Fuse Sizing Philosophy?

[Gary Lewis]

I need some help figuring out what size of fuses to use to replace fuse links. If you've been following my efforts on Big Blue's Transformation you've seen that I am implementing a power distribution box (PDB) that has both relays and fuses.

So I'm going to replace the several fuse links that are right around the location of the PDB, which is right behind the battery and next to the alternator. But, I need to figure out what size of fuses to use.

The links that are to be eliminated are shown in the schematics below, but they are:

• Fuse Link F: A 16 ga link that provides power to the trailer brake controller

• Fuse Link L: A 16 ga link that supplies the headlight switch as well as some fuses, like door locks

• Fuse Link M: A 16 ga link that supplies power to the main fuse box as well as the auxiliary battery

• Fuse Link S: A 20 ga link that supplies power to the fuel pump through the inertia switch

• Fuse Link T: A 20 ga link that supplies power to the fuel pump when cranking

One approach would be to add up all the fuses that the new fuse will support and use that number. But Fuse Link M supports fuses that add up to 135 amps, and on top of that it provides charging current to the aux battery.

Another approach would be to size based on the wire size. I don't know yet what the wire size is for Circuit #37, but I kind of doubt it is large enough to support 150 amps.

And another approach would be to fuse for the amount of current that is anticipated to flow.

What suggestions do y'all have?

[ArdWrknTrk]

A fuse link is meant to protect the harness and prevent a fire.

But it will take surge loads far beyond what a fuse will.

I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown.

You don't trust the experts that make Billions of fuses?

[myrl883]

A fuse link is meant to protect the harness and prevent a fire.

But it will take surge loads far beyond what a fuse will.

I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown.

You don't trust the experts that make Billions of fuses?

I've done some reading on this, but it gets deep fast. I don't have the engineering knowledge to answer.

From what I have read, it isn't a good idea. Fuse links act as a slow blow fuse, and the wiring in our old trucks is heavier than a more modern car, so the instant failure of a standard fuse isn't needed to protect the wiring.

I've decided that no more often than fuse links fail, I'm leaving my trucks as they were built and leaving the fuse links in place. The only one that I've changed as an upgrade is the one that protects the charge circuit. I changed from a 16 gauge to a 14 gauge fuse link when I did my 3G conversion.

[ArdWrknTrk]

I've done some reading on this, but it gets deep fast. I don't have the engineering knowledge to answer.

From what I have read, it isn't a good idea. Fuse links act as a slow blow fuse, and the wiring in our old trucks is heavier than a more modern car, so the instant failure of a standard fuse isn't needed to protect the wiring.

I've decided that no more often than fuse links fail, I'm leaving my trucks as they were built and leaving the fuse links in place. The only one that I've changed as an upgrade is the one that protects the charge circuit. I changed from a 16 gauge to a 14 gauge fuse link when I did my 3G conversion.

So, a 10G wire then?

My understanding is that fuselinks are sized 4 gauges smaller than the circuit they protect.

SAE J156 is VERY deep, indeed

[Danny G]

A fuse link is meant to protect the harness and prevent a fire.

But it will take surge loads far beyond what a fuse will.

I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown.

You don't trust the experts that make Billions of fuses?

This^ they make fuse links that look like fuse blocks instead of wires.

[85lebaront2]

A fuse link is meant to protect the harness and prevent a fire.

But it will take surge loads far beyond what a fuse will.

I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown.

You don't trust the experts that make Billions of fuses?

This^ they make fuse links that look like fuse blocks instead of wires.

Try this information and see if it helps:

These are from a Chrysler site hence the odd circuit numbers. I also realize some of them don't jive.

[kramttocs]

I've done some reading on this, but it gets deep fast. I don't have the engineering knowledge to answer.

From what I have read, it isn't a good idea. Fuse links act as a slow blow fuse, and the wiring in our old trucks is heavier than a more modern car, so the instant failure of a standard fuse isn't needed to protect the wiring.

I've decided that no more often than fuse links fail, I'm leaving my trucks as they were built and leaving the fuse links in place. The only one that I've changed as an upgrade is the one that protects the charge circuit. I changed from a 16 gauge to a 14 gauge fuse link when I did my 3G conversion.

