all about automotive car fuses

Know-How Notes: Automotive Fuse Guide

When you have a bad temper, people say you “have a short fuse”, in the world of automotive electrical components, a short fuse could mean a mini-ATC (technically ATM), GMA, or even an AGA style fuse. If you are not a regular in the automotive fuse section at your local NAPA store, you are probably scratching your head at the alphabet soup in that last sentence. Not to worry, we are here to help you make the connections you need without bogging down in the mire of complex electronical jargon.

A fuse is a mechanical breaker device that protects your vehicle’s electrical components and wiring in the event of a fault. A fault can be anything from a dead short (where the positive voltage shorts to ground), to a damaged component. When faults happen, the power feeding the component must be disconnected immediately. The most common cause of automotive fires is electrical shorts. Pulling too much amperage through a wire always leads to that wire getting hot, and eventually it catches fire. If you have ever experienced a wire fire, it is quite scary, especially if you are actively driving.

Auto makers design circuit breakers into every component in your vehicle, and most of them are housed in one or two central locations. These are the fuse boxes. Some vehicles have individual fuse boxes in various locations, but most vehicles use a centralized master fuse box, much like the breaker box in your home. There are many types of circuit breakers, ranging from sacrificial (fuses) to resettable (circuit breakers), and then there is the classically dreaded fusible link, which is the bane of many a mechanic. This article will address and identify the most common types of automotive fuses that you may encounter.

Sacrificial Circuit Breakers (Fuses)

All of these fuse types are destroy themselves in the process of breaking the circuit. This is done through a piece of metal that melts at a certain temperature point.

Glass Fuses

Most American vehicles went away from glass fuses in 1982. These fuses are rated for 32v DC and cover from 4 amps up to 30. There are numerous designs for automotive glass fuses. SFE and AG* type fuses are fast-blow fuses, they pop as soon as there is more draw than they are rated for. Slow-burn fuses, like the MDL type, are timed fuses.

Various glass fuses have different diameters and widths, depending on the style. These SFE fuses have different widths so that they only fit the appropriate circuit.

Various glass fuses have different diameters and widths, depending on the style. These SFE fuses have different widths so that they only fit the appropriate circuit.

SFE – These are the original glass fuses for automotive use. SFE stands for Society of Fuse Engineers, and have different lengths so that the wrong fuse cannot be installed into the wrong placement. They are ¼” in diameter, from 5/8” up to 1 7\16” long.

Outside of SFE fuses are AG type, which stands for Automotive Glass with a suffix letter to note the type.

AG Type Fuses
TypePhysical Size
AGA (1 to 30 amp)1/4” diameter x 5/8” long
AGB.177” diameter x .588” long
AGC (0.125 to 50 amp)1/4 diameter x 1-1/4” long
AGS9/32” diameter x 1-1/4” long
AGU (1 to 60 amp)13/32 diameter x 1-1/2” long (sometimes referred to as Midget fuses)
AGW (1 to 30 amp)1/4” diameter x 7/8” long
AGX (1 to 30 amp)1/4” diameter x 1” long
AGY (50 amp only)1/4” diameter x 1 7/16” long
UK (35 to 50 amp)1/4” diameter x 1 1/4” long

MDL – This style is similar to the AGC fuse in size and rating, however AGC fuses are “fast blow”, meaning they pop instantly, whereas MDL fuses are time-delay fuses that burn slowly. This allows the MDL fuse to resist higher amperes for very brief moments, such as a high-draw from a compressor or fan kicking on.

This AGU fuse is a 40 amp, notes on the side. The high-amp glass fuses can melt the solder before the actual fuse pops.

This AGU fuse is a 40 amp, notes on the side. The high-amp glass fuses can melt the solder before the actual fuse pops.

Barrel (Non-Glass)

Bosch – Used in older European vehicles, these plastic barrel fuses have exposed metal fuse link. They
install similar to a AA-type battery between two flat springs.

Bosch Fuses
ColorRating
Yellow5 amp
White8 amp
Red\Green16 amp
Blue25 amp
Grey\Black40 amp
These Bosch barrel fuses are used in European cars, through the 1990s. Photo courtesy of <a href="https://commons.wikimedia.org/wiki/File:Electrical_fuse,_Bosch_type.png" target="_blank" rel="noopener">Wikimedia Commons</a>.

These Bosch barrel fuses are used in European cars, through the 1990s. Photo courtesy of Wikimedia Commons.

Lucas – The much maligned Lucas wiring system is used on British vehicles, and of course they use different fuses from everyone else with a convoluted rating system. These are ceramic barrel fuses similar to the Bosch style. Where many people go wrong is confusing the “continuous fusing” rating (where the fuse pops) with the smaller rating of “continuous amps”, which is where fuse operates normally. When replacing a Lucas fuse with a non-Lucas glass-type fuse, you use the smallest rating for the new fuse.

