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Fuse size?

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snhtown 
Copper - Posts: 178
Copper
Joined: August 31, 2006
Location: United States
Posted: November 30, 2006 at 12:04 AM / IP Logged  
hey guys i am thinking of putting in a dist block instead of runing two seperate power wires...the block i am looking at has 1-o ga input and 2-4 ga outputs, there is also a fuse in the block...if i am running one amp putting out about 2000 watts and another amp about 300 watts would i want a 230 amp fuse? it seems kind of high...10% of the wattage?
master5 
Silver - Posts: 1,123
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Joined: October 10, 2006
Location: United States
Posted: November 30, 2006 at 1:06 AM / IP Logged  

The easiest way without alot of math and guess work with efficency is too look at the fuses that come with the amp...That fuse tells you the max current that amp can draw before problems.

Add all the fuse values together and you have the max the amps can draw. So if for exapmle the 2000w amp has 2-80a fuses..and the 300w has a 30a fuse..190a would work but I would reccomend going slightly over in case of a surge..so a 200a would be fine. Remember the basics..if you use a fuse thats too small..it won't hurt anything..it will just blow when the current demands exceed it. If you use a fuse thats slightly larger..that also will not cause any problems in itself..the only thing that should blow the fuse is a short..or if the amp draws too much..now if the amp draws to much due to a defect..well no fuse will save the amp..it needs servicing or replacement regardless.

The main fuse is there for protection against wires or product burning up in the event of a short in the wire..or internal problem with the amp..it should not have an effect on it's performance.

haemphyst 
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Joined: January 19, 2003
Location: Michigan, Bouvet Island
Posted: November 30, 2006 at 9:58 AM / IP Logged  
Too much effort...
How big is the cable you are installing. The fuse is there to protect THAT anyway (the cable and the car...), it has nothing whatsoever to do with protecting the amplifers. At the distro block, fuse according to the OUTPUT wire size.
#8 - fuse at 100A
#4 - fuse at 150A
#2 - fuse at 225A
1/0 - fuse at 350A
Any decent quality amplifier will have it's own protection fuses within or on it anyway. THOSE are for protecting the amplifier - period.
See here... Our wonderful 12v mods and admins have provided all of us with an awesome tool, specifically to answer this question.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: November 30, 2006 at 10:35 AM / IP Logged  
Yes, looking at the total demand capability of your amplifiers and other parts of a system is one way to determine what size power and ground cable to use (and how large an alternator you are likely to need) but the fuse size is ALWAYS determined by the size of the power cable.  The fuse used at the battery or at a distribution block is to protect the cable downstream, not the amp.  Always fuse for the cable size.  Always use no larger fuse than the smallest wire in the power chain before the next fuse and always use a new appropriately-sized fuse if the cable gauge is lowered.
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master5 
Silver - Posts: 1,123
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Joined: October 10, 2006
Location: United States
Posted: November 30, 2006 at 9:21 PM / IP Logged  

I have a basic question. with the statement "the fuse size is ALWAYS determined by the size of the power cable" wouldn't it be more appropriate to state "the size of the wire as well as the fuse rating is determined by the max current draw of the amp(s)"?

In other words..if the system called for a 4ga wire..and we used a zero gauge..is it understood that we should intentionally put in a larger fuse then required soley based on the wire gauge and not based on the systems maximum draw potential?

Someone enlighten me please.

I think I slightly misunderstand the post regardless as it states "there is a fuse in the block"..why would a distribution block have only 1 fuse?? He is asking about one fuse..anyone else notice that?

Oh and I hope no one read into my previous post thinking I meant the fuse protects the amp..not so, I only meant to state that the main fuse will protect the wire in the event of a short or excessive current draw...the amps protection fuse(s) are there to protect from the amp burning up in the event of an internal problem or short..not prevent it the problem, and should always be a lower rating then the main fuse. Hope I am understood.

DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: November 30, 2006 at 9:52 PM / IP Logged  

master5 wrote:
I have a basic question. with the statement "the fuse size is ALWAYS determined by the size of the power cable" wouldn't it be more appropriate to state "the size of the wire as well as the fuse rating is determined by the max current draw of the amp(s)"?

