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What fuse should I get for my system?
Posted 03-03-08
A lot of time, people make the mistake of using the fuse included in their amplifier wiring kits. This fuse could be underrated for their system, causing frustration due to constant burn-outs, or even worse, the fuse could be overrated creating a potentially hazardous situation.
A car’s battery is capable of supplying very large amounts of current in a very short time. If a live wire touched a grounded component (The car’s chassis for example), current would immediately start flowing through it. This would heat up the wire to an extreme temperature, just like the heating element of a toaster. The plastic insulation would quickly melt off, and then the wire would be in direct contact with flammable components, like the car’s carpeting, creating the potential for a fire that could cause heavy damages to your car, even total loss.
As you can see, selecting the right fuse and wiring is critical to a safe system. Being a car audio site, we’ll focus on the most basic setup: A system that has one amplifier being fed from the battery.
First of all, you need to look at how much current your system will need. If you recall from our amplifier article, peak power is a mostly worthless measure of what an amplifier really does. As far as current is concerned, we need to look at RMS (Root Mean Square) figures.
Let’s take a JL 250/1v2 amplifier (Like the one we reviewed here). This amplifier is capable of producing 250 Watts (rms).
A word about efficiency before continuing:
In a perfect world an amplifier would take 100 Watts of input power, and transform this into 100 Watts of output power, 100% efficiency with zero waste. As you may have noticed, this world is far from perfect, so amplifiers will not behave this way. Some types of amplifiers are better than others at using their input current efficiently. Class D amplifiers will typically have an efficiency of about 80%. This means that 100 Watts of input are required by the amplifier to be able to produce 80 Watts of output. Class AB amplifiers will be worse, with typical efficiencies of around 60%, requiring 100 Watts of input to produce 60 Watts of output. The rest of the current becomes heat, causing the amplifiers to be nice and warm during operation.
If you want a full explanation, check out this link: http://sound.westhost.com/efficiency.htm |
The 250/1v2 is a Class D amplifier. We can use the 80% figure to estimate the average current draw of this amp. We know it produces 250W of output power. This means that the input power must be (250W / 0.8 = ) 312.15W. Your battery can supply current at 12 Volts. This means that to produce the 312.15 W, roughly 26 Amps must be supplied. (Current = Power / Voltage. You’ve read out articles on current & power right?).
Now that we know this, we can select the proper wire to run to the amplifier. A cable’s resistance is determined by its thickness, and length. Just like the wires in your toaster, they will get red hot if the resistance is too high, and it carries a lot of current.
As a rule of thumb, make cable runs no longer than necessary, and make them thick enough to handle the current that will flow through it.
A typical run of cable in a car will be around 10 feet long, so we must select a cable that’s thick enough to handle 26 Amps over 10 Feet. The following table shows what AWG gauge copper cable should be selected for a certain run length. The lower the gauge number, the thicker the cable.
| |
Length of run (in feet) |
Current |
0-4 |
4-7 |
7-10 |
10-13 |
13-16 |
16-19 |
19-22 |
22-25 |
0-20A |
14 |
12 |
12 |
10 |
10 |
8 |
8 |
8 |
20-35A |
12 |
10 |
8 |
8 |
6 |
6 |
6 |
4 |
35-50A |
10 |
8 |
8 |
6 |
6 |
4 |
4 |
4 |
50-65A |
8 |
8 |
6 |
4 |
4 |
4 |
4 |
2 |
65-85A |
6 |
6 |
4 |
4 |
2 |
2 |
2 |
0 |
85-105A |
6 |
6 |
4 |
2 |
2 |
2 |
2 |
0 |
105-125A |
4 |
4 |
4 |
2 |
2 |
0 |
0 |
0 |
125-150A |
2 |
2 |
2 |
2 |
0 |
0 |
0 |
00 |
From this table, we see that for our 10Ft / 26A run we must get AWG 8 cable.
AGU fuse sizes go in increments of 10A, up to 60A. For this scenario, a 30A fuse will be sufficient. Place the fuse as close as possible to the battery to protect the wiring, as well as the amp.
Recap:
Required Input Power = (AMP RMS POWER) Divided by (Efficiency)
Input Current = (Required Input Power ) Divided By (System Voltage)
Remember, a fuse is the most inexpensive insurance that you can buy for your car! You don’t want to make a mistake in picking the right one!
Further reading:
Excellent read on fuses, and how to properly protect a more complex system than what we described here.
http://www.bcae1.com/fuses.htm
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