What amp cable? Battery and fuse box related.

[saalro]

Hi, i'm currently in the process of relocating my battery for my track car and i'm having a slightly problem deciding what amp cable is needed.

The vehicle is a Subaru Impreza, registered in 2000 (W).

The system is set up as follows:

From the battery +, there is a thick cable connecting to the starter motor and a thinner cable connecting to the fuse box, so 2 cables in total from the +.
Where the thinner cable connects to the fuse box there is a 80A fuse.

From the -, there is a thick cable that bolts on to the engine as well as a thinner cable bolting to the chassis.

From the alternator there is 2 cables (Plus a 2 pin connector, but this isn't important.) that connect to the fuse box, the alternator is rated at 75A, so it charges at 75A.

My questions are:

If the alternator charges at 75A, does this mean these 2 cables from the alternator to the fuse box have to be capable of over 75A?
So is this "Thinner" cable of mine most likely to be 80A? This would also make sense as the fuse is 80A as well.
The diameter of the cable including the insulation is about 6mm.

As for the thicker cable, it's around 10mm thick.
Cable diameter isn't the most accurate way of measuring the amps it can handle, but i do not have a method of testing what amps my starter motor draws.
Does anyone know what amp my vehicle draws, vehicle is stated above.

How does the alternator work exactly, based on the 75A charge, why does all the cables from the fuse box not fry? How is the charged contained to the battery only?

Thanks a lot for your time.
Sam

 

[Deleted member 26818]

The 75A your alternator is rated at, is the maximum it can produce.
It will not produce this, unless you are running at high revs.
Yes the cables and the 2 pin connector should be rated at a minimum of 75A, as should your battery connection to ground (the cable from the - battery terminal to the chassis).
The cables from the + terminal on the battery to the starter motor and from the - terminal to the engine have to be rated at the CCA (Cold Cranking Amperage) of the battery. Which I believe for your vehicle is somewhere between 450A and 550A depending upon model.
If you reposition your battery, you will have to extend the starter motor and engine ground cables and they will have to be rated the same as your existing cables.
It would probably be advisable to extend the cable from the + terminal to the fuse box, but the cable from the - terminal on the battery to the chassis can be disconnected from the chassis and then reconnected somewhere else on the chassis close to the new battery position.

The battery acts as a reservoir for the current produced by the alternator, if the alternator produces too much current, then the battery can cook.
The other wiring doesn't fry because most of the current flows alsong the large cables as they form the circuit.
If any of the large cables become disconnected, the current could be diverted along the smaller cables in the rest of the wiring and fry them.

 

[saalro]

Thanks for the reply, really helpful.

What confuses me is the current starter motor live and earth cable does not look like a 450A - 550A.

Thanks a lot.
Sam

 

[Deleted member 26818]

The CCA is the maximum amperage a fully charged battery can sustain for 30s.
As such any cables will not have to be as large as a cable intended for constant duration.
The shorter the duration, the smaller the cable.
Also automotive cables are fine fillament or Tri-rated, which means they have a higner CCC (Current Carrying Capacity) than standard household cables.
Most of my automotive wiring experience comes from working on military vehicles and motorcycles.
You may find it more informative to search for forums dedicated to 'imprezas', 'subbies' or even 'track day' driving.

 

[telectrix]

and if the cable to the starter is considerably longer than the original, volt drop could be an issue. might be an idea to search for a wrecked merc. clk or similar which has the battery in the boot, and obtain your cable from there. be far cheaper than buying new cable.

 

[vaughant]

You don't need to extend the engine ground cable as you can just bolt a short cable link from any bolt on the block/gearbox to a bolt on the body as of course ground is ground on a car.
Same in the boot where the battery earth simply connects to the boot floor,it does on my BMW cabby anyway.
An e36 Beemer would be your best bet,you get the jumpstart block under the bonnet then which makes your cables look nice and neat then plus the beemers are ten to a penny in the scrappers (6cyl and above).

 

[Deleted member 26818]

You don't need to extend the engine ground cable as you can just bolt a short cable link from any bolt on the block/gearbox to a bolt on the body as of course ground is ground on a car.

I would be wary about doing this.
Many vehicle manufacturers use this method so it makes me wonder why Subaru don't.
Perhaps they ground the engine directly to the battery to prevent spikes due to high starting currents from affecting engine management systems?