Talk me off the Electrical Ledge

[BrewCityCobra]

So I'm in the process of designing and building my "feed harness" (e.g., the wires between the battery, starter, and feeding the main fuse panel) and came across a potentially unsafe situation that caught my eye and now I'm worried I may be over-thinking things. I do have previous experience wiring cars but for some reason I just can't get this idea out of my head. So I'm looking to the forum to talk me off the ledge (or not).

For background, looking to how the Ron Francis wiring harness is setup out of the box - none of the wires positioned upstream (e.g., on the positive side) of the primary fuse block are protected at all (unless I'm missing something). Basically the 4 gauge wire feeding the starter, and the three main wires connected to the starter lug (RED-BATTERY FEED, RED-ALTERNATOR FD, RED-IGN SW-SOL) are all in the wind and if their insulation was to become compromised they would fry.

OK - that isn't the biggest issue in the world and can be corrected by a pair of well-placed MEGA fuses. I suppose there has to be a reason the people whose job it is to actually engineer these things decided that the risks associated with those wires was sufficiently low to be left as is. Who am I to say otherwise, right?

Now comes my design which is a bit more complex to introduce additional safety features and a master cutoff switch. I included two MEGA fuses (200A for the Battery and 130A for the alternator) to protect the "feed" wires upstream of the main fuse terminal block. Now, if any wire anywhere on the entire car completely shorts out (including the feed wires in the engine compartment) - the Mega fuses should blow and all is protected….right? Great.

158711

Well, this is where my conundrum comes in. The two wires feeding the primary fuse box (RED-BATTERY FEED, RED-IGN SW-SOL) only appear to be 10 AWG wire in the kit and therefore can only handle about 60 amps. As such, a condition could exist where the insulation in those wires becomes compromised such that a current greater than 60 amps but less than 200 Amps could go through it - thus the 200 Amp MEGA fuse would not blow to protect the circuit and those wires could potentially fry and start a fire.

A potential solution to this issue would be to add include 60 Amp fuses in the 10 AWG wires which would eliminate any condition where those wires could be overloaded. But that seems excessive and made me start to second guess myself.

So part of me says - Hell, these two wires are normally not protected AT ALL, the fact I already have extra protection in place (e.g., the two MEGA fuses) is above and beyond what's required for a safe car so don't worry about it. The chances the wires ground out in such a way that the resulting current is above the wire capacity but below the MEGA fuse limit is effectively null. If anything, a shorted wire will almost certainly blow the 200Amp fuse.

But I can’t shake the idea that a condition may exist where those wires could be compromised.

So my ultimate question to the group - am I going crazy? Has anybody else addressed this issue? Am I OK to just keep the MEGA fuses as I have it laid out in the diagram and leave the 10AWG wire as is?

[jiriza84641]

Hey, don't stress out on this, your schematic looks good, I followed a similar setup from the forums (EDWARDB). He has a good wright upon the wiring for a coyote which would work for. other motors as well. The two wires that feed the fuse box are not going to draw the power it needed top melt the two 10G wires, since there is not much. circuits on the car. I have those one a buss bar that is linked to the battery disconnect and the fuse box 10G wires are connected to that.So, when I cut off the battery power to the fuse box shuts off. The Alt wire is also connected to the buss bar with a 130a fuse. The starter wire has a 4 gauge if my memory serves me right from the battery disconnect. The PCB for my coyote is connected to the hot side of the battery disconnect so that the ECU has constant power with minimal amp draw to prevent the ECU from having to relearn everything.

I am sure some will chime in also.

[BrewCityCobra]

Yeah, the Edward B layout was generally what inspired the layout I put together. Which is another indicator that it is probably fine and I'm just making a big deal out of nothing.

I guess I was just more surprised after you think about it a bit how the "upstream" side of most car circuits are effectively unprotected.

