Fusible Link and the Starter Motor
 

Hello all.

A friend of mine did most of the restoration on my 72 firebird. (I appreciate your comments even though it's not for my ERA Cobra -I feel like this is a safe place!). Bottom line: he did an atrocious job with the wiring and it falls to me to fix it. I don't have a huge amount of experience in wiring but I'm going for it.

A question about size of fusible link and wiring:

Let's say other than the starter motor, my electrical system draws 175 amps with everything on...My alternator is rated 220 amps total and 150 amps at idle.

Let's also say that the starter draws 350 amps when cranking.

What size fusible link do I need? And where do I place the fusible link? Does the starter need to be on a completely separate circuit without a fusible link?

Current set up :
1. Right now I don't have any fusible link
2. Battery is mounted in the trunk, LS3 engine in front with Holley Terminator X Max ECU
3. From the positive terminal on the battery, I run 2' 1/0 awg wire to kill switch mounted on the rear outside panel
4. From the kill switch, I run about 10'-12' of 1/0 awg wire to Starter motor
5. From the Starter motor, I run about 4' of 1/0 awg to the alternator
6. From the kill switch, I have another 1/0 awg wire that runs to a buss bar; from that buss bar, I power a fuse box/relay (fuel pump) in the trunk and another fuse box/relay panel (windows) in the cabin. There are other relays for holley ecu, vintage ac...
7. From the alternator, I run 1/0 awg wire to charging ports in the front lower valance and then to power one more fuse box/relay panel under the hood (2 fans, headlights, horn)

Thanks in advance for any comments or suggestions that you have.

Regards,
Phil

1) Your starter motor will draw about 200 amps while cranking. BUT, it will have an initial draw of several hundred amps (my old style Ford FE starter motor has an initial inflow of over 900 amps).

2) If you were to place a fusible link(s) in your system, it would be between the source of potentially very high current (meaning your alternator or your battery) and the likely source of a serious fault to ground.

3) You do not need a fusible link on your system. I do not have a fusible link on my system. I have glass fuses on the eight basic circuits, a higher amp auto reset circuit breaker on the the positive cable after the starter solenoid; and a manual reset 15 amp circuit breaker on the headlight switch feed off the ignition switch.

4) You never place a fusible link on the starter motor.

Hope that helps.:cool:

GM cars of your era had a fusible link on the big wire right at the starter. It was connected to the heavy wire and then had an eyelet to attach to the starter.

I am not really sure how many amps it will need to hold.

Newer cars use fuses, as Partrick notes. Most newer models use maxi fuses. If I remember correctly, the starter on a 2000 Silverado was protected with a 40 amp maxi fuse. I actually had to replace it in that truck.

John

Thanks for your post.
It helps a bit.

Not knowing how your car is wiring, it's a little difficult to understand when you write "on the positive cable after the starter solenoid."
Do you have the positive from your battery going directly to your starter?
If so, after your starter would there be a cable from your starter to your alternator?
If so, is that where your circuit breaker is...hooked up between your starter and your alternator?
If I am understanding all of that correctly, then are you suggesting that I should place either a circuit breaker or fusible link (I really don't think of those 2 things are difficulty effectively) between the starter and the alternator?

Personally I would just rewire the whole car IF the crimps / routing are not up to par (aka rats nest)
These LS swap harness kits from Painless appear the way to go, or a least a guide.
https://www.painlessperformance.com/wc/p.php?s=lscar
On the installation manual for this LS3 kit, Page 26 hows the starter / alternator wiring.
https://www.painlessperformance.com/wc/60524

I wrote a schematic, see pic below. First, you don't need a fusible link anywhere. But, if you insist on putting one on, only put it on the area marked in RED below. That is the area that has the potential for very high amperage should some sort of fault to ground occur. If you put a fusible link on the area marked in BLUE you will burn it up the first time your starter motor pulls amperage past the link's protection point. This is what I meant by saying "on the positive cable after the starter solenoid." Note that in my schematic I have a 50 amp circuit breaker protecting my system. Now, my alternator only puts out 60 amps tops, and my fans draw 30 amps under a hot air load, lights another 10, and if I turned everything I have on the car on I might be able to crest that 50 amp mark. What that would mean is that if the car was turned off I might be able to get that circuit breaker to pop off, but if the car is running all that load is being fed from the alternator, so the current through the circuit breaker is only a couple of amps at most to maintain the battery. Now, immediately after starting the car I might get 25 amps through that circuit breaker, for a short while, but never 50. And that breaker has never blown, ever. Now, suppose I had a 200 amp alternator and some sort of 100 amp load off the fuse box that only ran when the engine was on, maybe a Javelin missile launcher or something. In that case I still would not change the 50 amp circuit breaker because that load is served by the alternator. But if I wanted to shoot my missile when the engine was off, then I'd have to upgrade that CB to a higher amperage. Hope this helps you.