Same here. In the middle of doing a lot of electrical upgrades but I've never had any real fuse link issues and don't really see a problem with them so am leaving them in place both at the fender solenoid and at the junction block (now aux batt solenoid). A small portion of that decision is that if I cut the fuse link out I would have to splice a short piece of wire to reach my fuse block and I am avoiding inline splices where possible.

[Gary Lewis]

Same here. In the middle of doing a lot of electrical upgrades but I've never had any real fuse link issues and don't really see a problem with them so am leaving them in place both at the fender solenoid and at the junction block (now aux batt solenoid). A small portion of that decision is that if I cut the fuse link out I would have to splice a short piece of wire to reach my fuse block and I am avoiding inline splices where possible.

Ok guys, thanks for the input. You've given me a lot to think about.

Jim - You said "I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown." I'll go looking for that info. Did we talk about it recently?

Bill - Your Chizzler info is helpful. Summarizing:

• 20 gauge is 20 to 40 amps

• 18 gauge is 40 to 50 amps

• 16 gauge was omitted but 60 to 80 amps

• 14 gauge was omitted but might be 100 to 120 amps

• 12 gauge is 160 amps

Scott - I will have to splice short pieces of wire in as well, but am ok with doing that. I have another harness from which I can get the same wire so the sizes and colors will be consistent. But, at this point I'm not addressing any fuselinks at the aux battery solenoid.

All - I understand that fuselinks don't fail very often. But having gotten down to fuselinks L & M, which are buried pretty deeply in the harness, I'd sure hate to have to replace them while on a trip. (I'll post pics in a bit of where they are.)

And, as shown in the schematic below, I'd really like to replace the stuff in the orange rectangle below - Fuselink J and the shunt. But if Fuselink J is replaced by a fuse then it needs to support both fuselinks L & M. So why not replace them instead and not have anything for Fuselink J?

 

[kramttocs]

Ok guys, thanks for the input. You've given me a lot to think about.

Jim - You said "I believe Bussmann & Littelfuse produce ultra slow blow fuses meant to replace them, and equivalence ratings are shown." I'll go looking for that info. Did we talk about it recently?

Bill - Your Chizzler info is helpful. Summarizing:

• 20 gauge is 20 to 40 amps

• 18 gauge is 40 to 50 amps

• 16 gauge was omitted but 60 to 80 amps

• 14 gauge was omitted but might be 100 to 120 amps

• 12 gauge is 160 amps

Scott - I will have to splice short pieces of wire in as well, but am ok with doing that. I have another harness from which I can get the same wire so the sizes and colors will be consistent. But, at this point I'm not addressing any fuselinks at the aux battery solenoid.

All - I understand that fuselinks don't fail very often. But having gotten down to fuselinks L & M, which are buried pretty deeply in the harness, I'd sure hate to have to replace them while on a trip. (I'll post pics in a bit of where they are.)

And, as shown in the schematic below, I'd really like to replace the stuff in the orange rectangle below - Fuselink J and the shunt. But if Fuselink J is replaced by a fuse then it needs to support both fuselinks L & M. So why not replace them instead and not have anything for Fuselink J?

So you are talking about cutting at splices 102 and 103 and hooking them up directly in your fuse box?

With going to the 1 wire alt I heavily simplified the left hand portion of C610 and I did connect the Y (cab power) to a 60 amp fuse in my fuse block. I didn't modify those fuse links though.

Edit: post corrected/updated - I was incorrectly reading L and M as being under the dash - which did sound odd. What does T/O stand for in the location chart?

[Gary Lewis]

So you are talking about cutting at splices 102 and 103 and hooking them up directly in your fuse box?

With going to the 1 wire alt I heavily simplified the left hand portion of C610 and I did connect the Y (cab power) to a 60 amp fuse in my fuse block. I didn't modify those fuse links though.

Edit: post corrected/updated - I was incorrectly reading L and M as being under the dash - which did sound odd. What does T/O stand for in the location chart?

I'd say splices 102 & 103 are where I'm connecting and extending to the PDB. And, as you've figured out, they aren't under the dash. In Big Blue's case they are on the passenger's fender liner, and I'll show a pic in a bit when I get back from a meeting at church.

But you kept L & M and replaced J w/a 60 amp?