(Battery in boot) What gauge wire is best?

[Lloyd]

id be supprised if any car draws over 150A witch is a breaker readley advable

You'll find a new motor which is quite tight, or something else slowing the starter motor down will cause the current to increase. The slower a starter is turning then the more current it will draw basically (for a given voltage). The most current drawn by a starter will be when its not turning, which you'll probably notice as a spike in the current reading when you start cranking.
Pretty much what Crampy said anyway

Ah the CANZE days, so so many questions asked then on fusing at the battery.

And yes fuses can pass double their rated current, try doing it for long periods of time however. Part of the reason some people use circuit breakers also. And the fuse/breaker is there to protect the wire anyway, if you get a short to earth and you have the power/starter wire fused at 100A then you're going to get considerably more than 100A draw quite quickly and the fuse/breaker will trip anyway

[Alex B]

Also, may I ask, how that hell did you measure 425 amps current draw???

Clip on ammeter.

[Crampy]

Seeing as fuses can operate up to twice their actual rating, yea right!!

Actually they can depending on what type of fault load they are operating for. Shorts to earth through a fuse can routinely draw up to twice the fuses rated current through the seperation part of the fusing sequence. With slow slgith over current say 10a or so over the rating of the fuse wire it may take 10 hours to seperate or may not even seperate. there is a graph somewhere which i will upload if i can find it in all the piles of study crap i've got lying around still and it's not a linear graph either.
You can work out the time it will take a fuse to rupture via a formula that i can't recall right now. Safe to say breakers are far more fefined than fuses for rupturing time, overcurrent limiting and fine breakng where the current sits above the rated limit for x amount of time

Well there you go gepsk8.

What would I know though, I'm only an Avionics Technician in the RNZAF, not like I use them or anything.

A very good explanation Loudtoy, good work.

[Stealer Of Souls]

I might be mistaken...
But 4ga is approximately the same as 35mm...

4ga is about 22m^2

A bit off the current "discussion" of "to fuse or not to fuse".
Equivalent sizings...

Now. Someone correct me if I'm wrong.
8ga = 1/8 of an inch diameter
4ga = 1/4 of an inch diameter
2ga = 1/2 inch
0ga?
1/0ga?

Soo...
if 4ga is 1/4inch diameter
25.4/4 = 6.35mm diameter
pi x diameter^2 / 4 = area
3.14 x 6.35^2 / 4
0.785 x 6.35^2
.785 x 40.3225
= 31.65mm^2

[bubz]

Cable I am using is 12mm - 13mm in diameter(copper only) and is 95mm*2.

I am in the process of installing it at the moment due to an upgrade as a result of blown turbo.

What size breaker/fuse is advised for me in my GTT?

[postfach]

OK, so just a little input on my part, I may be way off but the cable getting warm/hot would be due to the amount of POWER going through the cable, in watts, correct?

So, at work we use double insulated 25mm² copper cable to get power to our screens. the power supplies have 6 48V outputs and a screen draws upwards of 300A. Thats a total of 14.4kW. So each ouput would average 2400W, 50A. These screens can run at this level of power constantly for hours/days whatever without cooking the cables, hell, I don't think the cables even get warm.

Using the same cable in a 12V system means to get the same amount of power, there must be 4 times as much current, so 12V * 200A = 2400W.

200A is plenty to turn your average engine over.

Surely this must mean, provided the cable is fused in some way to prevent extreme current draw due to something shorting/the starter stalling (effectively a short) that 25mm² cable (smaller than 4ga) is perfectly suitable for a battery relocation, considering you won't be turning the key for more than about 2-3 seconds at the most, and once the starter is turning it draws substantially less current than it requires to get it turning initially.

Assuming your starter should draw a maximum of about 120A, it would be fairly standard practice to fuse it at ~25% higher than that, to allow for periodic spikes, so use a 150A fuse/breaker

[Crampy]

I'm using a 100amp circuit breaker from Jaycar.
http://www.jaycar.co.nz/productView.asp ... &SUBCATID=

I have had no problems at all with running this in my car. I only ran one power wire up to the front and then had a distrbution block under the bonnet, which goes to the starter and then all the other original power wires.

Distribution block:
http://www.jaycar.co.nz/productView.asp ... BCATID=469

[bubz]

OK, so just a little input on my part, I may be way off but the cable getting warm/hot would be due to the amount of POWER going through the cable, in watts, correct?

