I want to move the battery from its present position on the front bulkhead, to a area as far back in the chassis as i can get it. What diameter wire should i use to cope with the length of cable to best effect? I am a little concerned about abrasion of the cable and thought about running it through a piece of alcothene or similar to protect it from earthing out, am i going over the top here do you think, it is just that i had a MGB gt as a kid and it did earth out and wrecked a battery.

The Earth wire will be local to the battery so no need to change, the live wire should be OK, but you could go up a size to be sure.

Abrasion should be no problem if you mount it safely.

If you routh the wire along the transmission tunnel make sure you mount it below the loom.

I have just done a similar thing on my R1 powered reverse trike, I have moved the battery and its wiring from the rear to the front, I took a good earth from the engine block and took a power lead from the starter motor. I used both my 1.5Mtr jump leads to do the job, plenty enough amp capability on these cables. The cable is also a nice quality and flexible. I luckily had a pair of cables kicking around, I think a decent pair long enough will set you back £20, much cheaper than the official battery cables, in my opinion more than capable of the job.

The thing to be aware of is the proximity of the battery and fuel tank, not a show stopper but something to keep in mind.

Get a bit of split convoluted tubing and put the battery cable in that before fixing to the chassis. You might even get a battery cable down a bit of non split tube.
I would also wrap a bit of rubber (I tend to use sections of old push bike inner tubing) between the cable and the P clips.
Just helps to stop it wearing thru and causing a major fire.

I've had a battery cable fire under a lowered mini cooper, and its bloody hard to get at to put out at 3am on a lonely back road.
Hence I take a bit of extra care even thought that was 35 years ago!

I would look for a cable that will take in excess of 200 Amps, and preferably 300+ Amps. Also thicker cable will reduce the volt drop between the battery and the starter. Dont worry about the volt drop for the rest of the electrics, as the volt drop would only be significant when cranking the engine. A lot depends on what engine it is and what the starter motor draws under load.

Consider using Welding Cable, as its good quality and very flexible. Downside is its not cheap.



EDIT:
As said above, make sure the location is not hazardous. A poorly secured battery adjacent to the fuel tank is not a good idea, should you have an accident.
Secure it properly and in a good location.

Whatever you do, do it properly. Its YOUR life as a minimum you are putting at risk by not doing a proper job.



[Edited on 24/12/2011 by wilkingj]