Hey Paul, need your expertise

[Bobby B.]

Frank and I were talking about the Blue Sea fuse blocks and I realized that after five years I should put some sort of fuse or breaker on it to keep it safe.

I don't have any space for another maxi fuse block, so I'm going to use a small automotive self-resetting breaker.  Frank has room, so he's going to use the maxi fuse on his.

Here's the question -- How to pick the rating to protect the fuse block?  Overall it's rated for 30 amps per circuit, and 100 amps per block (at least for the 12-fuse model).  What's the best way to figure out a safe fuse?

I'm running eight gauge wire, which would put me at a 50 amp fuse.  But should I run less to protect the block since each circuit is only rated to 30?  I wonder because there's never a time when everything is on at the same time, but oftentimes there are several items working at once.

Any input?

[Paul]

8 gauge cable is rated at 50 amps or 600 watts - that's how big of a breaker to get.

If you want 100A to match the maximum output of your fuse box you'll need to run 2 gauge welding cable!

Your equipment will only draw as much as it needs so you design the cable to handle that and a bit more. The 50A that 8 gauge cable is rated at has a big explanation with the exact type of cable, the density of the cable (loose or in raceway), and even a correction factor for ambient temperature.

So don't line up more than 600 watts (ever) on that 8 gauge cable. That protection is designed into the system.

Fuses and breakers are designed to fail in a second where the wire would fail in minutes or more following a short circuit. You want the part that melts to be inside a safe glass container rather than under the carpet where it will melt and possibly start a fire. You want to size the fuse to the circuit limit or maybe a bit larger. That 50A 8 gauge cable is a good place to hang a 50-60A fuse off of. Some loads will "surge" as they come on (like an amplifier) and then run at a lower level. A stereo amplifier might surge to 10 amps as it powers on for a second and then play at a good level at 3 amps.

I ran 4 gauge cable to my fuse box under the dash which is about 85A/1020W and protect it with an 80A breaker (Stinger). I wouldn't sweat a 100A breaker because you design the cable around the loads and I have nowhere near 1000W worth of load. A short circuit is going to trip that 80-100A breaker in less than a second where the wire is going to take much longer than that to heat up and burn.

A breaker has some advantages and disadvantages compared to a glass fuse.

Pros: less sensitive to surge loads (like a slo-blow fuse)
Re-settable - push the reset and you're good to go (but tripping does shorten the life)

Cons: More expensive (but start paying for themselves with each accidental trip - those big fuses aren't cheap!)
Slow (but still fast enough to protect the cabling)
Bigger

[Bobby B.]

Thanks man.

How large are those stinger breakers?  I was looking at this one, mainly because I just used the same type from etrailer -- I just installed a brake controller and 4/7 pin plug flush mounted in the rear shrock.

http://www.etrailer.com/Accessories-and-Parts/Superwinch/87-22873-11.html

Very small and cheap, but no mounting bracket on this one.  The others I installed (30A and 40A) I simply tapped in behind the battery and was hoping to do much the same with this.  Space now really is at a premium in there.

[Paul]

About twice and then some larger then the ones in the pictures in the link you posted. I found the Stinger on a car stereo site. Mine has a red push button open and a lever reset so that I can disconnect (when I remember) while working on the power.

Hey Paul, need your expertise

[Bobby B.]

Found an alternative to the larger Stinger breaker at NAPA.  $10, made in the USA. Part number 782-3001 if anyone needs one.

Same small size as the others, but 50a.  In the pic it's the one with the red leads.

Hey Paul, need your expertise