https://i.ibb.co/f0nkCFF/quick-wiring-pic001.jpg

patrickt,
Thanks so much for responding and taking the time to draw it out for me.
This is very helpful to me.
Regards,
Phil

You really need a fusible link on the power wire on the back of the alternator.
Blas

Does a fusible link wire behave differently than a mega-fuse block or a resettable circuit breaker?

A fusible link and a "normal" fuse both burn up when they are subject to a current that exceeds their safety limit, and they look like a little sparkler or fireworks when they burn up. This might be an issue for you if you happen to have your fusible link right next to the gas tank's vent tube, or if you're dealing with a boat where a fire is always a concern. The mega-fuse block is just a fuse block that is also covered to protect against something like this. When fuses burn up you have to take them out and replace them. When a circuit breaker blows, it can either reset itself and then retrip if the fault is still there, or stay off if it requires manually resetting. In deciding whether you want to add a fusible link off, say, your alternator, look at the line going from the alternator back to the fuse block and battery and ask yourself two questions: 1) How likely is it that wire could rub through on something, or in some other way have a serious fault to ground; and 2) If that happened, how bad is it likely to be? Remember, your alternator stops putting out current when your engine dies, or your electric system fails. Not so with your battery. As a side note, faults to ground often telegraph themselves long before they become "burn my car down" type problems by burning the wires back some, the same way that the overhead conductors in the power lines running through your neighborhood will burn back encroaching tree limbs. Sometimes it will trigger a breaker, sometimes it will do nothing, sometimes it'll shut your lights off and then a recloser will bring them right back up. But I don't want you relying on that (the smell of smoldering insulator has a very distinct aroma), if you are nervous about it all, just go ahead and put a fusible link on your alternator's BATT output and make it about 80% of the rated capacity of your alternator (assuming of course 80% is still well above your total amperage load of your lights, fans, radio, etc.). The biggest danger on the line that you realistically face is having the wire run through a grommet-less hole in the firewall or an un-shunted ammeter that somehow manages to ground itself to the dash. Batteries in the trunk are a little different as well since you have longer runs, more voltage drop and more chances to rub against something and fault to ground... like the vent tube for the gas tank.;)

thanks again patrick.

I wanted to find a circuit breaker but I couldn't find one with reviews strong enough so I figured a mega fuse has fewer moving parts/simpler design.

btw, i called the tech support line where i got my alternator (PowerMaster - they are terrific, a real person picks up the person, excellent tech support, they built me a custom alternator at a very fair price). they suggested that the way to wire this is to:
1. run the cable for the starter and alternator to the battery side of the switch;
2. add my mega fuse to the powered/switched side of the switch for everything else.

that seems to make sense to me (it also helps me with my issue with the kill switch)

Alright. Just in case you're ever in a bar room bet situation with it riding on "what's better at protecting my car, a fuse or circuit breaker?" There are times when a circuit breaker is really better than a fuse and times when a fuse is really better than a circuit breaker. Circuit breakers get old and pop off easier, and their derating curves can fool you under a hot hood, but, on the other hand, some circuit breakers can trip faster because of the magnetic field of a sudden fault to ground, as opposed to just heating up and tripping off thermally. Soooooo, never place a bet either way on a bar room bet as to which is better... you're just being set up to pay for the next round of drinks.:LOL:

Good advice!

Separately, my internet "research" tells me that at least one factory wiring setup (it was a 2004 Jeep) includes a wired fusible link between the battery and the starter...maybe because the usual period of cranking time is not long enough to burn up that link...?

If you have a desire to, you can safely fuse any load. But that doesn't mean that you have to, or should, fuse every load.:cool: