Is there a reason *NOT* to limit dist block fuses to match amps?

My gut feeling is that this is a non-issue for the most part, but there are two very different opinions on this and I haven't found any detailed explanation as to why some choose option two. First off, I know that the fuses are meant to protect the cables, not the equipment at the end (they have their own fuses).

Scenario:

All the cable is OFC, KnuKoncepts Kolossus. One run of 1/0 awg cable fused near the battery, going to a fused distribution block in the trunk which has a short 4 awg going to one amp (900W RMS roughly) and a short 8 awg going to another amp (2x100W RMS).

Option one:
Either use basic math to figure out the maximum power draw of your amps (accounting for efficiency), or simply sum up the total of the fuses on the amps. In this case, 2x40A on one amp, and 1x30A on the other. This would mean the dist block would have fuses rated at 80A on the 4 awg and 30A on the 8 awg. The main fuse on the 1/0 awg would only NEED to be fused for 110A (more likely 120A).

Option two:
Size the fuses according to the rated current limit of the cable. For Kolossus cable, that would be 150A on the 4 awg line, 60A on the 8 awg line and a whopping 375A on the 1/0 line.

My question is... is there any benefit at all to going with option two? If the fuses on the amplifier will blow if they legitimately draw more amperage than option one, then what is the point of using larger fuses that allow more power through the cables? It seems like it is safer to use the smallest fuse needed for the application, so that any issue that causes a draw greater than what the amplifiers should be using will cause the fuse to blow, rather than allowing up to 375A to flow through the cable before blowing a fuse. Wouldn't this increase the risk of fire if there were unseen imperfections or damage to the cable?

I don't have a good enough grasp of electricity to know whether an undersized fuse will somehow create enough resistance to have an effect on performance... but going by the wiring guidelines here, I can't imagine that a 1" long piece of nearly any thickness of metal would cause any significant voltage drop.

The only other reason I can think of to fuse to the cable size rather than the amplifiers would be to have room to expand later... but it seems odd to make sweeping recommendations that can potentially impact safety for the off chance that the average Joe is going to massively upgrade his system in the near future (while not also considering paying a dollar for higher rated fuses).

Please educate me. I genuinely want to know the reasoning behind this, because I see option two recommended constantly with no explanation as to why.

While I'm certainly no expert, I think your reasoning is sound. I suspect that you're not seeing any explanation for option two because there really isn't one other than "bigger is better," or the like.

I too am interested in hearing what the experts say about all this.

Long ago, in the absence of an adjustable gain, I used an undersized external amp fuse as a cheap and effective failsafe means of limiting amplifier output (via blown fuse), in order to prevent inadvertent damage to my speakers.

Fuses are to protect the wire from catching on fire ultimately. If the object draws more current than it should be, it will hopefully short circuit itself somehow, hence the fuses on the amps.

But that doesn't mean you need to go with the biggest baddest fuse on the wire. I also wouldn't go with matching the amps fuse, you are going to lose some current in the length of the wire due to the resistance in the wire, so you want to be able to have enough current at the amp that it isn't "starving for energy".

If you put an 80a fuse on your distro block and your amps can pull 80amps, 80amps won't ever make it to the amp with losses in the wire before blowing the distro fuse. How much is lost depends on length, wire gauge, materials etc.
Now the odds of this ever being a true problem is beyond my knowledge. But I highly doubt most would ever pull 80amps into their amps, especially in a sound quality forum where people have active speakers with tweeters. Tweeters pull like 5w at ear bleeding levels, if they pull even that much. Subs are a different matter though.

ozzuneuj, might need to check those ampere numbers again (not that high on the 1/0 size(?))
Also it takes *twice* the amount of current and 1 to 10 seconds before the fuse actually blows. Plan 2 is like wanting to tests if the comprehensive insurance company is just good or really good (dangerous). That means try to use a fuse half of what the cable can handle. i think your Plan 1 sounds much better.

Any fuse in the rated acceptable range of a wire will blow long, long before the wire gets hot enough to start a fire. So argument to use smallest possible fuse to protect the wire is moot. One reason for largest possible fuse is there may be less resistance across the fuse, since it has more surface. This could be better for transients. That's just me hypothesizing tho.

With fuse you protect cable, not components.

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Originally Posted by LBaudio

With fuse you protect cable, not components.

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I'd rather protect the components with a smaller fuse, which will give even more protection for the wire.

