Fusable link vs maxi fuse?

[svr]

Im using a 21 circuit wiring harness on my '57, have several modifications so bought a more universal painless harness. It came with a couple very small fusable links, but the instructions call for a maxi fuse to protect the complete harness. It doesnt say how big of a fuse to use, any thoughts?

[ray_mcavoy]

The fuse or fusible link needs to be sized according to the gauge of the wire it will be protecting.

The general rule for fusible links is to use a link that is 4 wire gauge numbers smaller than the wire. Some will say 2 sizes smaller because AWG skips odd numbers. So for example you would use a 14 gauge fusible link to protect a 10 gauge wire.

And here is a link to a chart that shows the recommended fuse sizes based on the wire gauge --> http://bdfuses.com/fusesnwires.php So using the same 10 gauge wire for example, you would want to use a 30A (maximum) fuse.

For protecting the main feed wire(s) in your new harness, you'll want to locate the fuse(s) or fusible link(s) on the end(s) of the wire(s) near where they connect to the battery.

[VetteVet]

The stock fusible link was rated at 30 amps but it would survive a bit more than that.
I am using a 40 amp maxi-fuse in my harness and I have it just off the battery in a battery shutoff circuit.

[SunSoaked]

Originally Posted by VetteVet

The stock fusible link was rated at 30 amps but it would survive a bit more than that.
I am using a 40 amp maxi-fuse in my harness and I have it just off the battery in a battery shutoff circuit.

This makes me wonder: If the harness can only pull 30 amps what is the purpose of high output i.e., 100/150 alternators? That means every accessory added must go directly to the battery and not thru the fuse panel, even the open ports?

[Dead Parrot]

There just weren't many high draw gizmos on most 57s. Couple of taillights, plain incandescent headlights, a few dash lights, a heater fan and maybe an electrical wiper motor.

The max output may not be of much benefit but getting an internal regulated alt that outputs higher idle current can be. On my 75, switching from a 10SI to a 12SI got rid of much of the idle dimming when running headlights, blower on max and wipers.

[VetteVet]

Originally Posted by SunSoaked

This makes me wonder: If the harness can only pull 30 amps what is the purpose of high output i.e., 100/150 alternators? That means every accessory added must go directly to the battery and not thru the fuse panel, even the open ports?

Knowing how the harness is wired is the key. The fusible link is only in the battery circuit for battery power. The alternator is wired to the main junction and each of it's loads is fused separately or through the fuse panel. That's why it is never a good idea to run a one-wire alternator output wire directly to the battery positive cable and then the fusible link wire carries all the current.

Originally Posted by Dead Parrot

There just weren't many high draw gizmos on most 57s. Couple of taillights, plain incandescent headlights, a few dash lights, a heater fan and maybe an electrical wiper motor.

The 57s had fusible links and all their current runs through the links but they only had a 30 amp generator, plus all their current runs through the amp meter.





The max output may not be of much benefit but getting an internal regulated alt that outputs higher idle current can be. On my 75, switching from a 10SI to a 12SI got rid of much of the idle dimming when running headlights, blower on max and wipers.

You are absolutely right and combining them with relays will get you enough power for brighter headlights and faster motors etc.

Here is my diagram to convert to the SI and eliminate the external voltage regulator. I probably have a 1000 posts on the forum about how to do it.





And if you want to use the nineties and up CS style use this one and just add a 35 to 300 ohm resistor in the brown wire to the L terminal on the alternator plug.

[lutronjim]

Grainger has some delay FMX fuses. I plan to use one of those to feed my cab instead of the fusable link.