Fuses: I've been in the electronics repair business since about 1986 and have come to believe that most people don't understand the function of a fuse, or they just like to let the smoke out of electronic devices (transistors, resistors...).

Note: Letting the smoke out of an electronic device is a process which converts a useful piece of electronic equipment into a paper weight.

Function: A fuse is generally inserted into an electrical circuit for 1 of 2 reasons, either to protect the power source which includes the wire that connects the power supply to the electrical device, or to protect the electronic equipment. The electronic equipment manufacturers specify a fuse rated to open the electrical circuit before damage can be done to the device or open the circuit if the electronic device fails in some way (electronic devices may pull excessive current when they fail). If a fuse larger than the specified fuse is used, a small mistake when installing the equipment may cause catastrophic failure of the equipment. WHEN, not if, WHEN you're thinking of replacing a blown fuse with a higher rated fuse ask yourself if you know more than the engineer who designed the equipment. Don't get in a hurry when installing electronic equipment. Take the time to go get the right fuse. 50 cents for a fuse is better than $50 labor plus the cost of the replacement parts for a repair job.

Example: In the diagram below, you see that there is a fuse between the battery and the amplifier. In this configuration, the fuse can be used to protect the wire and the amplifier. If the fuse is the proper one for the amplifier, all you have to do is make sure that the wire segments 'A' and 'B' are rated to pass more current than the fuse and you'll be OK. Wire segment 'A' must be as short as possible because it is NOT protected by the fuse.

In the next diagram, things get a little more complicated. As you can see, wire 'A' is used to deliver power to the distribution block. Wire 'A' is of a large enough gauge to power both amplifiers. 4 gauge wire is commonly used as a main power wire. Fuse 'A' must be rated to protect wire 'A'. Any fuse rated at less than ~150 amps is sufficient to protect a 4g wire and the vehicle (if the 4g wire is longer than ~15 feet long, you may want to limit the fuse to ~125 amps). Again, fuse 'B' protects wire 'B' and fuse 'C' protects wire 'C'. Wire segments 'X' and 'Y' MUST be as short as possible because, unless they are of the same gauge as wire 'A' (or larger), they could be a fire hazard.

In most cases, the wire size is reduced at the point of distribution. ANY time that the wire size is reduced, you must add a fuse in the line (at the point of distribution) to protect the smaller wire. Look at the following for more detailed info about changing wire sizes.

What you should know about the image below (Please excuse any redundancy. This is important.):

• Wire "A" is unprotected which is why you want the main (125 amp) fuse as close to the battery as possible. If this length of wire gets shorted, it WILL burn.
• Wire "B" is protected by the 125 amp fuse. It has to be at least 4g. Anything smaller wire would possibly burn before the fuse would blow in the case of a short circuit to ground. This wire feeds all of the power to the distribution block.
• Wire "C" is supplied power from the dblock. Since it's also a 4g wire, it needs no fuse. The main wafer fuse will protect it and the vehicle in case it's shorted to ground.
• Wire "D" is a smaller wire. It's also supplied power by the dblock. Since this wire is smaller (let's say an 8g wire), the main fuse can not protect against a short circuit. If the 8g wire shorts to ground, it will burn (and possibly take the vehicle with it). This would be a very unsafe condition.

• Fuse "A" is protecting wire "E". If the wire is an 8g wire and the fuse is rated at 50 amps or less, the wire is properly protected in case of an accidental short circuit.
• Fuse "B" is protecting wire "F". The wire is a 6g wire so it can handle more current than the 8g wire. Any fuse rated at 80 amps or less will protect the wire in case it's shorted to ground. AGU fuses aren't available for anything more than 60 amps so you'd have to have a MAXI-fuse dblock for anything requiring more than a 60 amp fuse.
• As you can see, wire "G" is a small wire. Some people might want to power a crossover (or other low current device) in the rear of the vehicle. If the wire was a really small wire like a 16g or 18g wire, there would be no suitable AGU fuse to protect it. In this situation, you'd have to have a secondary fuse holder with a 7.5 amp (or smaller) fuse to protect the wire. A better solution would be to use a 14g (or larger wire) and a 10 amp AGU fuse.
• Sometimes, people will insert a small wire (wire "I") in with one of the other larger wires (wire "H") to power the crossover. Since the port that it's tapped off of is likely powering an amplifier, the small AGU fuse is not an option. Since the port already had a relatively large wire in it (because it's powering an amp), the wire tapping off of the shared port probably can't be very big. This smaller wire would absolutely have to have a dedicated fuse holder as close to the dblock as possible. If wire "I" was shorted to ground and the dedicated fuse holder was absent, wire "I" would burn.

