Uhh....no...

The stock wiring is 40+ years old; and wiring doesn't get better with age. Your stock wiring might be good for 80 amps [without losing sleep over it].

If you NEED the amps, upgrade the alternator and upgrade the power lead from the alternator....or 'piggy-back' that wire with a parallel wire of similar size.

You don't fully understand the issues or you wouldn't be asking these questions. Several folks here are trying to give you good advice, but you 'wanna do what you wanna do'. It's your car. Good luck with your decision.

 

It doesn't matter if it's 100 amps or 140 amps, the issues discussed in this thread still apply. It's really not that difficult to run a 6, 8, or 10 gauge (fused?) wire from the alternator to the starter to use along with the stock harness.

 

Here is the question; do you need all 100 amps or 144 amps or are you just looking to get a better idle amperage output?

In my case I only needed a better idle amperage output. The stock 63 amp alternator only produces about 30 amps at idle. So I upgraded to a 110 amp 10si alternator that produces 60 amps at idle. This was so I could run two electric cooling fans in addition to all the other systems on the car without draining the battery.
So I'm still below the max rating of the stock alternator with all consumables on but I can produce it at idle.

If you want to run the 100 amp or 144 amp alternator but do not intend on actually using over 63 amps then wire in a 60 amp fuse, fusable link or circuit breaker on the charging wire near the alternator.

You can now use any alternator with that 60 amps being the limit, only now you can produce all of that apmerage at idle instead of a higher RPM.

That's how you retain the stock wiring.

Personally I like the circuit breakers, but they are sensitive to heat and may pop off early due to that heat. So the fuse (fast blow) is the safest solution. Keep some spares in the car.

Here is an example of how cheap these fuses are;

http://www.ebay.com/itm/5-TOTAL-60-A...sAAOSwnQhXore4

Couple of connectors or a fuse holder and you're in business.

Or here is another syle;

http://www.ebay.com/itm/5-TOTAL-60-A...sAAOSwnQhXore4

I don't believe that the stock 14 gauge wire would carry 60 amps for any significant length of time, but that is what is on my 77 from the factory.

 

Personally, I would never use smaller fuses to try and current limit. That just makes the system less reliable. On a properly designed system, the fuse should only blow if the wire is short circuited. Not when you start the car with a somewhat discharged battery.

 

Then use a circuit breaker.

If you discharged your battery to the point that it is going to draw 60 amps I'll wager that battery is not going to start the car.

You use a circuit breaker to protect the primary wiring in your house don't you? 200 amps maybe 300 for the main? Many smaller ones for every other circuit.
Half a century ago it was fuses.

The transformer on the pole for your house is capable of far more that the measly 200 to 300 amps you could max out at and yet it's somehow safe.... how can that be?
Each 15 amp circuit is tapped into a 200+ amp main yet the wire for that circuit is only 14 gauge, how can that be safe?

You say you wouldn't design a system that uses a fuse/breaker to limit current.

Your house is designed exactly like that, I'm guessing you don't loose any sleep over it.

Current maximum in the car is going to depend on the consumables in the car. I explained that above. You may only need the current to be higher at idle and this may require a higher output alternator. You can still limit the max output of the alternator via an external limiter, fuses or breakers.

 

Here's wire size calculator- if you don't want to just believe us forum members.

Like 7T1 siad- wire is no fine wine and doesn't get better with age...

Look at the chart- that's with a 5% drop ( and if you go to their website they like the smaller 2% chart https://www.wirebarn.com/Wire-Calculator-_ep_41.html)

AND the other variable is HEAT- this chart uses the standard 70ºF rating- and the rule of thumb is move up a wire size to handle the temperatures under the hood.

 

Nope. I will never recommend using a fuse or breaker that is undersized compared to the alternator output rating because you have no control over what current the alternator will produce.

I posted "to try and limit current", as in to artificially limit the current, as in to attempt to limit the current to a lower value then the circuit is capable of operating at.

If you want to make odd comparisons to a home system, then it would be like installing 5A circuit breakers without telling anyone and then getting mad that the house dwellers "randomly" overload and trip the breakers.

 

Alternators don't produce current, they supply it.

Current is current. AC or DC.

ALL fuses or breakers are current limiters, it's what they do. Don't confuse it with current regulation.

There is nothing "artificial" about using a fuse to limit current draw.

Tell me this, how is the current draw limited on you 12V cigarette lighter?

 

{electrically challenged...}

You really shouldn't give advice on electrical systems if you don't have a basic understanding of D.C. circuits. Fuses/breakers/fusible links are not in the circuit so that you can "limp along" with an overpowered or under-wired system. Wires need to be sized to handle almost every circumstance...save for the 'fool' short (you know...when you dig into the electric circuit with a screwdriver and didn't disconnect the battery) or a major component failure that goes to internal short [that's what fusible links are for]. Breakers/fuses are there "just in case"--not because the wiring is not capable of handling the normal output of the alternator.

To the OP: PLEASE, don't listen to this drivel.

