Discuss Cable Size And Current Capacity in the Auto Electrician Forum area at ElectriciansForums.net

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Eagle05

Hi, I have a question relating to cable size in 12v wiring on cars and the amount of current it can take.

After looking at a friends car on the ignition barrel one of the cables went back to a 40A fuse, yet the cable its-self was around 2.5mm. How is this possible? or am I missing something? As only knowing about domestic wiring the cable to me seems greatly undersized.

thanks
 
I suppose it is possible for a 40amp fuse to protect a 2.5mm cable, but it would need to be a pretty clever fuse (my version of WTF). With a mystery fuse marketed as 40 amps I think there is a real risk of 40amps + current causing a fire but not causing the fuse to blow. I wouldn't use 2.5 for more than 20amps in a car myself, and so a 20amp fuse would seem appropriate. I'd prefer to see 6mm cable for 40amp fuse. Even that is a bit light ...
 
Have a gander for AWG on Wikipedia. There is some interesting numbers in the table with regards to CSA and current carrying capacity.
 
Hi Guys - please also recall the ratings given for cables apply in specific conditions. I don't know what the ref conditions were for the 4mm 39A in the ref, but they may be something like "20C ambient, going to 70C when cable in free air unbundled". Since most car wiring systems are not unbundled and may already be at 50C in the engine bay, then some derating may be advisable. Just saying :)
 
If the cable has thin wall insulation the CSA might be larger than you estimate, if you have not examined the conductors. As above, there is no direct relationship 'cable size X require fuse size Y'. The fuse might be for short-circuit protection only, it might be necessary to support a motor starting current etc. You can't make a direct comparison with 230V practice - at 12V the short-circuit currents may be very low, the curves of the fuses are different, the energy dissipated at a fault is much lower, voltage drop is much more critical. Until a few decades ago many car circuits weren't fused at all.
 
Yes depending on the age and type of vehicle some circuits could have multiple fuses , or none at all .
The cable you describe may be activating a relay , from which the circuits are fused separately, and as said above will be for short circuit protection.
I agree that the cables on the ignition switch can be underestimated by visual inspection.
 
Thanks for the replies guys and sorry for the late reply.

I suspect the cable may have been slightly underestimated, however it did *just* fit into a 2.5mm through crimp, but then maybe it would with it being stranded wire as I guess you'll get more CSA for diameter over a 5 cored 2.5mm as its more compact, but I didn't' realise that 4mm was capable of near 40a, even with ideal conditions on ref method.
 
Could I recommend that you use 4mm or 6mm cable for a 40A fused circuit, anything between 0.5mm-1.0mm can be fused upto 20A and 1.5mm-3mm can be fused to 30A accordingly. I work in a company which specialises in 12V & 24V DC wiring for Quarry plant and mobile machinery. If the cable is in a Loom (kopex) use finewall cable to allow for extra wires to be added etc. For terminating use a good insulated Lug or a Ferrule and connect it in
 

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