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Subject Topic: Fuses or circuit breakers?

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cvame
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Posted: July 04, 2010 at 6:29 PM - IP Logged
Link to Post  Post Reply Quote cvame

Im in the middle of a complete wire up grade for more amps. Doing the big 3 with 0/1 added to the factory wires, and a 0/1 gauge main amp power wire to the rear for the 3 amps. Question: which is better, fuses or curcuit breakers? i have access to both.  and how do i decide what amperage to get for the main power wire from the battery for the 3 amps?

Thanks for any help

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oldspark
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Posted: July 04, 2010 at 8:12 PM - IP Logged
Link to Post  Post Reply Quote oldspark

The cable amperage is based on your total load.

I prefer breakers but it depends on the circuit.
Self resetting blades for (critical) headlights & lights; maybe also for other essentials (stop, hazard, flashers).
Self resetting breakers for auxilliary batteries.

Bigger loads depend on convenience, price etc. EG - a 100A fuse is smaller than a 100A breaker.

And breakers (like fuses) SHALL not spark or flame in hazardous environments (boots with fuel tanks etc).

Breakers must be de-rated for temperature - more so than fuses generally are.
And breakers must be robust & suitable for vehicle use - ie, shall not trigger or fail from vibration or G-forces (LOL).
I carry bypass fuses in case I'm racing over the Gibber Desert or the Gunbarrel Highway (hot & rough flats or tracks).


Whilst it is nice being able to simply reset a breaker instead of finding a spare fuse & changing etc, it is not nice having unreliable breakers nor blown breakers which are harder to source.
But breakers are great for intermittents (hence for secondary batteries an similar variable high-inrush loads).
But intermittent glass fuses are never fun (fatigue or internal breaks); ditto for cracked blade fuses (hence the "rotate all fuses" technique - ok also for blade etc breakers, but inconvenient for hard wired breakers).


Finally cost. Recently I noted that a 120A circuit breaker (with integral voltage display - wow!) cost the same as a 120A <whatever> fuse holder and 2 fuses. I'd go the breaker. Plus the breaker functioned as an isolation switch. (A twin 60A breaker of the same type was $10 extra.)
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cvame
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Posted: July 04, 2010 at 8:43 PM - IP Logged
Link to Post  Post Reply Quote cvame

Where do I find the amplifier's load rating? I know this may be a silly question to most, but all I can find is the recommended fuse sizes...
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KPierson
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Posted: July 04, 2010 at 9:01 PM - IP Logged
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There are three ways to determine proper fuse size that are generally acceptable:

1. Look at the chart in the bottom right hand corner of this page for your wire size and use the fuse/breaker listed: http://www.the12volt.com/info/recwirsz.asp

2. Add up all the fuses in the equipment you are adding and use a fuse equal to that number AS LONG AS THE FUSE IS NOT GREATER THEN THE NUMBER LISTED IN THE ABOVE LINK FOR YOUR WIRE SIZE.

3. Actually calculate the expected load and use that size fuse/breaker.

So whats the difference in the different methods? The #1 method is the easiest and requires the least work, but may lead to damage to your vehicle when you start getting in to bigger power wires. For anything 8awg or smaller it is probably fine. For anything bigger then that I would use a more appropriately sized fuse.

The second method is pretty straight forward. If you have 3 amps and one has an 80A fuse, one has a 30A fuse and one has a 25A fuse you know that the max current load can never be greater then 80+30+25=135A so you wouldn't need a fuse/breaker bigger then that. This will get you much closer to the actual load required on the wire.

The third method is the most difficult, but the "best" way. It may require some trial and error and you may blow a few fuses in the process. The theory behind it is to try to calculate the expected current on the wire and use the appropriate fuse. Say we are usin the same three amps from above. One is being used as a sub amp (the one with the 80A fuse) and the other two are for mids and highs. You can assume that the sub amp will be bridged and nearly maxed out so assume it actually needs an 80A fuse. Neither of the other two amps will be bridged and they'll both be playing at 4 ohms stereo. Assuming they are both rated for 2ohm bridged you could easily cut the fuses in half and be ok. That would give you 107.5A expected load and you could use a 110A fuse.

