Current Draw & Power Cable sizes.

[Tigz™]

Working out what size Power cable you need can seem a little daunting, too large and your adding weight, and making routing the cable ever so much more difficult.

Too small, and your amp will find it difficult to get the current it requires, and you may find your self spending a fortune on blown fuses.

To work out what cable you need, you first need to know what current draw you amp has.

There is a few ways of doing this.

Check your manual, or the advert.

Most manufactures provide this information.

If this information isn't available count up the amp ratings on the amps fuses

IE. 2 x 20amp fuses = 40amp max current draw.

Some large power amps do not come with internal fuses.

Again, you will need some info from your manual/google/on-line advert.

Then you just need to do a little maths.

Power output / Efficiency / Voltage = Amperage required

A rough guide with efficiencies is to say that a Class D amp is around 80% and a Class A/B is around 60% efficient.

ie. 1000w rms / 80% (0.8) = 1250 So 250w rms is lost through heat.

ie. 1000w rms / 60% (0.6) = 1666 So 666w rms is lost through heat.

So as an example:

TRU-T2100

Class A/B

Fuse Requirement: 60 Amp

4 Ohm Bridged: 400W x 1 (400)

Efficiency: 65.70% (0.65)

Rated Power: 14.4 Volts DC (14.4)

400 / 0.65 / 14.4 = 42.73 Amp

So why does it need a 60 amp fuse?

Well, what would happen if the voltage dropped?

400 / 0.65 / 12 volts = 51.28 Amp

So we know a drop in voltage can raise the amperage required, something else that can alter the amp draw is the resistance.

If an amp was to draw 80 Amp @ 1ohm, it would draw 40 Amp @ 2ohm and 20 Amp @ 4ohm.

Now lets say our amp has the 60 Amp fuse fitted, this is the max current draw.

The music we play through our Audio systems is not a constant, so its unlikely we will reach the 60 Amps.

There is much more on this, if you want to know more about fuses check this out.

http://www.littelfuse.com/data/en/Data_Sheets/257.pdf

Now onto the cabling.

We now have our figure we need.

We know the max current draw from our amp' is 60 Amp.

The next figure we need to know is the length of the power cable needed.

In a Lupo GTI the battery is in the boot, so a cable less than 4 ft. could be used.

Amazingly, you could get away with a length of 8 awg cable.

As the non GTI has to travel from the engine bay to the boot, the cable is much longer, so 6 awg would be sufficient. It would make more sense to use 4 awg for this though due to the availability of the stuff.

Making sure you observe safety procedures, the cable must be fused no more than 12 inch from the battery.

This table should help you work the cable sizes out.

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Edited June 4, 2006 by Tigz

[NOCK]

A rough guide with efficiencies is to say that a Class D amp is around 80% and a Class A/B is around 60% efficient.

ie. 1000w rms / 80% (0.8) = 1250 So 250w rms is lost through heat.

ie. 1000w rms / 60% (0.6) = 1666 So 666w rms is lost through heat.

Not trying to pick faults mate but think you done your maths wrong there shouldn't it be:

1000w RMS / 80% (0.8) = 200w RMS is lost through heat.

1000w RMS / 60% (0.6) = 400w RMS is lost through heat.

I might be wrong though obviously

NOCK

Edit: removed emboldening-what was I thinking!

Edited April 15, 2010 by NOCK