Should distribution block fuses be used?!?

lexdiamond20 · 05-18-2007, 10:06 AM

Was just on Crutchfield.com and read this:

Q: Do I need a separate fuse block too?

A: A safe system will have the right fuses installed at each amplifier and also on the power cable by the battery. But if you've ever taken a peek at some competition-style car audio systems, you may have noticed fuses at a third location — in a fuse block by the components. Sure this hardware looks good, but is it necessary?

Unless your amplifiers are among the few that don't come with fuses installed, the answer is no. The reason you see these "redundant" fuses is that in the early days of car audio competition, amplifiers frequently didn't include their own fuses, or if they did, often hid them behind panels that were hard to get to, especially in the heat of competition.

Today, the use of such fuse blocks is largely a matter of style. But that's OK — looking cool definitely counts. In the world of car audio, it's a close third, right behind being safe and sounding great.

I currently have a fused distribution block getting 4ga wire from my battery (fused at the battery) and sending out two 8ga wires to power my amp and invertor. Though both components have onboard fuses, it was my understanding that the power wire coming form the distr. block to the components needed to be fused in order to prevent fire in the car. Should I stick with getting fuses for the distr. block or will they be a waste of time as the pasted text indicates?

Thanks in advance for the help.

rationalpi · 05-18-2007, 10:26 AM

That depends on a lot of things.. the length of the cables you are running as well as the amount of watts you are pulling thought.

you can find the rating of wire online as to how many amp you can push thought what distance of what gage wire. If you are close to maxing out on the wires that are coming of distribution block going to the amps, you might want to fuse it.

The reasons you put fuses on the wire is to prevent running to much current thought the wire, which can make it hot and cause fires. If you have a decent fuse/breaker by the battery, you don't really need the fuse distribution block by the amps, but having a bit extra security is never a bad thing.

In my install i am putting 150 amp breaker at the battery and using a none fuse distribution block. But i am still putting a 15 amp fuse on the opus power supply line, just in case.

lexdiamond20 · 05-18-2007, 01:13 PM

Well, there are 2 wires at 8ga each going to the amp and inverter. The run on each wire will be about 2-9 feet as all components will be in the trunk. Im assuming I wont need a fuse for such a short run. The amp is 400 watts and the inverter is 350 so there wont be much of an amperage pull.

Ill have to check out another site for calculations. Any suggestions in the mean time?

RedGTiVR6 · 05-18-2007, 05:13 PM

any time there is a change in wire gauge, I fuse.

It's the smart thing to do.

Buying a fuse block and a fuse is extremely cheap insurance.

I'm fairly surprised Crutchfield would post something like that...then again....Car Audio and Electronics has published some pretty stupid stuff too....

DarquePervert · 05-18-2007, 05:16 PM

Quote: Originally Posted by lexdiamond20

Was just on Crutchfield.com and read this:

Q: Do I need a separate fuse block too?

A: A safe system will have the right fuses installed at each amplifier and also on the power cable by the battery. But if you've ever taken a peek at some competition-style car audio systems, you may have noticed fuses at a third location — in a fuse block by the components. Sure this hardware looks good, but is it necessary?

Unless your amplifiers are among the few that don't come with fuses installed, the answer is no. The reason you see these "redundant" fuses is that in the early days of car audio competition, amplifiers frequently didn't include their own fuses, or if they did, often hid them behind panels that were hard to get to, especially in the heat of competition.

Today, the use of such fuse blocks is largely a matter of style. But that's OK — looking cool definitely counts. In the world of car audio, it's a close third, right behind being safe and sounding great.

I wholly disagree with Crutchfield. The fuses are there for a lot more than just "good looks".

The purpose of fuses on power lines is to protect the wiring, not the components. The fuse will blow, preventing higher-than-rated current from flowing through that power line, which could heat up the wire to the point of causing a fire, melting insulation and causing a short, and possibly worse.

Quote:

I currently have a fused distribution block getting 4ga wire from my battery (fused at the battery) and sending out two 8ga wires to power my amp and invertor. Though both components have onboard fuses, it was my understanding that the power wire coming form the distr. block to the components needed to be fused in order to prevent fire in the car. Should I stick with getting fuses for the distr. block or will they be a waste of time as the pasted text indicates?

The fuses onboard your components are there to protect the component only. If there is a short (I hope not!) that sends too much voltage down one of the lines, the component fuse will blow first, protecting the equipment. There could still be current flowing down the power line, and if that current is more than the wire can handle, you still have the fire risk.

hithere · 05-20-2007, 04:39 PM

Yeah, the mistake in the above by Car Audio is that it assumes the fault at or after the amp (indeed where most such faults occur in well-run wiring)...but a partial fault to ground along some 8-guage wire run to an amp of, say, even .1 ohms will cause quite a bit of localized heating near the fault, certainly enough to melt the nearby insulation away, and may not be enough to cause, say, a 200-amp fuse at the battery part. But then again, there's nothing that protects the larger gauge wire before that from doing the same thing except the battery fuse and the fact that the wire is rated for more current and thus more well-insulated against such heating.

For these types of faults, there's really not much you can do to prevent ALL danger...but prudent wire running using proper gauges as well as fused components will get you 90% of the way there. Fusing separate wire sizes adds a small amount of protection, but add protection it does.

I have a later issue of CA&E that says fusing is there to protect wire sizes as they get smaller.

rationalpi · 05-20-2007, 10:50 PM

The cable that run from the distribution block is short, should only be at most 3 - 4 feet. which can support way more amp then the 20+ feet 4 gauge wire. So the fuse or breaker you have set up on you main power wire should be more the enough to keep the wire from causing a fire.

So as far as it being necessary, it's not. And in the current system i am building i am not putting them in because well the price for the none fused distribution blocks was just way better. But having the fuse will never hurt. And you know what, it does look nice..

hithere · 05-21-2007, 05:40 AM

Quote: Originally Posted by rationalpi

The cable that run from the distribution block is short, should only be at most 3 - 4 feet. which can support way more amp then the 20+ feet 4 gauge wire. So the fuse or breaker you have set up on you main power wire should be more the enough to keep the wire from causing a fire.

What does length have to do with it?

Caelric · 05-21-2007, 06:03 AM

The longer the wire, the greater the overall resistance of that wire, and the less current it can handle.

As for fused distro blocks, sure, they are not REQUIRED, but a fuse at the battery is not REQUIRED for a system to run, but it sure as heck offers a great deal of safety. A fuse at the distro block offers safety as well.

So, the question really is whether your car and audio setup is worth the extra $50 or so price premium that a fused distro block is over a regular distro block. My choice is that my car is definitely worth the extra $50 for the added safety.

Dave

hithere · 05-21-2007, 08:40 AM

Quote: Originally Posted by Caelric

The longer the wire, the greater the overall resistance of that wire, and the less current it can handle.

As length of wire increases, so does volume, and surface area over which heat is dissipated. At the same current, whether the wire is ten feet or ten inches, it will dissipate the same heat per unit volume or surface area, and thus fail at or very near the same current level.

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.

rationalpi · 05-21-2007, 08:52 AM

Quote:

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.

Caelric · 05-21-2007, 09:04 AM

hithere · 05-21-2007, 09:22 AM

Quote: Originally Posted by rationalpi

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.

Caelric · 05-21-2007, 09:38 AM

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)

rationalpi · 05-21-2007, 09:48 AM

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.