MS wrote:

Recently, a picture was posted showing wiring to connect a 3G alternator. It stated to use a wire capable of carrying 135 amps, which is the max the alternator can produce. It also showed using a 175 amp fuse.
If there was a direct short and the fuse blew, wouldn’t the wiring, rated for 135 amps, melt way before the 175 amp fuse blew? Seems logical to me that a fused system should blow BEFORE reaching the max the wire is capable of handling? I always heard the fuse is there to protect the device and wiring.
Can one of the electrical gurus explain it to me? I must be missing something.

Yeah, that seems to make the wiring the fuse. I run a 220A alternator with larger welding batt cables and now I need to check the fuse size as I cannot remember. Anyway one day I had removed the alternator and was re-installing when I got interrupted. I came back, in my terrible memory thought I was done, hooked the battery up, and cranked the truck up. It ran fine then there was a fireworks show and it died. Turns out I had slipped that battery wire on the stud, but never put the nut on so the cable backed off and touched the header. It burned through one of the header tubes before the fuse blew! If the fuse had been bigger than the cable I would have had a real mess. Here's a pic of the damage to my $1200.00 Kooks headers


 

The pic of those headers made me shudder. Perfect example of really simple mistake causing max damage. Ouch!

Bolted to Floor wrote:

Disclaimer, I am no guru, but I have wired up a few things in the last 30 years!!
 
Where do you start…. so…. when you are dealing with a house or building, the electrical codes spells out some common practices for over current protection. Like certain over current sizes will be limited to a wire size such as 15A  and 14 AWG, 20A and 12 AWG, or 30A and 10 AWG. There are exceptions driven by the type of load. During design, your typical 20A circuit should have no more than 16A of load, 80% derating factor. Call it a built in cushion.  When you have a load, something that pulls 135A, it gets sized at 125% of the load or 168.75A. Typical, you move up to the next nearest common size for overcurrent protection. This may explain why the diagram has a 175 A fuse on the drawing. Do a search and a 95A drawing shows up with a 100 A fuse… go figure. Did Ford put a main fuse on the cars with the 95A and 135A alternators?
 
Dealing with automotive electrical, I can’t make it that clear. The 175A fuse is good as long as the wire is rated for 175A rings true. And ampacity ratings are where I have an issue. The conductor size, conductor type, conductor strand, and insulation will determine the amount of load it can handle.
 
My Painless harness came with a 75A fuse and #10 TXL wire which signals red flags for me and sent me down a rabbit trail for answers. When I was getting the Mustang together, I never found a chart that shows ampacity ratings of the new automotive wire, type GXL and TXL (both have high heat rating).  In the end, I trusted Painless knows what they are doing and I used newer vehicles as a design reference for other additions. If the manufactures were using XX wire size and TXL/GXL insulation type for XX fuse size, then I could too.
 
The TXL/GXL wire is a better choice over the welding cables because of the insulation. The welding cables have a lower temperature rating.   
 
Ouch on the header tube. Hope is was an easy fix…… I had to dimple one of my header tubes to fit around the Borgeson box. Beating on a $960 set of headers gave me no pleasure.

Great info, yeah when I added outlets to my garage I always figured in the 80% rule for breakers. For auto wiring I've always used the load and length of the wire to calculate a "safe" gauge. A good example was my fuel pump, it can pull up to 25A and is waaaaay at the back of the truck. I used USA TXL/GXL (depending on the use case) whenever I could find it when building my EFI harness. I used Wirebarn a lot! https://www.wirebarn.com/Wire-Calculator-_ep_41.html Going back to what Steve posted the alternator example seems to go contrary to that.

Header was fixed by a friend, only cost me a 12 pack and some lost pride...

Oh, I meant to say my welding cable usage is for the really big 8 gauge and larger stuff. 
 

Last edited by Raymond_B (7/06/2022 11:59 AM)

Went to answer this earlier but got detained, maybe this was already covered. Basically a 175 amp fuse blows at 175 amps. A wire rated at 135 amps is rated so it wont over heat at 135 amps. It will not not burn up and melt in half at 135 amps. A wire rated at 135 amps should handle 175 amps for a short period of time no problem and the fuse would blow first.

One needs to remember that the amps that a wire is rated to carry is based on holding that current continuously 24/7/365 and it is related to the insulation type and the number of conductors in the “raceway”.  The NEC current carrying value tables list these factors.  However, while the NEC values are a good reference, the application in vehicles is different. 
A direct short on a wire rated for 135 amps (continuous) wouldn’t have time to melt the wire prior to the 175amp fuse blowing.  The real problem would be a situation of say the alternator putting out 150amps for an extended period of time (and it would have to be a fairly long time) such that the fuse doesn’t blow but the heat created in the wire carrying the current melts the insulation, or sets it of fire.
If one is going to run a modern high current alternator, they should follow the fuse protection that a modern vehicle would use.  

I just bought one from Amazon, probably all made in China anyway.
Later on I saw them at a local auto supply house.

oreillys had the holder and fuse when i was sourcing one.

I’d suggest “Blue Sea” AMI or Mega fuses, along with their associated fuse holder.
Fuses are available to 200amp for the AMI, and 300amp for the Mega.