(Battery in boot) What gauge wire is best?

[Mr.Phreak]

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

4ga is about 22m^2

[gepsk8]

well like any good auto sparky will say u cant fuse the battery to starter wire when u put battey in the boot u can have up to a 7m or 8m cable depending on the car and way u route the wire, u havent no problems yet, wait till the starter gets tried or battery gets a little flat, then find out who laughing then,
gear reduction start u will get away with small cable but it not worth the risk for lossing ur car.

[Loudtoy]

thats the thing u cant put a fuse for the battery cable to starter, cause it draws to much current, biggest ever car fuse i've seen is 425amps, which might work in good conditions but would blown as soon as u got a cold start or flatish battery.

Dude your doing something wrong.

I have a 120amp circuit breaker on the 4ga lead coming off the battery in the racecar and it doesnt trip. Hell I had an 80amp circuit breaker on the bettery in my old mini which had a direct drive starter and that never blew either.

Just ensure your using good quality oxy free cable , and remember to match it with the earths / grouding points!

Sounds fine to me also, i had an 80 on my road car, never tripped and had some flat battery moments when i left my stereo on for a bit long.
Gepsk8 you might want to go through everything you've done and make sure it's sweet, most current i saw cold cranking in my car was 75a for about 2 seconds before it fired.

[Loudtoy]

well like any good auto sparky will say u cant fuse the battery to starter wire when u put battey in the boot u can have up to a 7m or 8m cable depending on the car and way u route the wire, u havent no problems yet, wait till the starter gets tried or battery gets a little flat, then find out who laughing then,
gear reduction start u will get away with small cable but it not worth the risk for lossing ur car.

I've never heard an good auto sparky saying anything like that, maybe i don't know any good ones. Although i did just crimp join the new wire from the boot to the old +ve, earth the old -ve wire to the chassis and do the same thing to the new -ve wire in the boot and put the circuit breaker on the new +ve wire about 100mm away from the battery and no complaints, no tripped breaker, no problems with flat battery, tired starter motor which i replaced after it failed or anything like it. Pretty sure i was laughing at a joke a mate said while we were going for a drive to recharge the battery after it got down a bit.
So who was the good autosparky who said that to you??

[fivebob]

How can low voltage increase current?

Surely that would break the laws of physics (V=IR) unless resistance was lower, but I can't see how that could happen. Even then wouldn't the total energy have to increase to blow a fuse?

[gepsk8]

cause ur car doesn't start staight away because of low voltage meaning so u cranking for longer, to start ur car so more current flowing through the wiring, if u use lite crap it will ge hot damage the insulation, u will get away with lite wiring on smaller engine car but its not worth the risk if some thing goes wrong.

[fivebob]

That makes no sense at all when it comes to the question of whether or not you should use a fuse

[gepsk8]

u cant fuse the battery to starter wire !!

[Alex B]

Awesome argument. Yes you can, as people above have said they have done it and it works fine.

[fivebob]

u cant fuse the battery to starter wire !!

Why not???

I've seen it done plenty of times before, some cars even have them from factory

[ChaosAD]

Motors are very low resistance, so when overloaded they can draw a lot more current.
For a 400v squrrel cage motor, a 5% voltage drop means a 10% current increase, or something like that.

But ive seen the consequenses of a non fused amp wire shorting out.
They heat up prity damn quick

[Crampy]

Yes they are low resistance but they are also very high inductance.
So, when they are cranking the BEMF induced limits the current draw.
If that guy is getting 425amps or whatever, I'd say the starter wasn't even turning and it was just shorting the battery to earth.

I ran a 100 amp CB from my boot on the one wire I had running to the front. It ran everything including the starter off that one wire.

Also, may I ask, how that hell did you measure 425 amps current draw???
You can't do that with many meters I know of. You'd wax them in no time.

I'd say you might even be quoting the cold cranking amps that your battery is rated too.

If you drew 425 amps from a lead acid battery, you're only doing one thing to it, TURNINNG IT INTO A HEATER!!!
Batteries have an internal resistance and to draw that much current from it, you'd have to dissipate all of the I2R losses.

[big_boy]

just for arguments sake i just went & tested mine with an inductive current meter

motor= 5M-GE with a compression of 180psi ish give or take 5
starter = is an up rated from factory ND rated at .9kw at motor & 2.7kw at final gear
battrie is otpma red top 850cca 1000ca

cranked for 8 seconds with a voltage of 13.2 V.D. under crank was 0.2V giveing me 13V & CURRENT OF 118.1A max

tryed on an old NS70 battrie witch was holding 11.9V V.D. under crank was 2.6V giveing me 9.3V at the starter & CURRENT WAS 133.3A

so i would get away with a fuse

also a EE90 with factory starter draws 57.6A at 11.6V

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

[gmacrae]

I dont even know what guage cable i used, but the copper core is about 8mm thick

[DeeCee]

i used 1/0gauge / 50mm^2 and a 250A ANL fuse

and you can go up to 500A ANL fuse..

[neo]

haha awesome,

If anyones from the Car Audio SQ arena, (when CANZE was around) it was a REQUIREMENT to fuse the battery cable within 300mm of the battery.

[Crampy]

My mate is currently working through getting his RX7 certed and has the battery in the boot and was told he had to have a fuse or CB within 300mm of the battery also.

[Crampy]

just for arguments sake i just went & tested mine with an inductive current meter

motor= 5M-GE with a compression of 180psi ish give or take 5
starter = is an up rated from factory ND rated at .9kw at motor & 2.7kw at final gear
battrie is otpma red top 850cca 1000ca

cranked for 8 seconds with a voltage of 13.2 V.D. under crank was 0.2V giveing me 13V & CURRENT OF 118.1A max

tryed on an old NS70 battrie witch was holding 11.9V V.D. under crank was 2.6V giveing me 9.3V at the starter & CURRENT WAS 133.3A

so i would get away with a fuse

also a EE90 with factory starter draws 57.6A at 11.6V

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

Ahhhh good a definitive answer.
Seeing as fuses can operate up to twice their actual rating, a 100 amp fuse will probably do the job for all of those. Or you could chuck a 150 amp one at it.

There is no point going for a 500amp fuse, because the insulation on the cable would have melted and caused your car to catch fire before it would blow.

[gepsk8]

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

[Loudtoy]

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