So, why does all this matter you may be asking? The fuse/breaker is a safety device intended to offer protection. It provides protection in two ways - from trying to pull too much current through a wire and direct shorts to ground. If you try to pull more current through a wire then it can safely carry the wire WILL heat up and it will eventually start melting and possibly catch on fire. That is why the chart in the link is so important. NEVER use a fuse larger then what is listed in the chart (or larger then the wire manufacture recommends if that number is available). Direct shorts, though, also have to be considered, and this is why using the largest "safe" fuse isn't always the best idea. Fuses are rated to blow based on percentage of load. A 350A fuse may hold 650A for 10 seconds or more before blowing! (http://www.cooperbussmann.com/pdf/0810938a-26c4-44e2-a299-8962dd6ea9a7.pdf) and will allow ~1200A to pass for 1 second before opening up! This has the potential to damage your battery, your alternator, or any electronic device in your vehicle if you were to experience a direct short.

Sorry for the book but this is probably the most critical part of planning a system out!
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oldspark
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Posted: July 04, 2010 at 9:28 PM - IP Logged
Link to Post  Post Reply Quote oldspark

Geez KP - ever get told your replies are too long?

Do as I say, not as I do!

(I hope readers get the joke. If not, you may son find my rotting encyclopaedias.)
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cvame
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Posted: July 04, 2010 at 11:38 PM - IP Logged
Link to Post  Post Reply Quote cvame

No, not long at all. Thank you both very, VERY much. Better safe than sorry. I'd rather replace 3 blown amps than a new truck... Which is why I asked whether breakers or fuses were better...
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mrmoto
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Posted: August 13, 2010 at 1:29 PM - IP Logged
Link to Post  Post Reply Quote mrmoto

I guess I kind of like the long replies. It shows the poster has knowledge, ability and understanding as applicable to the situation. I would trust a long descriptive post over a reply that just says "use this..." There are so many variables in situations, it's sometimes hard to know what to choose. But it's easier to learn and understand from people sharing knowledge in their answer.
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oldspark
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Posted: August 13, 2010 at 10:37 PM - IP Logged
Link to Post  Post Reply Quote oldspark

MrMoto - I disagree with long replies showing that the writer has knowledge. I often find quite the opposite... maybe CEO or politician's answers are good examples?


But the knowledgeable will often present a short answer (which raises questions for the clever but unknowing) or a long one.
The longs ones should explain the complexities & the reasoning & the experience etc. Hence long.
But in doing so, they should be able to be evaluated by other readers and questioned or corrected.
And hence it should be educatative....

[ Other forums though just bash & flame the long-poster.. f.ex qualitative statements like "ya don't know what you're talking about" (yes I do, but I'm writing you moron!), or "whatever drug you're on..." (is obviously making me smarter than you), etc etc. That rarely happens here.
And then there are the alternatives. Or ambiguities. Or opinions (eg, "it's ok to parallel batteries" is an opinion... LOL - love to stir!) ]

And yes - I have merely paraphrased what you just wrote....
I just got pedantic over the opening 2 sentences.
But the rest is (IMHO) spot on!
I oft say - er, write - that I write long for educative and traceability or completeness reasons.
My other extreme is the one liner, or short jest, but I find most do not grasp its significance until several pages later (by others) if I happen to the reply "that's what I wrote in reply#x".


BTW - KP is usually a "short replier". His reply above is long for him (short for me) but is exactly what I mean - descriptive with alternatives etc. XLNT!
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mrmoto
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Posted: August 13, 2010 at 11:23 PM - IP Logged
Link to Post  Post Reply Quote mrmoto

I do agree with you oldspark for the most part. The long answers are not always indicative of knowledge on the topic. But it does give the reader info on the poster, so they are able to evaluate the person and their knowledge. I know I can look at an answer and form an opinion on their level of expertise on a given topic. For me, I always like to make a well informed decision on the info and the poster. That is much easier with more information. I guess I should have explained that more in my previous post, thus making it longer. posted_image
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installsbyeric
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Posted: January 23, 2011 at 2:47 AM - IP Logged
Link to Post  Post Reply Quote installsbyeric

Every install I do with any system I put the biggest fuse in as possible.  If I am using AGU fuses, I take out the one that comes with the kit, and put in a 100 amp fuse.  If I am using ANL fuse holder, I put in a 300 amp fuse.

Why do I do this?  two reasons.  1.  All modern amps have their own protection in the amp.  The fuse has nothing to do with protecting your amp.  The fuse is there to protect you and your car from catching on fire in the event of an accident or if your power wire was not installed properly and shorts out.  If you have no fuse, that wire will get red hot and your car or customers car can catch on fire if it got shorted out in a accident or if installed wrong.  Reason 2 is the fact that I want to feed the amps as much power as I can get in to them and try to cut down on voltage drop from adding the fuse holder in the first place.  The more voltage at the amp the more power your amp puts out. 


___________________________________
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