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This is ABSOLUTELY THE most important page on this site!

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 this first diagram, you have a simple system. You simply need to safely get the B+ (12v) line from the battery to the amplifier. One of the most important things to understand here is that wire segment A is not protected in any way. If it were to be shorted to ground, either the wire or the battery would be damaged and in either case there would likely be an electrical fire. This is why the wire must be short. 12-18" is the maximum recommended. The fuse protects the remaining length of wire. This generally extends from the front of the vehicle to the back of the vehicle. The fuse must be rated to protect the wire that's being used. The wire 'gauge' is generally the deciding factor when it comes to selecting a fuse. More information on the selection of the proper fuse will be covered on the 'Wire' page of the site. If the amp has no fuse on-board, you will need an external fuse. If you have only one amp, the fuse at the battery can be used to protect the wire and the amplifier. The fuse would need to be whatever was recommended by the manufacturer and would also need to be rated less than the suggested maximum current for the wire that you're using.

In the next diagram, things get a little more complicated. As you can see, wire segment 'A' is used to deliver power to the fuse within 18" of the battery. Wire 'A' is of a large enough gauge to power both amplifiers. 4 gauge wire is commonly used as a main power wire so it will be used as an example here. 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 but I generally recommend a 125 amp fuse with 4g wire. Fuse B protects wire segment C. Fuse C protects wire segment D.

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 distribution block. 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 distribution block. 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 distribution block for anything requiring more than a 60 amp fuse. AGU fuses greater than 80 amps are not available from the two 'real' fuse manufacturers (Bussmann and Littelfuse). There's likely a reason for that. The fuse configuration simply isn't well suited for greater than 60 amps of current. If you use an AGU fuse rated for more than 60 amps in a circuit that draws more than 60 amps for long periods of time, expect to have problems (melted fuse holders, fuses failing but not blowing in the center of the fusible element...).
     
  • 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 distribution block 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

Wire GaugeRecommended
Maximum Fuse Size
00 awg400 amps
0 awg325 amps
1 awg250 amps
2 awg200 amps
4 awg125 amps
6 awg80 amps
8 awg50 amps
10 awg30 amps
12 awg20 amps
14 awg15 amps
16 awg7.5 amps
These are the recommended maximum fuse ratings for the corresponding wire size. Using a smaller fuse than what's recommended here will be perfectly safe.

Fuse Opening Time:
A fuse does not blow when the current reaches its rated current. It is designed to pass its rated current without opening. A fuse will take varying times to blow under different conditions. A fuse will pass significantly more than its rated current for a very short time. It may take 10 minutes or more to blow a fuse at 25% over its rated current. The table below is an example of the specifications for a slow blow fuse. You can see that a 20 amp fuse may pass 40 amps of current for as long as 5 minutes before blowing although it probably wouldn't take a full 5 minutes to blow. The times for other fuses will be slightly different.

%of amp rating Opening time
110% 4 hours minimum
135% 1 hour maximum
200% 5 minutes maximum

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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 intermittently 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.

time/current graph

Plastic Fuses

Mini Fuses:
These fuses are 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.

ATC/ATO FUSES:
The fuses below are ATC/ATO fuses. The fuse on the left is a 40 amp fuse. The fuse on the right is a 15 amp fuse. These fuses (like most plastic fuses) are also color coded. The exact color may vary from one manufacturer to another but the basic color (blue, orange, red, yellow...) is indicative of the fuse's rating.

Too many people use the excuse that they didn't have a fuse holder when they were installing their equipment so they installed it without a fuse. Virtually everyone will have the push-on terminals that go on speakers. Those can be used with ATC/ATO fuses as a makeshift fuse holder. If you do this, use either a wire-tie or tape to hold the wires together about 4-6 inches from the fuse. This will reduce the chance that the terminals will pull off of the fuse. If your terminals are not fully insulated like the ones in the photo, use electrical tape to prevent them from making contact with anything else.

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.

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 older vehicles and mains powered equipment. 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 30 amp fuse. The fuse with the smaller fusible element is a 5 amp fuse. Although both of these fuses are similar physically, they are not interchangeable.

5mmx20mm fuses:
You probably won't find too many 5x20mm fuses in car audio equipment. 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.

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

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 (not water-tight but has a cover).

The following fuse holder is one of the better designs for a fuse holder of this type but the salesman told me that they still have more trouble with these than the open fuse holders. I have another sample ordered that isn't designed as well as this one. It's one that the shop won't use because it causes so many problems.

This is the end terminal. This one has heavy metal that grabs the fuse very tightly. This is good because it keeps the resistance low at the point where they make contact. Cheaper ones have thin metal 'fingers' that make contact with the fuse.

Many fuse holders use a set screw to hold the wire. This one uses a bushing to hold the wire. The wire is folded over the end of the bushing and then the terminal is threaded onto the bushing and wire. When tightened, it provides a good connection (typically better than a set screw).

The following images show the worst of the worst in terms of reliability. It uses the 'fingers' to bridge the connection from the fuse to the brass terminal. This will introduce resistance that will cause heating. The second and third images show the bushing that allows smaller wire to be used. The bushing just adds one more connection that introduces more resistance. The third image shows how the wire is damaged from overheating. The set screw was tight but it still overheated due to high resistance. The terminals on the previous fuse holder were much better than these and not that much more expensive (~$9 for the good one and ~$6 for the cheap one).

MAXI Fuse Holders:

There are several types of MAXI fuse holders. The best typically have screws to ensure that there is sufficient pressure on the tabs to keep the resistance as low as possible. The fuse holder in the photo below is an over-grown cousin to the standard inline ATC/ATO fuse holders. This is a very good holder but not fancy enough for most people installing a stereo in their vehicles.

The next image is the type most commonly used. The spring clips are not as tight as either the one above or the type with a screw but will work for most situations where the average current draw isn't near the fuse rating.

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 Bussmann® 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. (new photo coming soon)

Fuses of this size and configuration are referred to as either a MIDI, AMI or mini-ANL fuses. The one in the holder has a blue LED that illuminates when the fuse blows.

Mega®/AMG 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 you can see, there is no big fancy block of metal with set screws here. This type of fuse requires ring terminals. In commercial/industrial applications, this is a better option because there are fewer points of connection. You'll never see the bling type fuse holders where safety and reliability are of the utmost importance.

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.

You should remember:
1.NEVER replace a fuse with a fuse rated for higher current than the recommended fuse.
2.The physical size of a fuse is NOT an indicator of its current carrying capacity.
3.Just because a 30 amp fuse fits in the place of a blown 5 amp fuse, that does NOT mean that it's a suitable replacement for the 5 amp fuse.

 

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