Lucas Fuses
ColorContinuous ampsInstantaneous fusing ampsContinuous fusing amps
Blue1.53.53
Yellow2.2554.5
Red on Yellow2.565
Green376
Nut Brown4108
Red on Green51210
Green on Black51210
Red on Brown61412
Light Brown7.51815
Pink12.53025
White17.54035
Purple & Yellow256050
Yellow & Red307560
Lucas fuses are confusing to diagnose. This is a 50-amp model. They are labeled with paper inside the glass, which makes it hard to see if the fuse is still good. This one is an original fuse that came out of a 1964 Jaguar XKE

Lucas fuses are confusing to diagnose. This is a 50-amp model. They are labeled with paper inside the glass, which makes it hard to see if the fuse is still good. This one is an original fuse that came out of a 1964 Jaguar XKE

Blade-Type

Micro2 – Also known as APT or ATR, these fuses have ratings from 5 up to 30 amp. Measurements are 9.1l × 3.8w × 15.3h mm

Micro3 – Also known as ATL, these 14.4 × 4.2 × 18.1 mm fuses run from 5 to 15 amp

LP-Mini/APS/ATT – Fuses measure 10.9 × 3.81 × 8.73 mm, and are rated from 2 to 30 amp.

Mini/ATM/APM – Fuses measure 10.9 × 3.6 × 16.3 mm, 2 to 30 amp.

Standard/ATC/ATO/APR/ATS – Fuses measure 19.1 × 5.1 × 18.5 mm, .5 to 40 amp.

Maxi/APX – These much larger fuses (29.2 × 8.5 × 34.3 mm) can hold more amps than standard or mini fuses. 20 to 120 amp.

The classic ATC fuse is a plastic blade type. The amperage is noted by both the color and printed on the top. You can test these fuses through the exposed tabs with a meter.

The classic ATC fuse is a plastic blade type. The amperage is noted by both the color and printed on the top. You can test these fuses through the exposed tabs with a meter.

 

This is the big boy MAXI fuse, these are used for high-current applications.

This is the big boy MAXI fuse, these are used for high-current applications.

Cartridge

These are generally used for higher voltage applications and are time-delayed. Styles include MCASE 32V, JCASE low profile, and JCASE 32V. They range from 15 to 60 amp ratings.

High-Current

When you need to manage high current, you need a big fuse. These fuses are large and generally mount with bolts or in some case, large blades.

MEGA/AMG/BF2 – Current ratings from 60 to 275 amp, these are used for starters, alternators, and batteries. They are a bolt-down fuse and are slow blow. These fuses measure 19 x 68 mm.

AMI/MIDI – These bolt-down fuses look just like MIDI fuses but are about half the size. Ratings range from 23 up to 200 amp. These slow blow fuses measure 10mmx 41mm.

ANL – These fuses measure 20 x 82 mm and cover a range from 20 to 500 amp. Unlike the other bolt-down fuses, this design has slotted ends, so the bolts do not have to come out completely.

ANL fuses are also used for high-current applications, and can go up to 500 amps.

ANL fuses are also used for high-current applications, and can go up to 500 amps.

MRBF/ZCASE – This is a single MEGA type fuse that is used for very high current application, most commonly the starter. It is mounted with a single bolt. Fuse ratings range from 40 to 250 amp.

Class-T – commonly used for EV and hybrid vehicles, the class-T fuse is a high voltage fuse. These are available in amperage ratings from 10 up to 50-amps, but the voltage capacity is much higher than 12-volts, as these are specifically for the battery packs, which are in the 250 to 450-volt range. Class T fuses are fast blow, and have the fastest blow speed of all high current fuses.

Color Coding

All non-glass sacrificial fuses are color coded for their amperage. The codes change based on the fuse type. The most common fuses you will encounter are blade type, so we have the basic color codes for them here:

Blade Fuses
ColorRating
Dark Blue0.5 amp
Black1 amp
Gray2 amp
Violet3 amp
Pink4 amp
Tan5 amp
Brown7.5 amp
Red10 amp
Blue15 amp
Yellow20 amp
Clear25 amp (Grey for MAXI)
Green30 amp
Blue/Green35 amp (Brown for MAXI)
Orange40 amp
Red50 amp
Blue60 amp
Amber/Tan70 amp
Clear80 amp
Violet100 amp
Purple120 amp

You will notice that the color do repeat. In most cases this is because the fuse types have limits, such as a MAXI fuse does not go below 20 amps.

