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You really don't have to keep the second charging wire the same size as the original. You can make it as large as you want it to be. But, the new wire needs to be properly protected with the correct size fusible link for that wire as if it was going to stand alone. You also need to maintain the existing fusible link for the original charging wire. Whether the two wires are the same size or not makes no difference when there is an electrical short. Whichever wire shorts first, that wire becomes the path of least resistance and it's fusible link will blow first. At that time the remaining wire becomes the path of least resistance and all load will now pass through it which will blow that fusible link (assuming the load is great enough at that point). The combination of the two wires does need to be able to handle the total load on the charging circuit.
Fusible links need to be two wire gauges smaller than the wire they are protecting. For example, a 10 gauge wire needs a 14 gauge fusible link.
As I've alluded to but maybe not made clear, the factory actually used a parallel path situation with one wire much smaller than the other. The ammeter circuit is in parallel with the original charging circuit. The ammeter wire is smaller but is protected at both ends by fusible links. The ammeter wire does not extend from one end of the charging circuit to the other because some brainiac calculated the length so that the ammeter would read the full current flow through the charging circuit even though it only sees a small portion of that current.
DC
Running 2 different sizes of wire with protection for each wire placed at the solenoid DOES NOT properly protect both wires. You need to look at the pictures Richard posted again. If the smaller wire shorted, the larger wire will back feed the short from the alternator end of the smaller wire. You could put protection on both ends of the smaller wire, but that seems silly when another parallel wire of the same size will get the job done.
The parallel ammeter circuit has protection at both ends of the wire. No matter where a short in the ammeter circuit happens it's protected.
Running 2 different sizes of wire with protection for each wire placed at the solenoid DOES NOT properly protect both wires. You need to look at the pictures Richard posted again. If the smaller wire shorted, the larger wire will back feed the short from the alternator end of the smaller wire. You could put protection on both ends of the smaller wire, but that seems silly when another parallel wire of the same size will get the job done.
The parallel ammeter circuit has protection at both ends of the wire. No matter where a short in the ammeter circuit happens it's protected.
Silly is not just changing it out to the correct size, as any OEM would do. Even if the same size wire is in paralled, one will favor the other. The conductivity of one will favor the other.
In industrial use, wires smaller than 2/0 can only be paralled if either wire carry the load. It is only done for voltage drop.
Only 100'x2/0 runs can be done for a "total " current carrying capacity. The reality is that only then is the vd per run is negligent.
These are rules guided by ul testing, and amperes is amperes. DC the rules are quite similar.
My 2 centavos from seeing burned up " but ohms law says."
Running 2 different sizes of wire with protection for each wire placed at the solenoid DOES NOT properly protect both wires. You need to look at the pictures Richard posted again. If the smaller wire shorted, the larger wire will back feed the short from the alternator end of the smaller wire. You could put protection on both ends of the smaller wire, but that seems silly when another parallel wire of the same size will get the job done.
The parallel ammeter circuit has protection at both ends of the wire. No matter where a short in the ammeter circuit happens it's protected.
I see what you're saying about the backfeed. I suppose there could be an oddball situation where a short blows the smaller fusible link but there was not enough current to actually blow the fusible link on the larger wire. A direct short fed by the battery should always take out both fusible links. I don't see making both wires the same as solving the issue. If both wires are same size as the factory original wire, that might not be enough total wire for a heavy load on a larger alternator. Doesn't seem to be anything wrong with protecting both ends of either wire. But, I do agree it's better to just have one wire.
I talked to an engineer buddy who went down this road in an old pickup. He used Painless Wiring products including an alternator wiring upgrade kit. Painless recommended to him to leave the original charge wire in place and to parallel it with the new larger charge wire. They further recommended only one fuse for the two wires. That surprised me. They told him that if either wire shorted, the available current (basically full starting current) from the battery would quickly blow the fuse or fusible link. He asked about using separate fusible links on each wire. Painless told him that would also work but there might be a slight delay in how long it would take both fusible links to blow in a direct short situation. They emphasized to him that the fusible link serves as a fire protection device and it's main purpose is to take the battery out of the circuit. There is nothing about properly installed parallel charging wires that prevents this from happening.
Here are 3 links from the industry that support what my buddy was told along with copied content from those links:
"Next install the new 4 gauge cable in addition to your original main charging wire. Stack the new cable together with the original charging terminal at the alternator post." https://www.dbelectrical.com/product...t-4-gauge.html
Then lets look at evolution of Corvette wiring in general, and look at the engineering. Bear with me.
