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Thread: Should distribution block fuses be used?!?

  1. #11
    Newbie rationalpi's Avatar
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    As a matter of fact, the longer the distance to fault, the more resistance in the wire, and thus, the less fault current the wire will experience. A longer wire is somewhat safer in this regard.
    You said it yourself there, "more resistance". Which directly mean more heat. Basic physics. Resistance produces heat. Ever touch a resistor. They get hot.

    (Copper wire)
    gauge - resistance per foot
    4 - .000292
    8 - .000739

    4 feet of 8 gauge will have 4(.000739) = 0.002956
    20 feet of 4 gauge will have 20(.000292) = 0.00584

    Put it at close to double the resistance. hence almost 2 twice as much current can run thought the short 8 gauge. Which means the the fuse on by your battery on the 4 gauge will blow before you even come close to pulling the max current for the 8 gauge.
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  2. #12
    Maximum Bitrate Caelric's Avatar
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  3. #13
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    Quote Originally Posted by rationalpi View Post
    You said it yourself there, "more resistance". Which directly mean more heat. Basic physics. Resistance produces heat. Ever touch a resistor. They get hot.
    You said it yourself...per foot resistance. Current handling has nothing to do with the length of the wire. You're confusing the application of the gauge of the wire, which determines the current it can carry for any length, with the application of the length of the wire, which determines the overall resistance of the wire as applied. Resistivity is different from resistance.

    Resistance does not produce heat, power does, in this case, a current through a resistance. The ability of a substance to dissipate that power safely is dependent on its material construction, it's size and shape, and the medium surrounding it (air, metal, etc.).

    A wire is typically manufactured with a set of parameters that is as close to uniform throughout its length as can be provided cost effectively.

    Example: I connect a battery at 12 volts to ground through 1 foot of a wire that carries (for ease of math) one ohm resistance per foot of wire. The battery will produce in the wire 12 amps of current, and these 12 amps will yield (I^2 * R power dissipated as heat) 144 watts of heat. This heat will be dissipated over 1 foot of wire, yielding 144 watts per foot.

    If I connect the battery to two feet of the same wire, I get double the resistance, and only 6 amps of current flows. In this example, I get 72 watts of power expressed as heat dissipated over two feet of wire, yeilding 36 watts per foot of wire...much less heat energy for each foot of the wire to dissipate.

    In other words, as I increase length, I increase resistance...but I decrease the amount of power produced in a fault situation. Go ahead and use the specs you provided if you don't believe me: As you increase the length, the amount of current produced becomes less, and less, because, as you say, it has resistance per foot, and each foot adds to the wire's total ability to dissipate heat, in the same way a larger heat sink cools a processor better due to having more surface area to dissipate heat over.

    The only reason we don't use power per unit length/volume/surface area to describe what capacity a wire can handle is that it is much, much easier to specify it in terms of amperage...amperage is taken to be the same throughout the wire as it is conducted, and a given current will produce a given amount of heat per unit length/surface area/volume, and current is less likely than power to be confused (between power at the load which changes with voltage applied, power dissipated as heat, rated power of the load, etc).

    As a third example, let's take things to extremes: We both know that air itself can be made to conduct electricity (we've both, I presume, seen lightning). However, air has an extremely high amount of electrical resistance (and it isn't ohmic in nature). If, as you say, "high resistance equals high heat", then why haven't you produced heat of fusion by charging a car battery and exposing it to air? Answer: The high resistivity of air is such that very, very little of the battery's power is siphoned off simply by it's exposure to it at the terminals. You have to leave a battery sit a while before such leakage currents will kill it.

    Now, if we were to hold current to be the same through many different lengths of the same wire (this is not what happens in a fault situation in a car, the battery is, or tries to be, a source of constant voltage, not current), we have a situation where more heat is produced the longer you make the wire....however, according to (I^2 * R), the amount of power produced varies linearly with resistance, which in this case varies linearly with length...but the ability to dissipate this heat safely also varies linearly with length. In this case, everything having to do with the length of the wire drops out of the equation. Same current, More resistance, more heat...but more wire to dissipate the heat through.

    In a fault situation in your car, the battery will attempt to hold the same voltage (but will likely fail miserably), and the amount of current produced by the battery will shoot towards some maximum (representing the battery's maximum ability to react chemically to produce current), where the slope of the current rise will generally be determined by the resistance of the wire/fault, and the sharper the slope, the more quickly the fuses involved will part.