If anything is unclear about this section, please email me and I'll try to clarify. As I said above, this is the most important page of the site. I want everything to be perfectly clear.

Using multiple small wires in place of a larger wire: Some people may want to use a bunch of smaller, individually insulated, wires (like ten 14g wires) in place of one larger wire (like a 4g wire). This may be OK as far as current carrying capacity is concerned but the problem comes in when you have to fuse it. A 4g wire can handle about 125 amps. A 14g wire can handle about 15 amps. If one of the strands of the 14g wire is shorted to ground (like where it runs through the firewall), the main 125 amp fuse would not blow and the wire would burn. To properly protect the multiple strands of insulated wire, you'd have to use ten 15 amp fuses in individual holders (each wire would have its own fuse). I know that this may be an 'off the wall' situation but I've had several emails about this (generally concerning two or three 8g wires and a large wafer fuse) so there are, at least, a few people who don't fully understand this.

Suggested Fuse Sizes:

This graph lets you see the estimated time that it might take for an ANL type fuse to open. The data used to create the graph was found on the Bussmann^® web site. You can see that it would take as much as 10 seconds to blow at double its rated current flow but it would only take approximately 0.1 seconds to blow it if 1000 amps of current passed through the 100 amp fuse. If your amplifiers would intermittantly draw 200 amps, the fuse probably would not blow. If they drew 200 amps for more than 10 seconds, the fuse would likely blow. If the wire was shorted to ground, the current flow would be extremely high (probably more than 1000 amps) and the fuse would blow almost instantly.

Graph of opening time vs current flow through a 100 amp ANL type fuse.

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Plastic Fuses

Mini Fuses:

These fuses are becoming more widely used in automobiles and head units. They are available in sizes ranging from 2 amps to 30 amps. As with all of the plastic fuses, the color of the plastic casing is an indicator of its rating. The fuse in the following picture is a 7.5 amp fuse.

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ATC/ATO FUSES:

The fuses below are the newer style ATC/ATO fuses. One fuse is a 30 amp fuse, the other is a 3 amp fuse. Notice the element width in each.

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MAXI FUSES:

The image below shows a Maxi fuse and the more common ATO/ATC fuse (for size reference). The Maxi fuse is available in current ratings between 20 and 80 amps. These fuses are, in my opinion, the best choice for systems/amplifiers drawing between 40 and 80 amps of current.

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Glass Fuses

AGC FUSES:

The Image below shows 2 glass AGC type fuses. These fuses measure 1¹/₄" X ¹/₄". This type of fuse is commonly used in the power wires of the head unit. You should notice that their physical size is the same but their 'fusible elements' are VERY different. The fuse with the wider element is a 10 amp fuse. The fuse that has a very small element (hard to see) is a 3 amp fuse. Although both of these fuses are similar physically, they are NOT (NOT) "NOT" interchangeable.

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5mmx20mm fuses:

You probably won't find too many 5x20mm fuses in car audio equipment designed for use in the US. The smallest fuse in the next image is a 5x20mm fuse. These are much more common in home audio equipment than car audio equipment but I included them so that you will know what they are if you encounter them in the future.

AGU fuses:

The largest fuse in this image is the glass AGU fuse which measures 1¹/₂" X ¹³/₃₂". These fuses are available in current ratings between 10 and 80 amps. AGU fuses can be used to protect the main power wire of small to moderately high power systems. They can also be used to protect amplifiers capable of producing up to about 750 watts RMS.

