Cross Sectional Area

[zoidberg]

I'm entangled in a web of conflicting advice regarding optimum Cross Sectional Area ( CSA ) for new 'lecky cabling going into the once-upon-a-time boat - specifically, that needed for the Start and Other battery circuits.

Various tables suggest I should be more-or-less fine with 25mm CSA cabling, for the voltage drop is likely to be better than 0.017V for the cable lengths. I have just about decided to go with 35mm CSA, for there may be more battery capacity added later on.... and I don't really understand 'starting surge current' and twin paralleled batteries capable of giving 2 x 845 Cold Cranking Amps, which sounds a lot. That may well spin the engine up, but might it be a tad OTT for the wiring?

And what about the terminations at the ends of the 35mm CSA cabling? Surely those crimped-on ring terminals, at their narrowest, are far less 'capable' than the adjacent 35mm copper cabling and the M8 bolt(s) they're attached to?

i haven't read a squeak about correct 'sizing' of these little weak links. Can someone advise....?

 

[MikeCC]

35mm is generally adequate and will take around 250A continuous. The batteries may offer 845CCA but it's what the starter draws that you are interested in. It will only take what it needs. It might be momentarily more than the cable capacity but not for an extended period.

The crimped terminals are only carrying the current over a very short distance. There's an awful lot of them out there, so let's hope they're up to the job. Compare the thickness of a fuse wire to the cable it's protecting as an idea.

The critical thing with the terminals is to ensure they are robustly crimped. Some crimp / tool combinations are better than others and high-end applications will often use both from the same manufacturer to ensure reliable connections.

 

[PaulRainbow]

Pretty sure i'll say 35mm with crimped lugs, protected by heat shrink. Lugs to be properly crimped, i use an 8 ton hydraulic crimper.

But, remind me of the battery sizes, engine and cable lengths please ?

 

[Lon nan Gruagach]

Aha, the weak link crimps (again)
Let's have a look at the two issues, volt drop and capacity.
Volt drop :
The crimps are, compared to a cable run, very very short, they would need to be in the order of 1/100th the size of the cable to have any significant effect.
Capacity :
Current capacity in a cable is a function of how much it can carry without bursting in to flames. Every inch of cable must be able to dissipate all the heat it generates. A crimp, however, has a stonking great heat sink on one side (the battery) and is usually open to the air, no thermal blanket insulation.