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Fusible link for 12v alternator #1309687
Sun May 05 2019 05:38 PM
Sun May 05 2019 05:38 PM
P
Phak1  Offline OP
Wrench Fetcher
Joined: Feb 2019
Posts: 69
Upstate NY
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?


1952 Chevrolet 3100 w/59’ 235
Re: Fusible link for 12v alternator [Re: Phak1] #1309697
Sun May 05 2019 06:33 PM
Sun May 05 2019 06:33 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 18,533
Dellrose, TN
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 murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Fusible link for 12v alternator [Re: Phak1] #1309703
Sun May 05 2019 06:56 PM
Sun May 05 2019 06:56 PM
P
Phak1  Offline OP
Wrench Fetcher
Joined: Feb 2019
Posts: 69
Upstate NY
The alternator is 63 amps.


1952 Chevrolet 3100 w/59’ 235
Re: Fusible link for 12v alternator [Re: Phak1] #1309714
Sun May 05 2019 08:35 PM
Sun May 05 2019 08:35 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 18,533
Dellrose, TN
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Fusible link for 12v alternator [Re: Phak1] #1309719
Sun May 05 2019 09:19 PM
Sun May 05 2019 09:19 PM
P
Phak1  Offline OP
Wrench Fetcher
Joined: Feb 2019
Posts: 69
Upstate NY
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


1952 Chevrolet 3100 w/59’ 235
Re: Fusible link for 12v alternator [Re: Phak1] #1309720
Sun May 05 2019 09:28 PM
Sun May 05 2019 09:28 PM
D
drdoug  Offline
Shop Shark
Joined: Aug 2018
Posts: 116
Prosper, TX
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.

Re: Fusible link for 12v alternator [Re: Phak1] #1309730
Sun May 05 2019 10:11 PM
Sun May 05 2019 10:11 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 18,533
Dellrose, TN
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Fusible link for 12v alternator [Re: Phak1] #1309745
Sun May 05 2019 11:15 PM
Sun May 05 2019 11:15 PM
D
drdoug  Offline
Shop Shark
Joined: Aug 2018
Posts: 116
Prosper, TX
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.

Re: Fusible link for 12v alternator [Re: Phak1] #1309814
Mon May 06 2019 03:18 PM
Mon May 06 2019 03:18 PM
P
Phak1  Offline OP
Wrench Fetcher
Joined: Feb 2019
Posts: 69
Upstate NY
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!

Last edited by Phak1; Mon May 06 2019 03:36 PM.

1952 Chevrolet 3100 w/59’ 235
Re: Fusible link for 12v alternator [Re: Phak1] #1309839
Mon May 06 2019 05:55 PM
Mon May 06 2019 05:55 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 18,533
Dellrose, TN
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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