Quote:

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Originally Posted by ckirocz28

I'd rather protect the components with a smaller fuse, which will give even more protection for the wire.

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This is standard practice in OEM industrial equipment, for example, where fuses rated (depending on type) 1.3x or 1.5x motor FLA are used in combination with a thermal overload device, where actual wire ampacity may be more like 3x or 4x motor FLA.

Here's how I think of it:

Each DC-powered component (amp, EQ, processor, etc) should (but may not!) have a fuse to protect it... and really it's the manufacturer protecting your car. Basically, that's the manufacturer is saying "This amplifier/EQ/processor/etc should only draw ____ amps of current at MOST." If it were to draw more than that, that could be a sign that something has become short-circuited inside of it, or it could be a sign that the installation wasn't correct (for example, hooking up a 1 ohm sub to a 2 ohm stable amplifier), and to protect itself [and therefore your car] from overheating and possible fire, the fuse blows before that can happen.
Plus - power wire has some resistance, so a little extra current can be burned as heat - not 10% but good to consider, even without math.

Sometimes, your processors or amps will NOT have fuses built into them - in that case though, the manufacturer will tell you what size fuse to install, and you want to install that fuse right before that component in the car, as close to that component as possible. Fused distribution blocks are nice for that... and even if your amps or processors have fuses built in, you could still use a fused distribution block. In that case, I would add maybe 10% so that the fuse in the amp or processor blows before your distribution block fuse blows, but even if you use the same size fuse you should be fine.

The same is true for your power wire. You are running a wire ALLLLLLLLLLLLLL the way from the battery at the front of the car to your trunk/hatch. Sure, you size that wire so that it has enough current capacity to supply all your DC-powered components in your trunk/hatch, and certainly the fuse shouldn't be MORE than that wire is capable of handling... or you risk the wire melting and causing a fire before that fuse even blows!
...and what is that underhood fuse for anyway? "Protecting the wire" is true, but what does that mean? That means a bunch of things:
* Protecting the wire from drawing more current than the wire is rated to carry - the wire and insulation can melt if it does. If you try to run a 5000w RMS amp on a run of 8 gauge power wire - it could melt, that could set your car on fire.
* Protecting the wire from a short circuit - could be any damage over time, any puncture, where the insulation of the wire is compromised and the wire is grounded. That would cause as much current to flow as the resistance of the short circuit creates, and again the wire could melt and set your car on fire.
So some people figure out the wire gauge and length of the wire they installed, and for sure it is a good number you NEED to know, to know what the maximum current that gauge/length of wire can safely flow - yes.

But you shouldn't use that theoretical "maximum current capacity" to fuse your wire, at the battery, IMO.

I believe you should sum up all the DC components fuses, and maybe add somewhere between 0% and 10%. The reason for adding 10% is to allow the individual guilty components fuses to blow - you wouldn't want to be operating normally and finding your underhood fuse popping, and not knowing what component was really causing it, if even a component, right?
But the reason some people add 0% is because of that theory mentioned above "you won't ever get close to any single component's fuse rating, in normal use - much less ALL the combined components drawing max amperage" - so even if you think along those lines, you'll probably still be fine and find your individual components would blow before your main line underhood fuse would pop.

I tend to prefer the "add 10%" method, because I also prefer the "use larger gauge power wire than you need to anyway" approach, and so I like to have plenty of extra current carrying capacity in the power wire, so that even adding 10% is still way below the max capacity of the wire.
Either way - you ALSO want to confirm that the sum total of all your DC-powered stuff back there (plus 10% or not!) doesn't exceed that maximum current capacity of that size/length of wire.

By using THAT number for your underhood fuse (or something close to it), you have a safer upper limit where that underhood fuse will pop, while still having enough of of a buffer that the underhood fuse won't pop even if you are cranking everything to max volume and drawing as much current as all those devices are designed to use, at most. I feel safer having a lower underhood fuse than one that might be questionably testing the maximum current capacity of a wire, leaving some "what if?" fear about a not-quite-short-circut (like an insulation worn thin by being routed near something metal like a seat slider or hinge) making the wire melt without the underhood fuse popping.

TL;DR-
In other words - if your amplifiers and processor fuses add up to 120A, and your wire can technically flow 300A without melting - it's still safer having a 150A fuse up front than it is having a 300A fuse up front... and that presents no limitations or penalty on your amplifiers or processors.