Fuse size question.
Thread Starter
Safety Car
Joined: Jan 2004
Posts: 3,783
Likes: 15
From: Northern California
Fuse size question.
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.
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.
Last edited by GKK; Sep 3, 2006 at 02:20 AM.
Thread Starter
Safety Car
Joined: Jan 2004
Posts: 3,783
Likes: 15
From: Northern California
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.
I'll be using a 150 amp ANL fuse for a 5 ft. length of 4awg cable.
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
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
Thread Starter
Safety Car
Joined: Jan 2004
Posts: 3,783
Likes: 15
From: Northern California
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
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
Last edited by GKK; Sep 4, 2006 at 12:34 PM.
Safety Car
Joined: Apr 2003
Posts: 3,793
Likes: 9
From: VA
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.
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.
Le Mans Master
Joined: Nov 2004
Posts: 8,718
Likes: 20
From: Dyer IN
Cruise-In IX Veteran
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
Safety Car
Joined: Apr 2003
Posts: 3,793
Likes: 9
From: VA
Originally Posted by pentavolvo
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
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.
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