Link to Kris's article on gtfours.
http://www.gtfours.co.uk/default.htm?ht ... attery.htm

I have done a couple of battery relocations following guide notes from different people. Some people specced a low CSA battery cable to the starter which although safe dropped the voltage by the time it reached the starter and it was very noticeable that the car was cranking slowly.

OEM wires are much much shorter so you can use thinner cable IMHO.

I should point out I know bugger all about safety or wiring but can only talk from my experience. Within Kris' article he indicates "big is beautiful" and I would agree that slightly oversizing the cable is not a bad thing.

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

3 aspects to choosing wire size.

1. Safety (overheating). Thinner wire will get hotter for a given current. The cable must be protected by a fuse at the battery end of a lower (or equal) rating to the cable. If not, in an overload condition, the cable can catch fire before the fuse blows.

2. Performance (voltage drop). Voltage drop is proportional to current, and is per metre. Thinner cables drop more voltage per metre for a given current. Bigger voltage drop means slower cranking and also less smoothing by the battery of alternator ripple / surges from ignition, injectors etc.

3. Weight. Thicker cable weighs more and costs more.

IMO. unless completely obsessed with weight for a track build, point 3 is minor enough to be ignored.

In more detail:

1.(overheating/fusing) Minimum cable size per current is determined by where the cable is run. i.e. running under the carpet but above underfelt or sound deadening will get much hotter than running in free air. In our favour, high current is normally fairly short bursts and not long enough to cause major heating - i.e. cranking and initial charge into battery. The fuse still needs to be rated for worst case though, and must be at the battery end as this is the only item capable of producing current greater than the wire size we are looking at. (if the cable chafes through in the middle and shorts to ground, it's no use having the fuse at the other end)

The following guide sizes for cables from the UK 17th edition wiring regs.
(current - size in free air (30C) - size in thermally insulating wall)
80A ------------- 25mm2
100A ------------ 35mm2
120A - 25mm2 - 50mm2
150A - 35mm2 - 70mm2

2. (voltage drop)

From 17th edition:

10mm2 - 4.4mV/A/m
16mm2 - 2.8mV/A/m
25mm2 - 1.75mV/A/m
35mm2 - 1.25mV/A/m
50mm2 - 0.93mV/A/m
70mm2 - 0.63mV/A/m

e.g. A 3m length of 35mm2 will drop 1.25x100x3 = 375mV = 0.375V


As above, bigger is better. You will get better starting, especially cold, and less electrical noise which will affect ECU and audio equipment. IMO minimum of 35mm2 with 100A fuse

_________________
If at first you don't suck seed, try drier grain.

When it comes to high power cables....
"Are they big? I likes em big"

With overspecced cables the worst problem is weight (partly offset by wallet lightening lol)
With undersize cables you risk death, destruction and general mayhem.

Nibbles - does it make any difference if we are talking AC or DC or do the 17th edition regs apply to both?

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

Hi,

many good informations thanks, i should say that i'am electrician so i know the risks of undersized wires

i want to know the oem power wire size just to choose the correct fuse

and, today i pressed some threads in the bood to mount the battery bracket

[img][img]http://img804.imageshack.us/img804/8690/batterie2.jpg[/img][/IMG]

[img][img]http://img852.imageshack.us/img852/2919/batterie1.jpg[/img][/IMG]

greetz

_________________
KEEP COOL UNDER PRESSURE

street legal?.....perhaps in tschibuti

the main am1(I think) fuse is 100A

I read from the DC column but at low frequency (50Hz) it won't make much odds.

_________________
If at first you don't suck seed, try drier grain.

This fuse is linked to alternator and charging circuit, not the starting process.

FFSA (french automobile sport federation) stand with 25mm² on a 2000cc engine (our is 1998cc and a turbo, equal to 3000+) with 5m max lenght.


when cranking over 40sec with fans and WTA pump on (idiot ) my 100A fuse broke before my 25mm² cable get hot (stays under 50°C), i now use a 130A fuse, no more WTA pump and always shut down fans before cranking.

According to FFSA i am OK and starting my engine is fine, i don't say this is what you have to do, just report it works fine like that

I would presume that being a competition car, you have no carpets ? 25mm is rated up to 120A continuous in free air at 30 deg., and would be a little higher if in contact with cold metal or colder air. Under carpet is a very different matter.

Also, as I said above, our high current loads are short duration only so wire will not have time to get as hot. For safety, the cable/fusing should be rated for a worst case scenario of continuous current being drawn in a fault condition (e.g. starter solenoid jams or alternator fries).

Bear in mind, also, that a fuse rating is the current it can carry continuously with many on/off cycles. The current at which it blows is substantially higher, often at least 1.5 times for long duration currents and over 3 times for short duration surges.

e.g. http://www.cooperindustries.com/content ... 24_ANL.pdf

For our application it is better to use a lower rated fuse with a slower characteristic.

e.g.
http://docs-europe.electrocomponents.co ... 07bc57.pdf

http://uk.rs-online.com/web/search/sear ... &R=2260866

_________________
If at first you don't suck seed, try drier grain.

Sunny, I can see one problem with your choice of battery location.
It is directly above the fuel pump located in the tank below.
If the fuel pump fails then it will be a case of dropping the tank down from underneath the car.

_________________
Vice Chairman
1990 White UK Celica ST185 - Project DTM
1997 Cream BMW R1200C (as seen in James Bond "Tomorrow Never Dies")
2005 Jaguar X-type 3.0 Sovereign AWD Automatic
Blood Bikes - https://www.freewheelers.org.uk/

Hi,

muddy, i do not see a problem when the pump fails i have to dismount the tank but this should not gave any problem with the tray?

i should also say that i never dismounted the tank on a ST205

greetz

_________________
KEEP COOL UNDER PRESSURE

street legal?.....perhaps in tschibuti

you can only get the fuel pump out by dropping the tank, the sender is the only wotnot that comes out from above

_________________
ST205




"And God said, Let the earth bring forth grass, and the earth brought forth grass and the Rastafarians smoked it."

Depends if you make a bigger hole in the boot floor and make a larger cover panel.

_________________
If at first you don't suck seed, try drier grain.

dropping it isn't a major drama unless like a tw@ you forgot you parked it up 3/4 full and decide to just drop it onto your chest. Then it's "interesting"

That sounds familiar!

_________________
René - Carlos Sainz 2433•5000
Turbocelica.nl | ST185 CS restoration and full rebuild

If it ain't broken.. it soon will be