The Stovebolt Site Forums The Shop Area The Electrical Bay Fusible link for 12v alternator

My 1952 3100 has a single wire 12v alternator wired to the ammeter. After reading some post about adding a fusible link to the alternator circuit, I have a few questions.

1) What size fusible link should I use? I have 10 ga wire from the alternator to the ammeter. 2) Where is the best place to locate it.
3) Should the battery also be protected with a fusible link?

What's the amperage capacity of the alternator? One fuse link, between the battery cable connection at the starter, and the ammeter, is all that's really needed. Any other fuse of any type in the main charging system is simply a "belt and suspenders" approach to electrical circuits. The only purpose of a fuse, or a fusible link, is to protect the wiring harness in case of a short. The fuse link should be able to carry the maximum output of the alternator, plus about a 10% overload. If you've got something stupid like a 100 amp alternator, that's going to mean you need to rewire the entire alternator/ammeter/battery charging system accordingly, with 6 or 8 gauge wire and a 125 amp fusible link. That's ridiculous!
Jerry

The alternator is 63 amps.

A 10 gauge wire with a 12 GA. fuse link between the ammeter and the starter cable terminal should be all that's necessary in that case. Don't worry about fusing the wire from the alternator to the ammeter.
Jerry

Jerry,
Pardon my ignorance, how long a piece of fusible wire do I need. Could I use one of those fusible links that look like big fuses and are rated in amps.

Phil

Max current rating for (low voltage) 10 gauge is 55 amps (60 deg C, free air)... so at minimum, protect the charging circuit with a fusible link sized for the 10 gauge wire. That would be a #14 fusible link.

Or, to fully support the 63 amp alternator with some margin, consider replacing the #10 wire with #8 for the charging circuitry - wire to ammeter from the battery terminal on the solenoid, ammeter to V reg (if it still has an external regulator) and to the output of the alternator. # 8 wire has a current rating of 73 amps (60 deg C, free air); protect with a #12 fusible link.

Fusible links are selected by gauge size of the wire they are intended to protect. The guideline is to select a fusible link that is 4 gauge sizes smaller than the wire, generally 3-4" in length. Here is a good review: Fusible Link FAQs.

The current ratings of various wire gauges can be found in the Handbook of Electronic Tables and Formulas for American Wire Gauge; here is a convenient link for the AWG Current Limit Table.

It would be an almost impossible situation for the full output of the alternator to attempt to go to the battery, as its first duty is to supply current to whatever accessories are being operated- - - -lights, ignition, heater blower, etc. Then whatever is left over is routed to the battery for recharging after the starting load is applied. The high-current surge is usually only there for a matter of a couple of minutes, maximum. Theory is fine- - - -practical experience is better.
Jerry

Jerry, you are right that it would not be normal for the full output of the alternator to go to the battery. I don't think I implied that it would. But regardless, to protect the existing wiring harness against unintentional shorts or higher loads, the as-is wire should be protected based on its gauge - 10 AWG typically protected by #14 fusible link, and typically 3-4" in length.

Perhaps my suggestion about considering a change to 8 AWG was confusing? What I was thinking about (and didn't articulate - fair enough) was that add-ons can quickly increase the load, and since the alternator can provide more than 55 amps comfortably, an upgrade to 8AWG would allow the system to take full advantage of that (for lights, sound, whatever), with decreased voltage drop, whether the current comes from the alternator or battery for those accessories. It was intended to be something to think about - not a go-do - which is why I stated "consider."

BTW, this is my field of professional expertise. I'm not a machinist, nor have I had decades wrenching professionally, but I have worked in a shop - engine work in particular - working evenings and nights back in the '80's to early '90's. My first tear down of a 235 was in the late '60's. Electrical systems are another matter. I made a 40-year career out of this area, starting as a tech while going to grad school. Many years later, I used to lecture the engineers who worked on my projects: "get off the carpets and down to the linoleum [factory floor]." I heartily agree that practical experience is priceless.

If I understand correctly (utilizing both of your vast practical and technical experiences), keeping the 10 gauge wire (as I don’t have any other loads than original equipped), I will splice in a 3” to 4” piece 14 gauge fusible link wire at the starter solenoid terminal to the 10 ga. wire going to the ammeter. Please correct me if I am wrong.

Thank you Jerry and Drdoug for your time and expertise!