From left to right: 5x20mm, AGC and AGU fuses

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Poor Quality AGU fuses:

The biggest problem with the AGU fuse is the quality. I am NOT saying that there are no good quality AGU fuses but there are a large number of manufacturers that make fuses of questionable quality. The lower quality fuses have a problem with the solder that connects the end caps to the fusible element. There is too much resistance in the solder connection which (with current flow close to their rated current) creates enough heat to melt the solder connection which leads to a melted fuse holder or a fuse with higher than normal resistance. The higher resistance will cause more of a voltage drop than a new fuse. If you can find some BUSS or LITTELFUSE brand AGU fuses, you won't be as likely to have problems. All of these problems can be alleviated by using Maxi fuses in the first place.

AGU fuse holders:

The problems listed above may be compounded if a poor quality fuse holder is used. I would strongly warn against using in line waterproof fuse holders. I said earlier that the AGU fuses may have a problem with heat buildup in their end caps. Using a fully enclosed waterproof type of fuse holder will prevent heat from escaping and will make the problem even worse. Using an open type of snap in fuse holder MAY help a poor quality fuse function properly. Using a good quality fuse in an open type snap in fuse holder will very likely work precisely as it should with little or no chance of overheating. The following is an open fuse holder. There are 2 quick connect terminals (inside yellow circles) and 1 screw terminal on each side of the fuse. The screw down connections are preferred for high current applications. The quick connect terminals will lead to an increased chance of overheating (due to higher terminal resistance).

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ANL/ANE Fuses:

ANL type fuses are the most common types of fuses used to protect the main power wire in car audio systems. ANL fuses are available from Bussman^® in sizes from 35 amps to 750 amps. The following picture shows a 150 amp ANL type fuse. The second picture shows the fuse holder for the fuse. You can see the 2 screws on the top of the block under which the fuse would be fastened. You can also see one of the set screws (on the side of the block) that holds the power wire in the holder. There is also a bushing (can't be seen in this picture) in the hole where the wire resides. The bushing allows a smaller wire to be securely held in the terminal block. If you use large gauge wire (4g), you remove the bushing so that the larger wire will fit in the block. The fuse is shown larger than actual size. The fuse holder is approximately actual size.

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Mega^® Bolt-on Fuses:

These fuses are not really popular in car audio but are widely available and may be an option for high current protection in the main power line (from the battery). They are available in sizes ranging from 100-250 amps. As far as I know, they are only made by the Littelfuse^® company. The fuse is about 2.68" long.

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Note on Blown Fuses

When a fuse blows (even one that has been perfectly capable of handling the current requirements of a given piece of equipment), many people will replace it with a fuse of equal size without thinking (which isn't necessarily a bad thing). Then, if it instantly blows again, they go to the next larger fuse size (now, that IS a bad thing). They don't think for a minute that the fuse that just blew was the same as the one that worked fine for a long time. They don't think that something just might have changed which is now causing the amplifier to pull significantly more current.

If you have a piece of equipment (especially an amplifier) that has played fine for a while with a given size fuse and the fuse blows, do not replace it with a larger fuse. I'd actually suggest temporarily replacing it with a fuse ~1/2 the rating of the one that blew. If you have a 200 watt amplifier that's been running fine with a 30 amp fuse and the fuse suddenly blows, replacing it with another 30 amp fuse is fine but I'd suggest trying a 15 amp fuse first. With the volume set to its minimum position, the amp should power up and idle with the smaller (lower rated) fuse. It should also play cleanly at a low to moderate volume. If the fuse blows with no volume, there is very likely a problem with the amplifier. If it does not play cleanly, there may be a problem with the speaker(s) or wiring. If it blows the fuse at very low volume, there is most likely a shorted speaker or a short in the wiring.

For amplifiers... What happens many times is that a component fails (often an output transistor) and the fuse blows. Since the fuse has likely been passing a fair amount of current (remember, the amplifier was playing loud enough to fail) the fuse's element was at a higher than ambient temperature and allowed the fuse to blow relatively quickly. When you insert a new (cool) fuse, it will take more to blow it even if it was identical to the one that originally blew. If the power supply components in the amp are operating at the upper end of their safe operating area (common with budget amplifiers), the new fuse might not blow before the power supply components (remember that the amplifier probably has a shorted output transistor). If the power supply components are destroyed (along with the output transistors), the repair bill may be significantly higher (maybe $100 instead of $60). Checking to make sure that everything is OK with the smaller fuse will assure you that no more damage is done.