 

Sure, except the circuit quits working when the fuse blows making it a EXTREMELY lousy way to limit current. Building circuits where fuses can blow even though components are operating within their normal operating capabilities is NOT the way to build a reliable vehicle.

Produce and supply don't mean the same thing? "The alternator produces the current required by the car." vs "The alternator supplies the current required by the car." Sounds the same to me.

But sure, the higher current ones do supply current and are capable of supplying well over 100A of current, so it makes no sense to put a 60A fast acting fuse on the output. A high capacity alternator is easily capable of supplying a current that can blow a 60A fuse. So, you have a circuit where the fuse could just blow randomly while the alternator is working normally and then the car quits charging. Not a very good solution...

Sorry, but I will never build or endorse such a circuit. There is no excuse for such a poor design since it makes zero sense when the goal is to build a reliable car.

Also, fuses and circuit breakers are not current limiters. If they current limited, then they would limit the current to a defined level and not allow anything higher. Just to give an example, a 60A MEGA fuse will allow >80A continually and >200A for 1 second. It certainly doesn't limit the current to 60A.

 

Using a 60A fuse on a 100A alternator with the intention of limiting current is a terrible idea for several reasons.

1. It won't actually limit current. Either the fuse will blow or it won't. If the fuse blows, your car is no longer charging (which defeats the purpose of having a fancy alternator). If it doesn't blow, then you're still seeing whatever amperage the alternator is producing to supply the demand.

2. Fuses (and circuit breakers) don't magically blow the instant their rated value is exceeded. The fuses heat up until the piece of metal gets hot enough to destroy itself. I own 2 DeLoreans. One of the problems in that car is that some of the fuses are rated too conservatively, leading to fuses that don't blow and just melt taking the fuse box and other wires with it. Running continuous current through a fuse near its rated value is a bad idea. There needs to be a decent margin between the expected current draw and the rated amperage of the fuse.

3. Fuses are designed to protect wiring, not limit current. They're designed to blow when a short occurs, whether through one of us being careless or a component shorting out. The cigarette lighter circuit is on a 5 amp (for example) fuse because the wiring can't handle much more. The fuse blowing in the case of a dead short is preferable to wires melting. In the case of the alternator supply wire, a fuse blowing when the alternator is working normally may be preferable to melted wiring, but (as cited in my example in point 2) there's no guarantee the fuse will blow in that situation, and if it does, it's a nuisance. None of those outcomes are optimal.

I know much of this has been similarly stated in previous posts, but it's a bad enough idea that I felt I had to chime in.

 

On my '75, I added a #2 or #4 wire (been a long time) direct from the alternator feed to the battery with a fusible link at the alternator. The stock wiring is still hooked up and the amp gauge works, just reads low.

 

Wow! quite the band wagon here.

First of all none of you understand how electricity actually works.

Current is a demand factor in the electrical scheme of things.

No power supply be it a battery or a generator, alternator,or the power line to you house can PUSH current down the line. It has to be pulled by a demand. SOMETHING has to be demanding the current.

It's like water in a tank. It's not going anywhere unless you enact some force to move it.
You can think of a alternator as a water pump. The pressure it can produce is like voltage.
The volume is can supply is like amperage.
So I can jack up the voltage (pressure) in a supply line but unless I open the line somewhere down the line, water (amperage) is not flowing.

The only accurate statement in the three posts made is that a fuse acts to protect the wiring it's serving. Yes that is true. It does it through current limiting via a sacrificial piece of metal. So what is so hard to understand?

No supply is not the same as production. Do I really have to expalin that?

Your alternator produces 14 volts. That it does and will continue to do at all times regardless of the load on the system up to it max current capacity.

The current that flows from the alternator has to be demanded by a component. It CANNOT flow or be pushed without being demanded by a user conponent.

so when a component demands some current your alternator supplies it, no more no less...period, end of story gentlemen.

All fuses and breakers are used to protect the wiring it serves through CURRENT limiting. Period, end of story.

If you have a component that can potentially demand more current than the wiring it's attatched to can carry then a limiter comes into play. That limiter is the fuse or circuit breaker.

Maybe one of you can understand this and maybe not, but many others will read it too.

In your car is a cigarette lighter. Since the manufacturer cannot know what might be plugged into that lighter they need to put a limit on how much current can be pulled through that circuit due to the SIZE OF THE WIRING. Hence the fuse for the cigarette lighter.
Notice they don't put in a 0 gage wire since that is size appropriate for the amount that the generating system on the auto can supply.

In your house most of you circuits are 15 amp or 20 amp. Since there may be several outlets on one circuit and the wiring is the limit of the current that can be drawn on, a circuit a circuit breaker is used to limit the current that can be drawn on that circuit.

The builder of the house has no way to know how much current draw may be placed on a circuit. But he does have a way to limit it due to the gauge of the wire used. A circuit breaker.

ALL wiring current is limited by the fuse or breaker on the circuit due to the current carrying capacity of the wire used. The potential of the generating sorce is not how the circuit is limited.

Why is that so hard to understand?