    A shorter cable to your amp is desirable so that you don't induce an undue voltage drop before feeding the loads you put on the end of the wire...in a fault situation, however, the longer the distance to fault, the less fault current and the less heat produced.

    The numbers in this example are exaggerated for ease of use, but the principle is the same.
    I have too much time and too little aggravation in my life, so I built a carPC. ;)

  4. #14
    Maximum Bitrate Caelric's Avatar
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    Ok, I was mistaken in my comment. The length of the wire does matter, because a longer wire has more chances for it to get frayed and then short to ground. A longer wire also most likely goes through confined spaces more often (firewall, etc...) which might cause a break through the wire insulation, and thus a short to ground.

    However, you are correct in saying that a longer wire has more ability to dissipate heat, but if there is a short to ground, that doesn't really matter. I was thinking that you want to use a thicker gauge wire on a long run, and you do, because there is a voltage drop for a given current, or conversely, it will draw more current for a given voltage because of the increased resistance of a longer wire.

    However, that really has nothing to do with whether a given wire should be fused or not. The real question is whether a distro block should be fused, and as I stated earlier, I believe yes, it should, as it is cheap insurance for a pricey investment (car, audio system, carPC)
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  5. #15
    Newbie rationalpi's Avatar
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    the number you are showing are actually going to back up what i was saying.. See the problem is that as the wire length gets longer, the amount of watts you need to go thought the wire doesn't drop, so you still need x amount of watts to power your amp. Which the longer wire, as your number go to show, can't support.

    There is a really good example of this on weldingsupplys.com about 1/4 of the way down the page, they have wire length and the amperage they can support. based on gauge and distance.

    So as you can see 100 amps can be carried 50 feet on 4 gauge, while 0/4 can carry that same amps 350 feet. and the 0/4 gauge can carry 550 amp on 50 feet of wire.

    So this goes to show that was i was saying earlier. To be on the safe side, you have to fuse your wire at the battery. And as long as the wire is fused at the battery, your distributed wire is save as well. This is because your short wire can support way more amp then your long wire running to your distribution block. IF you have the proper fuse/ breaker on your main power wire, you don't have to worry about fusing your distribution block. And just make sure that the amps and your pc power supply are fused to protect themselfs.
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  6. #16
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    Quote Originally Posted by rationalpi View Post
    the number you are showing are actually going to back up what i was saying.. See the problem is that as the wire length gets longer, the amount of watts you need to go thought the wire doesn't drop, so you still need x amount of watts to power your amp. Which the longer wire, as your number go to show, can't support.
    We're talking about fusing for fault situations...if you have to fuse based on current draw from a properly connected amp being a danger, then you're using the wrong gauge wire for the amp to begin with. On welding supplys.com, they're suggesting higher gauges based on length in support of the load and not dissipating power needlessly, like the voltage drops in a thin gauge wire to an amp (note the title of the chart: it's "Suggested", not "Rated"); this has nothing to do with fault current should the wire short to ground, where it is indeed impossible to predict on a chart what the current could potentially become (we don't know where the fault is)...I would make the same suggestions they do. Welders have high electric bills as it is. What they're saying to customers is, "Yes, it's cheaper to buy thinner gauge wire, and it might support your load, but you'll be wasting more power the longer you make it."
    I have too much time and too little aggravation in my life, so I built a carPC. ;)

  7. #17
    Newbie rationalpi's Avatar
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    You don't fuse to protect the amps. they are already fused to what they should be at internally. When you fuse the wire, you are fusing for current draw to prevent shorts in the wire from catching your car on fire.

    When you ground a live wire to create a short, there is a pull of as much current as the wire can handle. That's why you fuse the wire.

    So no matter where the fault happends, you have a fuse for your gauge wire at the battery.
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  8. #18
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    Quote Originally Posted by rationalpi View Post
    You don't fuse to protect the amps. they are already fused to what they should be at internally. When you fuse the wire, you are fusing for current draw to prevent shorts in the wire from catching your car on fire.

    When you ground a live wire to create a short, there is a pull of as much current as the wire can handle. That's why you fuse the wire.

    So no matter where the fault happends, you have a fuse for your gauge wire at the battery.



    Though, unfortuantely, not all amplifiers are fused internally...just another reason to purchase high quality car audio amplifiers.

    Always check your amplifiers before installing to check if they are fused internally.
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  9. #19
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    I think we're really getting off track when it comes to supporting various wire gauges, however.