No, not in my opinion.  The max draw of the amps can be used to determine how large a power and ground cable is required (like I stated in my previous post) but the fuse used in the power system is to protect the wire (and you) from over current.

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stevdart 
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Posted: November 30, 2006 at 10:24 PM / IP Logged  

master5 wrote:
..if the system called for a 4ga wire..and we used a zero gauge..is it understood that we should intentionally put in a larger fuse then required soley based on the wire gauge and not based on the systems maximum draw potential?

The consideration of fuse value is based on the gauge of the wire it protects.  If the fuse is NOT chosen with this consideration, other influences were allowed to interfere with the reasoning process.

Why use a fuse or breaker value of 300 to 350 amps on that 1/0 wire if the pull of the gear doesn't support such a large value?  You have to answer this yourself with the simple question:  Why is there a 1/0 wire installed when all that was needed was 4 gauge?  That's an example of faulty reasoning based on the wrong consideration;  it's based on the pull of the equipment, not the wire size.  Or is it the associated cost of the fuse or breaker that prompts the choice in fuse value?  There again, invalid reasoning.  And how valid is that 1/0 wire anyway when you've installed a 4 ga. eqivalent fuse in the run?  ...or with a breaker that will shut off when the thermal limits of a 4 ga. wire has been reached?

The intent here is to provide the basic principles of why and how.  It should be known that the fuse protects the wire, and any mention of the current system's demand on the fuse's ability  should not be addressed as it has no bearing on the principle of this wire/fuse safety system.    A professionally-installed system should always have the correct safety devices in place for the wires they serve.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
master5 
Silver - Posts: 1,123
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Joined: October 10, 2006
Location: United States
Posted: November 30, 2006 at 11:57 PM / IP Logged  

Ok thanks guys..I get the picture. The thing is in the real world where I am in the trenches performing all kinds of installs on a daily basis (not talking about a comp vehicle where there are rules and judging)..I might for example have some "kid off the street"  come in to install all his "stuff".

Now lets say he has a generic 200watt amp and it has a 40a fuse from the manufacturer mounted on the amp. And he has a 4ga. wire long enough for the job. Now what I would do if he needed to buy a fuse from me is sell him a 50a or 60a fuse and be totally confident this will work.

If I go strictly by the logic above..I would want to use a 150a fuse..or sell him a smaller wire. Now I understand that a 4ga is overkill for this amp...but so would a 150a fuse, and the last thing I would do is reccomend a smaller wire..can you see my point as well? Just using common sense alone I have no "fear" of using a wire gauge that is larger then required..but I might have a concern with using a fuse that is 3 times the rating required to run this system..still keeping in mind it's sole purpose is to protect the wire/vehicle.

When I am designing a system myself from scratch....I determine the wire gauge based on the total draw of the amp(s), and use a fuse based on the total current as well...as long as the fuse is protecting the wire..and not too small where it will blow during normal use..what harm is it doing?

I am not looking for an aurgument by any means but some of you seem so adamant about this that it is causing me to question if i am properly fusing my installs.

while we are at it can I blow a sub by underpowering it?........KIDDING  posted_image

DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: December 01, 2006 at 10:19 AM / IP Logged  

master5 wrote:
Ok thanks guys..I get the picture. The thing is in the real world where I am in the trenches performing all kinds of installs on a daily basis (not talking about a comp vehicle where there are rules and judging)..I might for example have some "kid off the street"  come in to install all his "stuff".

We all live in the real world please stop being so condescending.

Your approach is OK as long as you remember that the fuse is intended to protect the wire and you always use fuse sizes SMALLER than the max capacity of the wire.  Anything else is dangerous and unprofessional.  Period.

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master5 
Silver - Posts: 1,123
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Joined: October 10, 2006
Location: United States
Posted: December 01, 2006 at 1:21 PM / IP Logged  

I apologize if I came off as condescending..I didn't mean it to sound that way. I was using the term "real world" for the particular example where I see amplifiers installed with wire gauge that is larger then required..not that I actually live in the real world and others don't as a comparison...

and I appreciate your response and agree with it 100%.

Thanks

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