Resettable Circuit Breakers

Circuit breakers come in two types: manual or automatic reset. Manual reset breakers are mostly used for aftermarket applications such as car audio or lighting. Some manufacturers use auto-reset breakers for certain devices that have high current draw during start-up, such as cooling fans or electric motors. The benefit of an auto-reset breaker is that they reset themselves in the event that the initial draw is too high due to heat or other issues. This means your cooling fans will still work if they experience a brief high-draw issue, without have to replace the fuse. An auto-reset breaker is not a good idea for electronics because it will constantly try to reset in the event of a failure or short, and that is not a good thing.

These re-settable breakers are useful for many applications.

These re-settable breakers are useful for many applications.

Manual breakers operate just like a sacrificial fuse, but instead of replacing the fuse, you simply click the switch or button on the breaker and you are back in business. These are beneficial for aftermarket circuits where blown fuses are common (car audio, added lights, etc), as high-current fuses are not cheap.

This is a buss bar made of several auto-reset breakers. These are often used in OEM applications where there may be a short high amp draw that is normal, but would trip a standard fuse.

This is a buss bar made of several auto-reset breakers. These are often used in OEM applications where there may be a short high amp draw that is normal, but would trip a standard fuse.

Is it bad?

Most sacrificial fuses are obvious when it is bad, but not always. These fuses have a metal bar that literally burns up, disconnecting the power source from the rest of the circuit. If the little bar is melted, the fuse is bad. This does not always happen where you can see it. This is a particular problem that effect glass fuses. You cannot see the entire fusible element; the metal caps on either side prevent it. Unlike blade fuses, the element is soldered to the cap. If the solder melts before the element, you won’t see the break, but the circuit is dead anyway. This is most common with high-amp glass fuses.

You don’t have to pull a fuse to see if it is bad. Using a test meter (not a test light), set to continuity (where the unit beeps when the probes are touched together), and touch each lead to the ends or to the test terminals of the fuse. Your meter should beep (or read “0” if it is a silent meter). Any other reading means there is problem with the fuse and it should be replaced. High-amp glass fuses are notorious for melting the solder at lower amp ratings, so you may not have an actual problem, it could just be a bad fuse. This is why glass fuses are no longer used in most automotive applications.

This is the continuity setting on a multimeter. It is your new best friend.

This is the continuity setting on a multimeter. It is your new best friend.

Replacement

Replacing a bad fuse is fairly easy in most cases, but you do need to take a few precautions. First, make sure the vehicle is off. You don’t want to replace a fuse while the vehicle is running or the equipment affected is active. This can cause the fuse to pop even if you have rectified the actual problem due to the initial connection. If the problem has not been rectified, the fuse is going to pop anyway, so it is best not to have it in your hand when that happens. If your circuit is always live (has power even with the key off), then you should disconnect the ground terminal of the battery.

Bladed, cartridge, and non-glass bladed fuses can be really tricky to get out of their sockets. Most vehicles come with a small plastic tweezer tool to remove the fuses, but you can use needle-nosed pliers in the likely case that the tool is missing.

Bolt-down fuses like ANL and Class-T types require tools. A socket or nut driver is the best option, as a wrench can accidentally bridge the gap on the fuse terminals and again, that is rarely a good thing to do. The most difficult to remove fuse is a glass-type barrel fuse. These fuses are delicate. When the fuse goes bad, the glass can break, but moreover, it can break when you are trying to get it out. Because the spring terminals are tight, these fuses require some effort to remove. It is a good idea to use a non-conductive tool to pry one side of the fuse out of the holder. Plastic or wood is perfect. Don’t use pliers on the glass tube, it will break.

When is it safe to use a bigger fuse?

The short answer is almost never. The fuses are there to protect your vehicle’s equipment, if there is a fault, you need to find the fault rather than just up the fuse rating. There are a select few instances where you can go up to a larger fuse, but only when you absolutely know that the wiring is capable of handling the extra current.

Example 1: You have a car audio system with an amplifier; The amp is wired with a 4-ga wire that is 16 feet long. The previous amp had a 40-amp fuse, the new amp requires 60 amps. The wire capable of handling the larger load, go ahead and increase the fuse size.

Example 2: You replaced the head light bulbs with fancy new high-output bulbs. DO NOT increase the fuse size for the headlights. In fact, you need to install a relay and re-wire the headlights with larger gauge wires. The current draw of high-output bulbs is more than the factory wires can handle and will result in a fire.

Armed with the knowledge we have provided, you should be able to test and replace any fuse with ease. Make sure you follow the safety protocols to protect yourself and your vehicle. One more cautionary note- NEVER EVER bypass the fuse for a component. Coins, aluminum foil, etc are not suitable circuit breakers in any situation. DO NOT bypass the fuse, that is a sure-fire way to end up in a catastrophic situation.

Check out all the ignition & electrical parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on fuses, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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