[edwardb]

Since I'm referenced here will weigh in. :p I agree about putting a megafuse between the alternator and the battery. DD's typically have a fusible link so protection there is an industry practice. Just wondering if the 130amp fuse you show is adequate for whatever alternator you're planning. That's maybe pretty light for many alternators these days. Something to check. A starter can draw a bunch of current, and depending on the situation, 200amp might be light there too. Probably you know this, but DD's (and boats for that matter) typically don't fuse this circuit. The subject is discussed quite a bit, but normally not done. Safety is one of the reasons cited. Seems counterintuitive, because safety is your concern obviously. But tripping a fuse that prevents vehicle starting is also seen as a safety issue. With 2 gauge wire, and very careful routing and tie-down of the cables, you will be OK. If helps you sleep at night, put one on that circuit. But personally I don't and the car you're driving every day from whatever major manufacturer doesn't have one either. As for the 10 gauge wires going to the panel, keep in mind that every circuit downstream tied to those wires is protected. Don't mean to be Captain Obvious here, but remember current is drawn, not pushed. So they shouldn't ever draw any more current than the 10 gauge can handle since any faults would blow a fuse. If you add any circuits (you may have seen some in my builds) then yes add a properly rated fuse or circuit breaker on the added circuit and make sure the total (panel plus whatever you add) doesn't exceed the capacity of the 10 gauge. But a fuse on those lines is redundant and just adding a point of failure for no reason.

[CraigS]

I'd say that your thought here is correct.
"The chances the wires ground out in such a way that the resulting current is above the wire capacity but below the MEGA fuse limit is effectively null. If anything, a shorted wire will almost certainly blow the 200Amp fuse."
However, on the Lexus cars I used to work on, the heavy wiring bat to starter, bat to fuse box, etc. was contained inside hard plastic specifically formed insulators. This pic happens to be of engine injector harness but you can see the formed insulators w/ the tabs for mounting them to the engine.
https://forum.ih8mud.com/attachments/wiring-harness-jpg.2500370/
You can easily add some wire loom to provide similar protection for your wiring. When I moved my battery to the front I used split loom to cover the + cable. First I installed a size that was pretty tight, and over that I installed a size or two larger and I oriented the splits of the two layers 180 degrees apart.
https://www.delcity.net/store/Loom-&-Loom-Clamps/?utm_term=wire%20loom&identifiers=kwd-73461321283468:loc-190&Campaign=Loom%20-%20Broad&CampaignId=361477659&AdGroup=wire%20loom&AdGroupId=1175378814561811&AdId=73461365088024&Network=s&msclkid=9632a2c920691cba0261c375a340c0a4&utm_source=bing&utm_medium=cpc&utm_campaign=Loom%20-%20Broad&utm_content=wire%20loom

[Nigel Allen]

I put a 100amp fuse on my alternator feed. It is a bit undersized, but what I could get my hands on. Rather than find out on the side of the road that it was too small, I did testing by discharging the battery right down to a point where it could just start the engine. Once the engine started, I switched on all loads (headlights and heater blower) I then ran the engine at 3000 RPM for over a minute. This would have worked the alternator hard in the worst case scenario. Fuse held up just fine.

I have never fused a starter motor. However I did measure starting current for another member:

https://thefactoryfiveforum.com/showthread.php?38139-Coyote-Starter-Current-Draw&p=437900#post437900

Hopefully this info helps.

Cheers,

Nigel

[LateApex]

On the topic of the battery disconnect, did you entertain the idea of putting the disconnect on the ground side?

I note that the kit's (+) cable to the Starter is 4 AWG. I am looking at wire gauge charts to better understand voltage drop, and specifically when I might add another 5 foot of wire to the PS foot box.

Regardless, I'll likely be wiring my Terminator-X directly to the battery ...

