How important are fuses?

Fuses are very important!
Fuses are there to prevent fires in the case of short circuits. (or too much current draw in general....)

Most amplifiers (and headunits and processors etc) these days have their own fuses, to prevent the chance of fire due to a short circuit inside the amplifier (or headunit etc), or in the speaker wiring. (or blown speaker/crossover)
However, this fuse will only protect the amplifier (or headunit etc), it is also important to make sure the power cables are fused to prevent fire.

In the case of cars where the negative terminal on the battery is connected to the chassis of the car, it is important to fuse the power cables connected to the positive terminal on the battery. It is important to place the fuse as close to the battery as possible, because if the cable shorts out between the battery and the fuse, then it will not be protected.

Note, in some cases a circuit breaker of decent quality may be used instead of a fuse.


A fuse just blew in my system, what should I do?

The first thing you should do, if a fuse blows is make sure that there are no short circuits. For example, if an amplifier fuse blows, then you should first make sure there are no short circuits in the speaker wiring (or that the speaker is blow, or the crossover is damaged). If, for example a power cable fuse blew, then you would also make sure there are no short circuits along the power cable.

The reason for this is, if you don't verify the problem first, then it is likely that when you replace the fuse, the new fuse will also blow.


What size power cable should I use and what size fuse should I use?

There are two factors that you need to consider when selecting a power cable.

First, cables can only pass a certain amount of current, before they heat up too much. Therefore, if your system requires high current, then you would need to use lower gauge (thicker) power cables.

Here are some general values for cable gauge vs maximum fuse size (current demands)

Wire Gauge - Fuse Size
2/0 gauge - 430 amps
0 gauge - 345 amps
1 gauge - 265 amps
2 gauge - 215 amps
4 gauge - 135 amps
6 gauge - 80 amps
8 gauge - 55 amps
10 gauge - 30 amps
12 gauge - 20 amps
14 gauge - 15 amps
16 gauge - 7.5 amps

These are general values, but should be fairly safe as they are rather conservative.

Secondly, it is important to make sure the total resistance of the power cable is not too high, to minimize the power loss through the cable. Too much power loss will result in too much voltage drop. It is generally recommended to keep the voltage drop below 0.5v.

To calculate the voltage drop, you first need to calculate the total resistance. This is of course the total length of the wire multiplied by the resistance per metre (in ohms).

Here are the approximate resistance per metre values for common gauges. (but if possible, try to use the resistance values quoted by the manufacturer of the wire that you buy)

AWG (American Wire Gauge) - ohms per metre.

2/0 gauge - 0.0002580
0 gauge - 0.0003251
1 gauge - 0.0004961
2 gauge - 0.0005161
4 gauge - 0.0008192
8 gauge - 0.0020643
10 gauge - 0.0037268

Next, you use ohms law to calculate the voltage drop

Voltage = Current Multiplied by Resistance
V = I x R

For example, if we had, say 100 amps of current draw, 5 metres of 4 awg cable.

I = 100 amps
R = 5 x 0.0008192
R = 0.004096 ohms

V = I x R
V = 100 x 0.004096
V = 0.4096 volts

So you would have a voltage drop of about 0.4 volts if you had 100 amps flowing through 5 metres of 4 awg cable.

Note the ground wire should generally be the same gauge as the power wire.

Next, you need to work out what fuses to use.

Remember, that the fuse (at the battery) is there to protect the cable. This means that the fuse rating should not exceed the amount of current that the cable can pass safely.
Secondly, rather than fusing the cable by the maximum amount of current it can pass safely (continuously), for example 125 amps for a 4 awg cable. Instead, it is slightly safer to use a fuse that is as small as possible, but that will still be able to pass enough current for the amplifier(s) without regularly blowing.

There are two common ways of working out an approximate maximum continuous current draw value.

The first method is to simply add up the fuse values on the amplifiers. So, if for example, you had two amplifiers, one had two 25 amp fuses and the other had four 20 amp fuses, and the cable needed to be 5m long, then you would ideally use at least 2 awg with a 130 amp fuse. While it may be unlikely that this system will draw a maximum of exactly 130 amps while playing music, it is better to err on the side of larger cables and smaller fuses, therefore if a 130 amp fuse was not available, a 120 amp fuse will most likely work fine anyway.