That protects the rest of the wiring harness from serious shorts, as the fuse link burns out before there's major damage to the rest of the wiring. It's a very unusual circumstance, for that to happen, but it's good insurance. One further piece of advice- - - -Since it's difficult to get a really good crimped connection on fuse link wire, and soldering is also problematic, I like to use a "Western Union Splice" which consists of about 10 turns of tightly-wrapped wire in each direction so there's a large amount of contact area between the wire and the fuse link. Then insulate the connection with self-vulcanizing rubber tape. You'll get a virtually zero-resistance connection that way.
Jerry

Phak1, you got it. Jerry's info on the connection style is good stuff.

Here's an image if the Western Union (aka Lineman's) Splice, in case you aren't familiar. They do provide a superior joint - strong and low resistance. In cases where the wires to be spliced will be soldered, I especially like this type of splice, since it isn't reliant on just solder for mechanical strength and low resistance.

Would a waterproof heat shrink tubing in place of the self-vulcanizing rubber tape work as well?

Yes, as long as there are no wire ends sticking up to pierce the shrink tubing. I'd do both- - - - -a "belt and suspenders" approach to holding up your pants guarantees you won't moon anyone!
Jerry

Thanks all for your help. Great forum, great people!

For your consideration; I was concerned to not damage the original ammeter after I installed an alternator so I ran a wire direct from the alt. to the starter battery connection.I left the original wiring hooked up to power the dash ect. as usual. I know this renders the ammeter mostly useless but it also guarantees no damage to the ammeter or wiring as my 49 is mostly stock. It would be simple to wire the idiot light or a volt meter if you think you need to.
Sorry I don't remember what amp my alt. is, its under 100, for sure, I wanted one I could buy off the shelf at any parts store.
In this case I suppose you would want to put your fuse-able link on the starter connection and attach it to both the original wire harness and the jumper wire.

A simpler method of protecting the ammeter would be to put a shunt across the terminals to bypass some of the current flow while still allowing it to indicate charge/discharge conditions. I like to use a piece of stainless steel radiator hose clamp bent into a horseshoe shape on the ammeter terminals to carry most of the current. It has just enough resistance to shunt enough of the current through the meter to make the needle deflect.+/-.
Jerry

I like that, improvements that go unseen.

Jerry,
Can you expand a little on how to fabricate this shunt.

Are you talking about the clamps that are about 1/2” wide?
Do you use the part of the clamp that is solid or the part that goes thru the screw?

I assume when you said “bent into a horseshoe”, you would also bend out tabs to attach to the terminals.

You've got the basic idea. Cut a strip of the clamp that's solid, about 2" long or a little more, and bend it into a "U" shape with tabs bent at right angles to the main loop. It will resemble the "Ohms" symbol used on wiring diagrams. Drill holes in the tabs to fit over the ammeter terminals. Install the shunt onto the terminals, then connect the wiring as usual. All the connections EXCEPT the wire going to the starter cable connection get attached to the downstream side of the ammeter. That way, all the current going in and out of the battery except the starter current passes through the ammeter and/or the shunt. The shunt prevents the ammeter from deflecting full scale during heavy current situations like just after starting, while still allowing enough current to take the original path to indicate a charge/discharge situation. It's a simple, inexpensive fix that protects the original ammeter from overloads.
Jerry

Thanks! I’ve got my gauge cluster out so I’m going to do this upgrade now. I report back how it went.
Thanks again!

Go ahead and attach the alternator output wire to the downstream side of the ammeter, along with the wiring to the ignition switch, light switch, fuse block (if you have one) etc. That way, the ammeter will show a battery charge/discharge indication after the alternator supplies all the current demand by the rest of the vehicle.
Jerry

My first attempt to fabricate the shunt didn’t fair well. Although the stainless cuts easily with aviation snips, it is too hard for titanium coated drill bits in a battery powered drill. After regrouping, I was able to cut slots with a Metabo (side grinder) and a cutting wheel. Installation was easy. I placed the new shunt on first followed by the wire lugs. It worked just as the original without the shunt. It still pegs the needle on initial start but it comes down after a few seconds.

I believe this as a success. A big thanks to Jerry for steering me in the right direction.

"I love it when a plan comes together!" That's a good-looking installation- - - -too bad it's going to be out of sight behind the dash soon!
Jerry

I finally got to install a fusible link in the starter to ammeter 10 ga wire. I bought a 14 ga fusible link wire that had a 3/8” ring terminal already installed. That left one “Western Union Splice” to make the connection. I practiced first on some scrap 10 ga and the end of the fusible link wire that was too long. I didn’t get anywhere near 10 wraps on each side as suggested but I do believe it is a sound connection. Only time will tell. Thanks to all who helped me tackle this task.