[BrewCityCobra]

I put a 100amp fuse on my alternator feed. It is a bit undersized, but what I could get my hands on. Rather than find out on the side of the road that it was too small, I did testing by discharging the battery right down to a point where it could just start the engine. Once the engine started, I switched on all loads (headlights and heater blower) I then ran the engine at 3000 RPM for over a minute. This would have worked the alternator hard in the worst case scenario. Fuse held up just fine.

I have never fused a starter motor. However I did measure starting current for another member:

https://thefactoryfiveforum.com/showthread.php?38139-Coyote-Starter-Current-Draw&p=437900#post437900

Hopefully this info helps.

Cheers,

Nigel

Wow, thank you for that information. Cant' really argue with solid data. I'll make sure to take this into account. I looked through that thread as well and there is all sorts of good stuff in there.

[BrewCityCobra]

On the topic of the battery disconnect, did you entertain the idea of putting the disconnect on the ground side?

I note that the kit's (+) cable to the Starter is 4 AWG. I am looking at wire gauge charts to better understand voltage drop, and specifically when I might add another 5 foot of wire to the PS foot box.

Regardless, I'll likely be wiring my Terminator-X directly to the battery ...

So I considered having a "ground" cutoff switch but ultimately decided it would just require more wires (I'd have to run 2 gauge all the way up to the firewall) and I was worried it wouldn't stop the alternator from operating since it, too, is grounded to the chassis by itself (people can correct me if my assumption there is incorrect). By having the setup I have laid out above it guarantees the engine must stop if I open the battery cutoff switch which was my ultimately goal. Also allows me to completely cutoff the starter if something were to happen to it.

[BrewCityCobra]

Since I'm referenced here will weight in. :p I agree about putting a megafuse between the alternator and the battery. DD's typically have a fusible link so protection there is an industry practice. Just wondering if the 130amp fuse you show is adequate for whatever alternator you're planning. That's maybe pretty light for many alternators these days. Something to check. A starter can draw a bunch of current, and depending on the situation, 200amp might be light there too. Probably you know this, but DD's (and boats for that matter) typically don't fuse this circuit. The subject is discussed quite a bit, but normally not done. Safety is one of the reasons cited. Seems counterintuitive, because safety is your concern obviously. But tripping a fuse that prevents vehicle starting is also seen as a safety issue. With 2 gauge wire, and very careful routing and tie-down of the cables, you will be OK. If helps you sleep at night, put one on that circuit. But personally I don't and the car you're driving every day from whatever major manufacturer doesn't have one either. As for the 10 gauge wires going to the panel, keep in mind that every circuit downstream tied to those wires is protected. Don't mean to be Captain Obvious here, but remember current is drawn, not pushed. So they shouldn't ever draw any more current than the 10 gauge can handle since any faults would blow a fuse. If you add any circuits (you may have seen some in my builds) then yes add a properly rated fuse or circuit breaker on the added circuit and make sure the total (panel plus whatever you add) doesn't exceed the capacity of the 10 gauge. But a fuse on those lines is redundant and just adding a point of failure for no reason.

I appreciate you chiming in.

You bring up a few good points that land exactly where my "spidey sense" started to kick in. For example, with regards to the alternator circuit. I was able to locate a kit by "painless wiring" for high output alternators (between 140 to 190 amps) and it included 6 gauge wire and a 200 amp fuse. I thought - great - this is exactly the circuit I need to reproduce, I have a 160 amp alternator so I can use 6 gauge wire and put a 200 amp fuse/circuit breaker on that line and I'm all set.

The thing that bothered me was that I can't find anywhere that says 6 gauge can handle 200 amps. Blue Sea System's chart says you should have an 80 amp fuse for a 6 gauge circuit - 80!!!. So the engineer in me just couldn't merge the two. I kept thinking - "Why is it suddenly acceptable for a 6 gauge wire to be protected with a 200 amp fuse because it is attached to an alternator?" I think the answer is "stop thinking about it - professionals say it is OK so just accept there are elements at play here you don't understand." So, yeah, 200 Amp for the alternator it is and I’m going to update my circuit to eliminate the “battery 200 Amp” fuse as that appears to just be a pain in the *** waiting to happen.

