GKK

On the Big3 wire upgrade, what size fuse should I use from the alternator wire to the battery post?

I've been reading on numerous websites but, still can't decide. One site says: You should use the largest fuse you can for the wire size, fuses are restrictive to current flow. Typically you want the fuse value to equal 80% of the wires load carrying capacity.

How do I determine the load carrying capacity of a 5 foot long 4 awg wire? edit: (I found that a 5 ft. length of 4 awg wire can carry a load up to 190 amps so, 80% of 190 = 152. a 150 amp fuse should be used. Information from this site: http://www.alternatorparts.com/wire_size_chart.htm

Another site says to use only a 60 amp or less because the electrical system doesn't use that many amps and if it does there is something very wrong with it.

I asked a guy at the local high end car stereo shop and he said to use a 200 amp fuse.

Which is correct? Thanks for the help.

GKK

I've found that 80% of the load carrying capacity of the cable used, is the correct formula to figure out the correct size fuse to use as in my previous post above.

I'll be using a 150 amp ANL fuse for a 5 ft. length of 4awg cable.

SIXFOOTER

That is not correct. The fuse should be about 125% to 150% max load for the device your powering. If you connect a 150 amp fuse to a wire from the alternator to the battery, you bypass the fusible links designed into your electrical system to protect it, and quite possibly created a fire hazard, if you get a short in the alternator it could burn and never blow the fuse, same thing if the battery shorts internally

GKK

I'm confused again.

I plan on leaving the stock 8 gauge alternator wire with the fusible link attached (don't know what size the fusible link is) and install a 4 gauge wire from the alternator post to the positive side of the battery.

SIXFOOTER, can you explain further your statment , "The fuse should be about 125% to 150% max load for the device your powering".

I'm upgrading to a CS144 140 amp alternator and it's powering a mostly stock "91" Vette with a 305 watt audio system.

Anyone that has done the Big3, have you added a fuse on the alternator wire or just left it without? Also, if you've added a fuse, what size fuse on what gauge wire?

Thank you

VenkmanP

80% of wire ampacity would be correct for your alternator to battery lead.

If it was, for instance, an amp lead, then both would come into play - you'd want a fuse which is the lower of (~20% smaller than the wire ampacity) or (~20% bigger than the expected load).

The 190A number is interesting. In household use, #4 wire is rated at 70-85A. I guess we are more careful about not burning down houses than cars...

Googling around, I found several examples of people installing #4 for the alternator-battery lead, and they used 150-175A fuses.

pentavolvo

150-175amp is a pretty normal size fuse for 4ga. Also the reason it is fused smaller in home use is because of higher voltage and generally its a much longer run of wire so resistance comes into effect

GKK

Thanks everyone! Your information helped a lot.

VenkmanP

Voltage and length have absolutely nothing to do with overcurrent protection of a wire.

A 1' length of #4 used in a house at any voltage (well, actually low-voltage wire ampacity isn't regulated - for sake of argument 120V or 240V) has the same limit of 70-85A. The exact limit depends only on type of wire and installation method.