fuse size between battery and junction block

[gwyon]

I added a second junction block on the firewall of my 1985 GMC Sierra.  I'm doing the relay upgrades, so I'll use the new junction block to power the headlights, power windows, and wiper motor.  I want to add a fuse close to the battery to protect the wire between the battery and the block.  What size fuse should I use?
 
Thanks.

[bd]

That's kind of a loaded question since there are several undefined variables.  The following scenario should help convey the idea.  Certainly, other scenarios exist.  Assuming four, DOT approved headlamps at ~60 watts each for estimating purposes, the maximum headlamp draw will be ~240 watts, equating to ~20 amps; add another 10 - 14 amps for running lamps; windshield wiper motor on high speed ~20 amps, and power windows rolling up/down ~25 amps.  So driving at night in heavy rain with the headlamps on high beam while rolling a window up conceivably could draw as much as ~80 amperes, if only for a few moments. 

You definitely don't want to lose the function of safety items like headlamps, w/w or power windows, especially at the same time at night, so you need to plan conservatively.  Allowing for a 20-amp margin, the B+ cable feeding the junction block should be able to safely carry 100 amps at a maximum voltage loss of ~5%.  The supply cable's insulation should be able to tolerate high temperature, abrasion and typical automotive chemicals, so specify SXL wire.  Assuming a B+ cable length of ~10 feet, 8-gauge stranded copper wire is the absolute miniumum wire size to consider, with 6-gauge being a better choice.  To protect the junction block B+ cable, install a 100-amp ANL, MIDI or MRBF fuse near the battery, or substitute a 6" long, 12-gauge fusible link for the 8-gauge cable or a 10-gauge fusible link for the 6-gauge cable.  In addition, each appliance (headlamps, w/w, power windows) needs to be independently protected by an appropriatly rated fuse or circuit breaker.  Bear in mind that the headlamp switch has two B+ feeds that are independently protected, one for the headlamp circuit and one for the running lamp circuit.  The instrument lamps are redundantly protected by the 20-amp running lamp fuse and the 5-amp instrument lamps fuse.  Select a junction block with at least a 5/16" diameter stud.

[gwyon]

Wow, thank you, bd.  I have some nice 6 gauge marine grade battery cable that I'll use.  I've never installed a fusible link before.  How long should the link be?  And will split loom and some zip ties be enough to protect the cable?

[bd]

References:

How to make a Fusible Link - Fusible link is a very specific type of high strand count wire that is insulated by ultra-high temperature insulation.  DO NOT substitute any other type of wire for use as fusible link!  The industry standard length of a fusible link is 6" regardless of gauge (note that rare exceptions exist for very specific applications that do not pertain to your project).  The fusible link is always four wire gauges smaller (numerically larger) than the wire being protected (add 4 to the wire gauge being protected).  For example, 6-gauge wire is protected by a 10-gauge link, 14-gauge wire is protected by an 18-gauge link, etc.

Diagnosing dash instrument lights - Study the images and descriptions of the headlamp switch connections under Supplemental Information.  Both the red and orange wires separately feed battery power into the headlamp switch. 

If your intention is to install a headlamp relay kit while maintaining the factory wiring for relay control (a good approach), connect the B+ supply leads from the relays to the junction block and deduct 14 amps from the load calculation for the primary bus feeding the junction block, dropping the estimated maximum draw to 66 amps.  In this scenario, the primary bus fuse can be decreased to ~90 amps, but maintain the previously suggested wire sizes, with 6 gauge being preferred.

[gwyon]

Thanks for the help, bd.  I really appreciate it.