[LateApex]

So I considered having a "ground" cutoff switch but ultimately decided it would just require more wires (I'd have to run 2 gauge all the way up to the firewall) and I was worried it wouldn't stop the alternator from operating since it, too, is grounded to the chassis by itself (people can correct me if my assumption there is incorrect). By having the setup I have laid out above it guarantees the engine must stop if I open the battery cutoff switch which was my ultimately goal. Also allows me to completely cutoff the starter if something were to happen to it.

I have been thinking about that "sneak circuit" too (related to the alternator providing a power source regardless of whether the battery is cut-off). The one-wire alternator kinda makes that tough :-)

One other question: In your schematic, what is the intent of also routing the starter power through the cutoff switch? Is it not sufficient to connect only battery feed and ignition switch to the switched side of the cutoff?

[BrewCityCobra]

I have been thinking about that "sneak circuit" too (related to the alternator providing a power source regardless of whether the battery is cut-off). The one-wire alternator kinda makes that tough :-)

One other question: In your schematic, what is the intent of also routing the starter power through the cutoff switch? Is it not sufficient to connect only battery feed and ignition switch to the switched side of the cutoff?

With regard to the "sneak circuit" - I certainly could be over-thinking it but I figured it is basically a horse-a-piece to put it on the positive or negative side. In the end, the wiring seemed easier for positive and the sneak circuit issue we discussed earlier wasn't an issue with positive. So that's why I went on Positive.

As for running the starter through the switch - I just wanted to make sure that if anything happened anywhere I could shut it off - that included the starter motor. By no means is that required. But to that point, I made sure to purchase a safety switch that could handle 350 amps to accommodate that design choice.

[bobl]

I had an instance during my build where I had a positive cable accidentally touching ground when I turned on the batter switch. It welded the contacts in the switch and i couldn't turn it off. Fortunately it melted the battery terminal which is the weakest link in the circuit. I added a mega fuse right at the battery because I proved it can happen.
I also bought a heavy duty battery switch and installed it right after the mega fuse. It shuts everything off. It's worked great for 5 years.

[BrewCityCobra]

I had an instance during my build where I had a positive cable accidentally touching ground when I turned on the batter switch. It welded the contacts in the switch and i couldn't turn it off. Fortunately it melted the battery terminal which is the weakest link in the circuit. I added a mega fuse right at the battery because I proved it can happen.
I also bought a heavy duty battery switch and installed it right after the mega fuse. It shuts everything off. It's worked great for 5 years.

Bobi,

Out of curiosity - what size did you make that fuse on the battery?

[bobl]

I believe it is 250 amps. I mounted it so it is easy to reach laying under the rear of car. In the event of a failure I can always swing the cable to the other post to keep from being stranded, assuming the problem was fixed.

[phileas_fogg]

...The thing that bothered me was that I can't find anywhere that says 6 gauge can handle 200 amps...

I use this chart for calculating max current for a given gauge & length of wire: https://www.allfordmustangs.com/attachments/wiringdiagram-jpg.62425/. You'll find other charts that give different max values for the same constraints; some are conservative & others less so. This one is towards the conservative end of the spectrum.


John

[Alan_C]

IMO, not overthinking at all. Although my build is not a MK4, I appreciate all the Coyote install information here.
I have a dual run from my battery in the trunk of my 70 Mustang Mach 1. Nigel did the cranking AMP test for me that resulted in running a 1 AWG cable from the battery to cutoff switch and a 250A mega fuse then to the starter. The secondary run was 1 6 AWG has a 100A mega fuse and run to the power distribution box. My AAW harness is connected to the PDB.
We debate the need for the fusing, but should an accident occur, I don't want to be cursing as my car burn to the ground.
I have also been accused of over thinking and engineering stuff, but I am a retired engineer and that is we do.