The first batch of 53's had to be rewired because they did not start. Someone forgot the car was plastic.
The c3's worked just fine the way the were wired. Years later, look at all the threads here.
My blinkers wont work when the fan is on.
I turn on my defroster and the fan goes out.
I push in my lighter and the lights dim.
My dash lamps are go nuts when I honk the horn
What appears to be sound engineering is failing because no one took into account the different failures.
Now advance to the C5 with all its computers, and millivolt signals.
The harness has a ground going everywhere. The issues are gone.
By your own account, your ammeter did not work correctly when you added a second wire.
Now you suggest to the guy take his brand new harness, and duplicate that failure.
To me, that is just insane.
I rewired my 81 with a ground buss in strategic places. Everything works, and works well. I have eliminated all the "sound" engineering I could find.
I now rest my case. I am over it. "Everything on the internet is true." Abraham Lincoln.
Connect a 100 AMP fuse to the positive battery terminal- then connect a small gauge wire to the fuse. Then touch the wire to the battery's negative terminal-
The small gauge wire will act as a "fusible link" and burn- HOWEVER a real fusible link has insulator that will not catch on fire!!!
True story- back in the day( early 90's) I worked for Kenwoood- the car audio company.
They ended up buying 4 cars- The radio's fuse holder- had a spring and a glass fuse- old school. The spring would glow red hot BEFORE the fuse blew- the fuse holder plastic caught fire - and so did the rest of the car.
It can happen- engineers ALL signed off on the design...
I'd either redo the wiring- or at least carry a fire extinguisher. We are just trying to give you helpful advise....
By your own account, your ammeter did not work correctly when you added a second wire.
Now you suggest to the guy take his brand new harness, and duplicate that failure.
To me, that is just insane.
Completely agree with your assessment of factory wiring.
To be clear, I think one single wire is best. But the discussion evolved into whether or not a parallel charging circuit could work. It can. May not be pretty but it can work without burning your car down. Sometimes, it's just a hell of lot easier to add a wire than to dive down the rabbit hole of cleaning up the factory system.
Coincidentally, I'm in the process of going down that rabbit hole. I've been working on an engine/transmission swap project. While the engine was out, I noticed a couple of wiring problems and decided to expose every single wire under the hood. Glad I did as I found many other issues. Bubba was there and some of the factory coverings of bare splices had deteriorated. While I'm this far, I will eliminate the factory charge wire. I already don't need the ammeter circuit as I've converted to a voltmeter. Turns out it's pretty easy to replace the factory charge wire on my '73. It runs to the horn relay and then over to a circuit breaker on the firewall. I still need that part of the circuit but I can disconnect the lead from the circuit breaker to the starter, disconnect the ammeter circuit at the horn relay and then run a new charge wire from the alternator to the starter.
It's been a good discussion as far as I'm concerned.
No need to try it. I'm fully aware the last thing you want to do is to have your wiring protecting your fuse. That's why I was surprised that Painless Wiring recommended one fuse connected to two different size conductors for my buddy's situation. That sure seems like a potential wire protecting the fuse situation to me.
And I get that you are saying that the backfeed situation will send excessive current to the smaller wire until the larger fusible link blows. I just think the factory wiring can handle the temporary and very short term excess current until that larger fusible link blows. It's basically the same current the smaller wire would see anyway if the larger wire were not installed. It's not like we're talking about a 22 gauge wire in parallel with a 1/0 wire here. It's more likely a 10 gauge with an 8 or something similar.
It doesn't have to be a short- there can be a fan- vacuum pump or higher amp headlights- wired off the horn relay- pulls too much current and the wire could melt/catch fire- without blowing the larger fuse
When you have a large wire connected to a smaller wire without being protected- it is not a good idea...
I have seen WAY to many upgrade the wire from the starter with a large gauge wire to the alternator connect the stock wire and not realize there is NO protection....
I am just trying to help out- not be confrontational- just explain it as I see it....and I didn't stay at a Holiday Inn last night!!!...
I am just trying to help out- not be confrontational- just explain it as I see it....and I didn't stay at a Holiday Inn last night!!!...
