wiring gauge

[dazzer]

Ok all you electrical Gurus. What gauge wire should I be using for a 120amp alternator? and indeed what other wiring might I need to change in addition to the obvious main feed out of the alternator.

This is on a 2000 MK1 that has had a 65amp running quite merrily on the standard loom. I've now removed the loom and was going to make a copy with a few extras but beef it up where necessary.

I can tell that someone is going to ask why do you need a 120 amp alternator? The reason being that i have a 24amp rad fan 2 x 7 amp blower fans (engine bay cooling), halogen lights x 4, uprated heater fan, rally trip, clocks, int lights etc etc and I'm looking at fitting front and rear heated screens. Currently on all night rallies with all the lights on in rally mode with the cooling fans running the 65amp cant keep up with the draw, result flat battery. So this time round I'm going for spare capacity. Choices in lightweight alternators 55 amp, 90 amp or 120amp.  

Cheers Dazzer    

[yorkshire_spam]

I'd go for something like a 196/0.40 (25mm sq) cable - typical rating is about 170 amps.
But I'm not an expert!

[RobPearce]

Well, according to this site you probably want at least 5AWG, which is 4.6mm diameter.

[Clive]

A thick one! I would guess about 10mm^2

Ifyur existing wire is copingOKwith 65A, then you need one where the copper is approx 50-100% larger diameter (if that makes sense)

[yorkshire_spam]

16mm^2 is about 110amps, not sure I'd be comfortable with something that size on an alternator with that rating.

[dazzer]

Thanks Guys

I'l err on the side of caution and go for thicker where applicable. I've got everything laid out ready to dive in, looked daunting to start with but its making more sense off the car.

Cheers

[RedRooster]

Not sure how long your wire run is, but lets say 3metres, with 25mmsq wire that will give you 0.4v drop across its length which is about 3.4% which is acceptable.

[Saltddirk]

Without measurements, about the same size as the wire to the starter. The starter draws 120 Amps on my roadster

[Dogsbody47uk]

Try  looking at the Vehicle Wiring Products website. They give guidance on wiring sizes needed. Cheers, Dave.

[bodders1]

Quoted from Saltddirk

Without measurements, about the same size as the wire to the starter. The starter draws 120 Amps on my roadster

Not exactly - your starter cable is designed to cope with the starter motor's STALL CURRENT.  i.e the maximum current the motor will draw when it is stationary.  The limiting factor in this case may well be the battery's own maximum current - usually printed on the battery as a CCA number (Cold Cranking Amperes).  This is what the cable to your starter motor must be able to cope with, and for standard fitment batteries on Triumph cars can be anything between 360 and 700 Amps.

Once the engine is turning over (even if it is not firing) the current drawn by the starter is less than the stall current.
Your measurement of 120 Amps will be the rated load current once the engine is turning over.

Here is a good page for calculating the size of cable you will need from the alternator back to the battery: http://www.solar-wind.co.uk/cable-sizing-DC-cables.html
Enter the percentage loss that you can put up with at maximum current draw (2 or 3 percent is acceptable) and the length of the cable run in metres (I'm guessing 1 metre) and it will tell you the cable size required for the job.  3% loss over 1 metre equates to a 16mm² cable cross-section (or 5 AWG in old money).

Hope this helps.
bodders1

[JohnD]

Do not forget the engine block earth cable!

The alternator earths through the block, and the OE eartrh was a length of mesh, often forgotten or rusted away.
Without it, the earth is through transmission, eventually, or the choke cable!

I had this problem, and in the end didn't need a bigger alt, juts a length of cable.
Make it the same cable as supplies the starter - it earths that as well - and take it right back to the battery earth point

John

[dazzer]

Thanks all for the info. Much appreciated!

Dazzer

[piman]

Hello Dazzer,

you don't have to use one cable of the rated size, you can double or triple a smaller cable which could make the terminations easier.
Not many of us have a 16mm crimp tool. Also ensure what cable you get is multi fine stranded for flexibility.

Alec

[dazzer]

Thanks Alec

Good point i was wondering how to crimp the cable sucessfully.

Cheers
Dazzer

[piman]

Hello Dazzer,

should you want to go with a 16mm sq lug say, the old way is to solder it. You'll need a blowlamp, hold the tip of the lug in a vice or mole grips, heat the lug and meld solder in it to the brim, then while still heating slowly drop the cable end in until it is fully home. Wrap cotton tape around the insulation as that stops it melting too much (cotton rag ripped into a strip works fine.)
Some modern lugs have a hole near the spade end, cotton tape again will keep the solder in.

Alec

[nicmk1est]

il second what he said above,,,solder it

[dazzer]

Thanks guys. Will do soldering iron at the ready.

[piman]

Hello Dazzer,

"soldering iron "

It's difficult to find a large enough iron for lugs, unless you can find an old sheet metal workers iron which were heated by a gas burner. Blow lamp or solder pot.

Alec

[dazzer]

Well not a soldering iron as you describe but i do have a big elecric fella as you refer. Looks to be pre war but serviceable that i,ve previously used on meaty jobs. It is electric and the light goes dim in the garage when its on!

[Rallyspit]

"I have yet to hear an argument that would move soldering ahead of crimping" [Google].

Here's one:

Soldering completely removes the possibility of moisture getting in the joint causing oxidation and a high resistance between the parts joined - crimping doesn't. Just make sure your cable and lug are clean and use flux and sufficient heat.

[ferny]

Quoted from Rallyspit

"I have yet to hear an argument that would move soldering ahead of crimping" [Google].

Here's one:

Soldering completely removes the possibility of moisture getting in the joint causing oxidation and a high resistance between the parts joined - crimping doesn't. Just make sure your cable and lug are clean and use flux and sufficient heat.

It also creates a hard point for failure and introduces an acid which will corrode the copper wire.

All my joints and connections when I did my engine conversion were soldered.

[piman]

hello Iain,

a properly crimped lug is better but there are crimping tools and crimping tools. The cheap long handled one with a single prod (for lack of a better description) are quite common but don't do anywhere near as good a job as the hydraulic type with decent shaped dies and I would say is not as good as a soldered lug. Not so easy to find to borrow for most people. A proper soldered lug is more than satisfactory carried out as I outlined earlier. Lugs were soldered for a great many years, and as I said, done properly they are a perfectly sound job. Incidentally acid flux should never be used on electrical connections, that's for tinsmiths.

Alec