The second method is to attempt to calculate the current draw based on the battery voltage, efficiency and power output.

To calculate current draw you can use the following formula:

Current = Power / (Voltage x efficiency)

Ie,
Voltage = 12.5v
Power output (at full volume) = 400w RMS
efficiency = 60%

Current = 400 / (12.5 x 0.6)

Current Draw = approximately 53 amps

Class A/B amplifiers will generally be 50-60% efficient at full volume. (before clipping)
Class D amplifiers will generally be 70-80% efficient at full volume.
Tripath Class D (sometimes refered to "Class T" when marketed) efficiency depends on if you are using it full range, or with subwoofers. When running full range, it will be less efficient than when running subwoofers. When running subwoofers, it will be similar to normal Class D amplifiers, around 80%.
As the volume decreases, so does the amplifier efficiency......

Lastly, do also remember that while playing normal music, at reasonable bolumes the continuous current draw will tend to be not be nearly as high as the maximum current draw values. However, it is best to take the maximum current draw values into account so that the cable and fuse will have a bit of headroom, should they need it.

References:
http://www.dave-cushman.net/elect/wiregauge.html (cable gauge vs diameter etc)
http://www.bcae1.com/wire.htm

More Notes on Fuses.

As mentioned, fuses are there to protect your equipment and power cables.

It is important to note, that a fuse will not magically blow as soon as the current draw reaches the fuse rating. They are generally designed to pass their rated current without blowing. If the current draw exceeds the power rating, then the fuse won't magically blow automatically, but will take a little bit of time before it heats up enough to blow. The time it takes to blow, depends on how much current is drawn through it and the design of the fuse. For example, you have a a fuse that has 15% more current drawn through it continuously, it might take over 10 minutes before it blows. If the current draw was double the rating, then it would likely blow in less than 30 seconds. If there was a short circuit and there was a very large current draw, the fuse will likely blow in a fraction of a second.

In car audio, there are several common types of fuses.

These are the various physical sizes of blade fuses, glass fuses and ANL fuses. Circuit breakers are also used occasionally.

Glass fuses tend to come in the smaller current ratings. There are two common physical sizes of glass fuses, these are 3AG and 5AG. 5AG fuses are occasionally used to fuse power wires and tend to come in values of between 10-60 amps. (80 amps tends to be rare)

Blade fuses also come in several physical sizes. These are mini blade, blade and maxi blade fuses.

Standard and mini blade fuses tend to be found on headunits, amplifiers and are also used by some cars in the fuse box. These tend to come in values up to 30 amps.
Maxi blade fuses are of course a bit larger and tend to come in values between 20 and 120 amps.

ANL fuses tend to come in the larger ratings. They are commonly found in values of 60 amps to over 250 amps.

Also, it is important that you use a decent quality fuse holder with a properly selected fuse. A poor quality fuse holder can cause increased voltage drop and at worst, may not be able to handle the current and therefore be unsafe.

Also, I should also point out that in the case of using a distribution block to split a thicker wire into a thinner wire(s), it may be necessary to individually fuse the thinner wire(s). Ie you use a distribution block to split a 2 gauge wire into several runs of 8 gauge, you had a main fuse of around 150-200 amps - then it would be important to consider fusing the thinner 8 gauge wires. If the 8 gauge wires are very short (ie, 10cm long), then you may get away with not fusing them individually, but if they are sufficiently long that they may be at risk of shorting out, then it is important to fuse them individually. Ie, if the 8 gauge runs were 40-50cm long then I would suggest fusing them individually. Obviously, you would make sure the fuse on the 8 gauge wires was no larger than 50-60 amps.

Other Tips:

If planning on later upgrading to larger amplifiers or adding more amplifiers later, it is often a good ideal install thicker wire in the first place, so you don't have to pull your car apart again and rewire it when you upgrade.