So, at work we use double insulated 25mm² copper cable to get power to our screens. the power supplies have 6 48V outputs and a screen draws upwards of 300A. Thats a total of 14.4kW. So each ouput would average 2400W, 50A. These screens can run at this level of power constantly for hours/days whatever without cooking the cables, hell, I don't think the cables even get warm.

Using the same cable in a 12V system means to get the same amount of power, there must be 4 times as much current, so 12V * 200A = 2400W.

200A is plenty to turn your average engine over.

Surely this must mean, provided the cable is fused in some way to prevent extreme current draw due to something shorting/the starter stalling (effectively a short) that 25mm² cable (smaller than 4ga) is perfectly suitable for a battery relocation, considering you won't be turning the key for more than about 2-3 seconds at the most, and once the starter is turning it draws substantially less current than it requires to get it turning initially.

Assuming your starter should draw a maximum of about 120A, it would be fairly standard practice to fuse it at ~25% higher than that, to allow for periodic spikes, so use a 150A fuse/breaker

Makes alot of sense.So with the cable I am using ,would it matter much if the key is turned for more than 3 seconds?

[postfach]

no, i wouldn't worry about how long you have to turn the key to start the car, my point was that the cable was fine for hours on end, so the small amount of time it takes to start a car certainly isn't going to make the cable so hot that it melts/burns stuff.

however if you turn the key and all you get is the click of a stalled starter motor, don't keep the key turned, because that would be just like shorting the positive to ground, and you could damage your battery/starter etc esp if you don't have a fuse or breaker on the cable.

[big_boy]

postfach i see what you saying but most cables ant the quailty you probly have at work most people say the max current for car audio wire is

8ga=50A
4ga=80A (120A for deaqsent OFC wire & some fuse up to 150A depending on length )
2ga= 150A (once again 200A for good stuff)
0ga=200A (250/270A good stuff)


i cant rembour the rateings for weilding wire im sure some one can tho i know its more than the above good quailty rateings for the double sheild stuff of the equilvent sizes

[Alex B]

Yes but thats legnth dependent.

[postfach]

Yes but thats legnth dependent.

if you're referring to what i said our cables are quite often 25m+

[big_boy]

im pridy sure thats at me & yes it is

[jondee86]

if 4ga is 1/4inch diameter
25.4/4 = 6.35mm diameter
pi x diameter^2 / 4 = area
3.14 x 6.35^2 / 4
0.785 x 6.35^2
.785 x 40.3225
= 31.65mm^2

This is only valid if you are working with a single solid 1/4" wire. To make
them flexible, electrical cables are made up from multiple strands; the
finer the strands, the more flexible the cable.

The area of the copper (mm2 cable rating) is calculated from the cross
sectional area of each strand x the number of strands.

From Ohms Law: DC Power (Watts) = Volts x Amps

Therefore if the starter is putting out the same amount of grunt, when
the volts drop the current increases. Nicely illustrated by big_boy when
he swapped in the old battery (posted higher up).

Cheers... jondee86

[Stealer Of Souls]

if 4ga is 1/4inch diameter
25.4/4 = 6.35mm diameter
pi x diameter^2 / 4 = area
3.14 x 6.35^2 / 4
0.785 x 6.35^2
.785 x 40.3225
= 31.65mm^2

This is only valid if you are working with a single solid 1/4" wire. To make
them flexible, electrical cables are made up from multiple strands; the
finer the strands, the more flexible the cable.

The area of the copper (mm2 cable rating) is calculated from the cross
sectional area of each strand x the number of strands.

Yep. My bad... Sorry. At work we deal mostly with compacted cables... so the total cross section of a stranded cable is very similar to the cross section of a solid core... DUH!

[Lloyd]

Therefore if the starter is putting out the same amount of grunt, when
the volts drop the current increases. Nicely illustrated by big_boy when
he swapped in the old battery (posted higher up).

The starter uses power, and that equation isn't exactly being used in the right sense here. Put 6V to a starter and it will turn over slowly, put 12V to it and it will turn over faster. 12V with the starter locked solid will be where it draws the most current, as it gains speed the current will drop. If you give it 8V for example, it will turn very slowly so will basically be closer to stall, and will therefore be trying to draw as much current as it can (resistance/impedance/whatever will be lower then if it was turning quickly)

P=VI is nice and simple, though doesn't take into account the effective changing resistance across the starter motor.

And I'm tired and cant be arsed reading back over that so it might have bits with little sense being made