If you total amperage with all users in use is below 60 amps ( as I pointed out) then how are you limiting the flow via the fuse? Your not. Your limiting it by what is on the circuit drawing current.
The fuse is only doing what all fuses do, protecting the wiring. Repeat, IT'S PROTECTING THE WIRING. This IS current limiting. You are limiting the current draw on that circuit via a fuse.

How much potential the alternator has is irrevelant.

You guys do understand that the alternator is also capable of more than 14 volts too right? Without the voltage regualtor you would see wild swings in voltage via RPM.

That's right. There are fast blow and slow blow type fuses. Slow blows are for circuits that may see occasional exceedences. But these exceedences are not long enough duration to damage the circuit and the slow blow eliminates the nuisance of changing the fuse each time a short duration exceedance occurs.

In the case of a standard fuse (fast blow) they pretty much do. But it's not magic.
Go ahead and put a 20 amp fuse between the positive and negative terminal of you battery. Start a stop watch. Let me know how long it took to blow.

They do both...right?

Sooo every time you've blown a fuse or a breaker it was due to a short? That's interesting.

Brother

 

Here's a hypothetical situation: The OP goes out to start his Corvette that has a 140 amp alternator with stock wiring. The battery is dead. Really wanting to drive his Corvette that day, he jump starts it. Now, the battery is demanding as much as the alternator can deliver in order to recharge. If the wire is on a 60 amp fuse, the fuse blows, and will continue to do so until the battery is charged by a trickle charger. If the wire has not been modified, it will likely blow a fusible link. If there is no fuse, the wire may melt. If the OP had run a separate large wire with a fuse on it, there would be no problem.

What you're describing is a dead short, and not at all a response to what I said. Of course it's going to immediately blow. But, if you were to run anywhere between a steady 18 and 22 amps through that fuse, it wouldn't blow, but it would get very hot and possibly melt. As I said in my previous post, I see this in the DeLorean world all the time because many of the circuits specify fuses with lower ratings than they should have.

No, they don't. If the battery is demanding all 140 amps that the alternator can give it, a 60 amp fuse isn't going to only let a steady 60 amps through. A device made to limit current would, but a fuse would just blow (or melt, depending on the actual alternator output).

Not what I said, but nice try.

I had the same line of thinking that you did when I installed my first 140 amp alternator. Several melted wires and many hours repairing my wiring harness later, I learned the error of my ways.

 

What have you added that makes you need more power?

When adding high current accessories you need to avoid tapping into the fuse box as the wiring that feeds it won't handle significantly more amperage.

The easiest way to use a larger alternator and add power-hungry accessories is to install a new alternator output wire (the one with the rubber cap). Let's say 8 gauge. Run it to an accessory fuse box that you install on the driver side fender liner--plenty of room for such. Then install the original (12 gauge) alternator output wire at the connection to the fuse box.

If you need additional ignition (on when engine is running) power then you'll have to run a small wire from an existing ignition circuit in the cabin to your new fuse box location and use it to power a relay coil.

I have a '99 Cadillac (a Seville STS). It uses a water-cooled alternator. Make certain the one you have doesn't have connections for coolant!

 

It's not a question of how electricity works, it's a question of properly engineering the electrical system. Applying requirements such as making the system reliable and ensuring it doesn't cause fires. What you're claiming will work doesn't fit these requirements. It shows a complete lack of understanding how to build a reliable charging system. You have completely ignored all the transient things that happen in a car.


It's comical how we point out that fuses don't blow instantaneously when you slightly overload them and your response is to twist the responses by making up a test that puts a very, very high current into the fuse as proof that they do open instantaneously. In what world is 100's if not 1000's of amps right around 20A?

The right test would be to put a load that will draw about 20A onto the 20A fuse and then let it run for a week or two and see what happens. If you want to make the test accurate for underhood use, then heat the fuse to simulate the engine heat.

What dmruschell posted is ABSOLUTELY correct. Run a fuse right close to it's rating and you run the risk of the fuse getting hot enough to cause damage. I've seen automotive fuses melt the fuse holder too. And if the fuse doesn't get hot enough to cause damage, you still run the risk of the fuse fatigue opening when running it right at it's rated value.

I use a 30+% derating on fuses, with more like 40%+ for under hood or hot locations. In other words, I would only put a 60A fuse on a circuit that has a 40A or smaller load. Under the hood, for a load that is under 35A. This may seem conservative, but this is what is required to make the circuit reliable. Running the same fuse at 50A or 60A is simply not reliable. I'd have to look for proof (and you would probably ignore it anyways), but the fuse manufacturers recommend a similar derating as this.

 

I have to tell you fellas....it's very difficult to convince a 'brick' of anything.

 

I'm not going to disagree with you.

The only reason I chimed in so far is so that someone in the future who may be looking for information about upgrading their alternator doesn't melt their wiring harness.

 

Calling yourself a "brick"?? I've been an electrical engineer for over 20 years and designing electrical systems for as long. So, you're simply not convincing me of anything with your over simplified theories.

 

You and 7t1vette (and I and others) seem to be in agreement on how electricity works. There's no need to convince you if anything when he's preaching to the choir. I'm guessing you thought you were responding to another user?