    In reality, the type of faults where the wire grounds out on a flat metal floor pan , where the wire's entire ability to dissipate heat is put to the test, are not the faults we need worry about. In these situations, the question becomes "will the smaller gauge wire burn up before the big fuse off the battery does?" The answer is most likely no for anything but the largest fuses and the smallest of wire sizes.

    The types of faults we need to worry about are ones that produce localized heating, where the wire goes to ground through a resistance that is very small, but nevertheless can be orders of magnitude more resistant than the wire (a knife edge on a body panel, a welded piece that's not directly connected to the floor pan of the vehicle, a less conductive material, etc.). In these situations, you end up with a high current flow, perhaps not enough to melt wire insulation or pop the big fuse near the battery, but one that produces a high degree of localized, concentrated heating very near the fault, due to the area of the fault being the principle voltage drop in the voltage divider produced by wire and fault. This type of localized heating is much more likely to cause a fire than is general failure of the wire throughout it's length. In these situations, the more fusing, and the smaller the fusing, the better...but there's nothing in these scenarios that suggests changes in wire size are the places to apply fusing.

    So, I don't really know: Is wire size changes the metric we use to place more fusing? I think such fusing offers a slightly higher degree of protection, but I also think that we, along with trade magazines and the like that talk about load as if it were a metric for fusing are mistaken. It's not the load (amp, carputer, etc.) that determines current in a fault, nor is it the wire gauge that necessarily accounts for the body of the resistances and currents involved.

    Fact is, there's no way to account via fusing for every possible fault without compromising your ability to perform the work involved (drive an amp, etc.). Using a larger fuse near the battery along with individual equipment fuses, provided you're using the proper gauge wires to begin with, will likely get you 99% of the way there in terms of safety, and the additional fusing provided by distro blocks is likely some small percentage on top of that.

    Quote Originally Posted by rationalpi View Post
    When you ground a live wire to create a short, there is a pull of as much current as the wire can handle. That's why you fuse the wire.
    Do the math, and you'll find that Ohm's law will yeild heat of fusion with a car battery and some thick copper wire going to ground...so why hasn't the car stereo hobby produced any craters the size of a small town, and why isn't the military producing bombs made of car batteries and copper cable? Point is, there's another limiting factor that will likely be reached before the wire's sustainability will come into question. Although on paper, a car battery looks like a constant 12-volt source, it is anything but when it comes to providing current to an extremely low resistance. Sooner than later, a fault causes the battery's maximum ability to chemically produce current to come into question. Ohm's law simply does not apply the way it does on a high school physics test in a situation like this. The main question we are faced with is how localized the heat produced is, and it's localized heating, not the heating dissipated throughout the length of the wire, that needs to be worried about the most.

    There's nothing you can do to fuse all the possibilities away.
    I have too much time and too little aggravation in my life, so I built a carPC. ;)

  10. #20
    Maximum Bitrate Caelric's Avatar
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    Quote Originally Posted by hithere View Post
    Fact is, there's no way to account via fusing for every possible fault without compromising your ability to perform the work involved (drive an amp, etc.). Using a larger fuse near the battery along with individual equipment fuses, provided you're using the proper gauge wires to begin with, will likely get you 99% of the way there in terms of safety, and the additional fusing provided by distro blocks is likely some small percentage on top of that.
    .
    Well, you could always just use a chain of fuses rather than wire; that would account for every possible fault. Obviously, though, thats very unrealistic.

    However, I think having a fused distro block is a small price to pay for the added protection, especially given your scenario of:

    The types of faults we need to worry about are ones that produce localized heating, where the wire goes to ground through a resistance that is very small, but nevertheless can be orders of magnitude more resistant than the wire (a knife edge on a body panel, a welded piece that's not directly connected to the floor pan of the vehicle, a less conductive material, etc.). In these situations, you end up with a high current flow, perhaps not enough to melt wire insulation or pop the big fuse near the battery, but one that produces a high degree of localized heating very near the fault, due to the area of the fault being the principle voltage drop in the voltage divider produced by wire and fault. This type of localized heating is much more likely to cause a fire than is general failure of the wire throughout it's length. In these situations, the more fusing, and the smaller the fusing, the better...but there's nothing in these scenarios that suggests changes in wire size are the places to apply fusing.
    The fused distro block will probably protect against this, if the short is one of the wires from the distro block to the amp(s), assuming the fuses at the block are smaller than the main battery fuse (as they should be!)

    Again, it's all a matter of price versus return on investment, and to me, it's a small price to pay.

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