rare nowadays). It doesn't matter if you fuse + or - but the engineers that built the vehicle decided to fuse the car's electrical system decided to fuse the + off the battery. You also mentioned the keyword "flow". A fuse will work on both ends and it really doesn't matter which end that you fuse, but you must fuse what ever side of the battery that is not common. The fuse on the power wire is there to protect the CAR not the audio equipment(your amps should have their own fuses for protection).

There was no mention of current flow or the debate that follows, atleast that I noticed. But your question sounds more like a textbook misunderstanding. Don't take that statement as flaming but I have heard it many times.

-------------
Certified Security Specialist
Always check info with a digital multimeter.
I promise to be good.
Tell Darwin I sent you.

I've been sick lately, sorry I won't be on much.

Posted By: saterdaynight
Date Posted: June 01, 2005 at 5:55 PM
ok, first, i dont take that statement as "flaming" whatever you mean by that, and second, i dont think there is anything realy to misunderstand, i was simply stateing that the conventional theory was just that, a theory. nothing works that way in real life. it was mentioned more out of political corectness than anything else.

anyway, while cars may fuse the positive side as you said, in my minds eye, if you wanted to protect a circuit best, you would put the fuse at the beginning of the whole thing so that current would have to pass by the fuse before it got to anything valuable (as in your circuit) and knowing that things flow - to + makes me think the fuse would be better suted on the negitive side???

prehaps i have forgotten/never learned some vital detail on the subject but it still dosent answer the original question of why the 18 inch placement. just about every manufacture recomends the fuse at the 18 inch mark, there must be some simple reason.

homer1 said it came from competitons... mabe, but i would think that would be irrelivent to manufactures because that is a competition rule. it has nothing to do with the average joe on the street that just buys the product that never sees a competition in its life...

mabe someone else would care to post on the topic?

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speed is my drug of choice, let me drive...

Posted By: auex
Date Posted: June 01, 2005 at 8:09 PM
saterdaynight wrote:

...you would put the fuse at the beginning of the whole thing so that current would have to pass by the fuse before it got to anything valuable (as in your circuit) and knowing that things flow - to + makes me think the fuse would be better suted on the negitive side???

Ok, why would it matter if you fuse the -(beginning) or +(end) of the current flow? Can you have a complete circuit with out both being present? Wouldn't the fuse blow at the same moment regardless of where it is fused? Maybe it is my misunderstanding but, can one side of the fuse have more of a draw then the other side of the fuse?   The moment the fuse has to pass a current over the value of the fuse then it will blow, it is this way regardless of where it is. But wait, the fuse on the power wire is to protect the car NOT THE AMP!!! Ok, so that paragraph was pointless.

Moving on, the 18 inch rule is only a rule for competition as you pointed out. It is a general rule of thumb when it comes to other installs, also it is generally recognized as the appropriate length by MECP. It would be best to fuse directly off of the battery.

-------------
Certified Security Specialist
Always check info with a digital multimeter.
I promise to be good.
Tell Darwin I sent you.

I've been sick lately, sorry I won't be on much.

Posted By: saterdaynight
Date Posted: June 02, 2005 at 12:54 AM
ok, thats cool. makes sence, no need to get mad about it or anything...

-------------
speed is my drug of choice, let me drive...

Posted By: sinfulpain
Date Posted: June 11, 2005 at 5:56 PM
saterdaynight wrote:

hello everyone.

not to rain on anyones responces here, but i noticed that it has been said that current flows from + to -.

i was taught that current flows from - to + so i went back to my electronics text book (electricity and electronics, by Howard H Gerrish and Willian E Dugger) and from it i will read (or rather type) what it has to say

"There are two theories about the direction of current flow. the ELECTRON THEORY OF CURRENT FLOW defines the direction of electron flow as moving from NEGATIVE TO POSITIVE in the external circuit. Anouther theory, the CONVENTIONAL THEORY OF CURRENT FLOW states that current flows from POSITIVE TO NEGATIVE in the external circuit. in both cases, the current always flows in the opposite direction of the current in the voltage source."

then it goes on to say that it will use the electron theory (- to+) and that is how every electronic circuit i have worked on has been.

so.... it stands to logic that IF by that theory current flows - to + then everyone elses theory about wire shorting wouldnt make much sence, aditonaly, if you have say a current or voltage or both spike (what blows the fuse) remember that current flows at the speed of 186,000 miles per second (the speed of light) anyways, so the idea is to put the fuse at the source of the circuit, so that it will blow BEFORE it (current) reaches anything else... so if the standard, is for current to flow from negitive to positive, why would you put the fuse at the end of the circuit (the + end)???????

so i feel that the question goes unanswered, and i would realy like to find someone that knows. now, on the outher hand, the regulations for competition makes sence i suppose, but not putting the fuse at the wrong end of the circuit, and i will bet, that if you contact any manufacture of amplifier (that you own) and ask what direction current flows in THERE design, they will tell you - to +.....

nateraly, all you beleivers that it flows + to - will tell me that (duhh, they put the fuse in the right place) but thats not what i was taught.

anyway, just my 2 cents...
Thanks I just got out of my autimotive electroncs college class and that was what we were tought.

Posted By: mmartin1873
Date Posted: June 11, 2005 at 6:18 PM

i think its more along the lines of... if you have a short you want the shortest distance of live wire running through the car...  that way say you pinch the wire, in a door jamb, and it frays... what would happen if the fuse was in the trunk, instead of in the engine bay...sorry bout my bad english i've only been speaking it for 26 yrs of my life :)