Richard
You are being helpful and I've come around to your way of thinking. Especially after my discovery last night.I gave this a lot more thought last night after discovering I had wired my car differently than I thought I had. I need to clarify how to more safely add a parallel charging wire. In a parallel charging wire situation, whichever wire is smallest should get fusible links at both ends for best protection. And, while it might be much more work, it's probably best to just replace the existing wire with a new larger wire & appropriate fusible link.
I actually wired my car with a second fusible link on the alternator end of the original wire. I completely forgot that until I found it last night while finishing up my task to expose all wires in the engine compartment for inspection.
My original thought during this discussion was that once the smaller wire was damaged by a short, it didn't matter if it got damaged further while waiting for the fusible link on the larger wire to blow since it likely needed to be replaced anyway. I was negating the possibility of that wire burning enough to start a fire even though the odds are low if the two wires are fairly close in size.
To clarify how I believe is the best way to add a parallel charging wire: If the new wire is the same size as the existing wire, simply install an equivalent fusible link near the starter end of the new wire. But, if the new wire is larger than the existing wire, install an appropriate fusible link near the starter end of the new wire AND install a second fusible link at the alternator end of the existing wire. Since a direct short on the existing wire can be fed by the battery from either direction once a parallel path is established, having the second fusible link on the existing wire minimizes the risk of a fire. This is exactly why the ammeter wire has fusible links on both ends. If someone adds a new wire that is smaller than the existing factory wire, it should have fusible links at both ends. Fusible links should be 2 gauge sizes smaller than the wire they will protect. There are modern fuses that can take the place of fusible links but that's another discussion.
It's hell getting older. It's either that or the Tequila. Sorry for any confusion.
Silly is not just changing it out to the correct size, as any OEM would do. Even if the same size wire is in paralled, one will favor the other. The conductivity of one will favor the other.
In industrial use, wires smaller than 2/0 can only be paralled if either wire carry the load. It is only done for voltage drop.
Only 100'x2/0 runs can be done for a "total " current carrying capacity. The reality is that only then is the vd per run is negligent.
These are rules guided by ul testing, and amperes is amperes. DC the rules are quite similar.
My 2 centavos from seeing burned up " but ohms law says."
Yes, adding a parallel wire and then tacking more protection here and there to try and make it all work and be safe gets to the point of ridiculousness vs just doing it the correct way and installing a single wire the correct size.
Smaller parallel wires can share fine because they inherently have a higher resistance per length which balances out the lower current running through them. Doing testing at work, I see smaller wires share and maintain more balanced currents than heavier wires do. I don't know the reason why the electrical code doesn't allow smaller wires to be paralleled, but I suspect it's about keeping things simple with single wire runs more so than it not working.
Last edited by lionelhutz; Jun 11, 2020 at 02:54 PM.
Yes, adding a parallel wire and then tacking more protection here and there to try and make it all work and be safe gets to the point of ridiculousness vs just doing it the correct way and installing a single wire the correct size.
It's not ridiculous. It's just another way of doing it. It works and is safe if done properly. Same as one wire. Doesn't make it any less correct. It's actually easier, faster and cheaper to run a parallel wire. When I first did it a decade or so ago, it took me less than 30 minutes to make the change. Fast forward to several weeks ago and it took me all afternoon just to unravel the wiring mess along the firewall and sort things out. And that doesn't count the extra hour I spent the other day on the harness near the horn relay.
What are you guys using to fuse larger charging wires? I have access to some free 4 gauge wire but finding 8 gauge fusible link is nearly impossible other than in large quantities. I see Maxi fuses, Mega fuses, Midi fuses, ANL fuses, and MRBF style fuses. They all need some type of fuse holder and preferably one that can be tucked out of the way somewhere. Also need to consider the close proximity to the header in the starter area. I found this one but it does not appear to be available other than by special order in large quantities: https://www.geppowerproducts.com/sta...e-fuse-holder/
The proper, correct way to do it is to use the correct size of wire. Adding a wire in parallel will always be a compromise solution, poorer than the correct way.
What are you guys using to fuse larger charging wires? I have access to some free 4 gauge wire but finding 8 gauge fusible link is nearly impossible other than in large quantities.
Thanks,
DC
I like using the marine stuff- seems to be better quality-
I wouldn't put any fuse down by the solenoid. But then, there aren't many cars that actually need to use 4 gauge wire for the charging circuit so a 10 gauge fusible link would likely work fine for that wire.
Last edited by lionelhutz; Jun 12, 2020 at 04:00 PM.
