Alternator Circuit Fuse

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RustyRatRod

RustyRatRod

I was born on a Monday. Not last Monday.
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I want to fuse my alternator circuit on Vixen, my 64 Valiant. Stone stock charging and electrical system, other than the addition of a stock Mopar electronic ignition system and a modern stereo system, with no external amp. Alternator is a square back 55 amp. I want to fuse the big charge wire coming off the alternator. @67Dart273, Del I know we've discussed this in the past, but please forgive me as I've forgotten. What amp fuse would you recommend? I HAVE a 100amp MAXI fuse and fuse case, but I can easily get something different if need be. I thought I would post here, rather than message or call for the sake of sharing and learning. Thank you drive through.
 
Fuse selection is based on the gauge of wire you are protecting not the alternator output. Once you have that number, a quick online search for the recommended fuse size for a given wire size will provide you an answer.

You should not source your 12v power from inside the cab of the car, so the amp draw doesn't pass through the bulkhead connector. Use a relay wired to an underhood source, and do the standard bulkhead and amp gauge bypass. Use good grounds for your high amp components too.
 
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CFD244 said:
If it's a stock set-up, isn't it already fused at the bulkhead with a link?
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Fusible links were not stock until the next year (1965). They were usually placed between the starter relay junction and the bulkhead.
They protect against battery shorts to ground.

Alternators are pretty much self protected from shorts to ground.
a) the field gets no power b) the ignition gets no power so engine stops turning.

66Valiant528 said:
Why not just run a piece of fusible link wire?
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He could. But every fuse and fusible link in the alternator output can become resistance when the current flow is high. Alternator output can be fairly high at times. Fusible link in particular is a purposely undersized piece of wire. Lets say lights, stereo, and heater fan are all running. That's around 5 amps for the engine and alt, 11 amps for lights, 2-3 amps stero? 5 amps for heater. Lets call it 25 amps while driving. A few more with brake lights and turn lights.
Do we really want 25 amps through a 16 gage wire for more than a few minutes?
A properly sized fuse will protect the downstream wires from overloads. I think thats what he's asking Del about.
 
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I 've seen a few of these rigged up in old trucks. :lol::lol:

GLASS_FUSE_ROYAL_2881_5_AMP__56708.1607721389.jpg
 
Fuse link is a form of slow-blow fuse .
It will take an overload for a short time, where a fuse will just blow .
 
I figured he'd be running a fat alternator wire, say 8 gage so a 12 gage link. I think that'd handle the load of our 25 amp draw. Probably handle more. I don't know a fuse pops and done. I have circuit breakers in my race car.
 
All the wires downstream are 12 gage or less.
 
Rob I assume you are upgrading the wire size? I would go with either at least no8 or maybe a pair or 3 parallel no10's for the charging line. Then you can figure the ampacity of that wiring and fuse it by the wire size. In other words you want to get the wire size substantially larger than the total current the thing will see, and then it can be fused with some reserve.
 
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Mattax said:
Alternators are pretty much self protected from shorts to ground.
a) the field gets no power b) the ignition gets no power so engine stops turning. .
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Not quite true. A shorted stator or diode can result in pretty much a dead short at the alternator output stud. That, in part is exactly why the factory fuse link needed to be there, but it is damn poor protection, BECAUSE the wiring was undersized for what it should have been, and the fuse link was "oversized" (or on the ragged edge) by the amount of current in the circuit.

Steve up there in Canada HAD a shorted alternator once that was driving him nuts. I experienced this in the early 70's with a loaner car with mine in the body shop. A diode FELL OUT into the alternator and ended up causing a short that MELTED THE ENTIRE underhood harness---and then---the link blew.
 
67Dart273 said:
Not quite true. A shorted stator or diode can result in pretty much a dead short at the alternator output stud. That, in part is exactly why the factory fuse link needed to be there, but it is damn poor protection, BECAUSE the wiring was undersized for what it should have been, and the fuse link was "oversized" (or on the ragged edge) by the amount of current in the circuit.
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Not following you here.
The fusible link limits the battery from grounding.
The alternator can't produce power unless it somehow the path through the rotor and the path to the coil is of similar resistance to the ground short (parallel paths).
I'm not saying its perfect or complete. Its on par with the fusible link.
This seems to be why most years there's no fusible link on the alternator output wire. That was the question-statement I was answering.
 
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66Valiant528 said:
Why not just run a piece of fusible link wire?
Click to expand...

I found that fusable link wires are limited in the sizes that are available when larger wires/cables are used. When I installed a rewound CS144 alternator I installed is rated at up to 225 amps at a fast idle and over 115 amps at idle, I installed a 2 gauge cable and a Marine Grade fuse block between the alternator to a distribution stud under the hood of my '70 E-Body. I also installed a 2 gauge ground cable from the alternator to a ground distribution lug.
 
Mattax said:
Not following you here.
The fusible link limits the battery from grounding.
The alternator can't produce power unless it somehow the path through the rotor and the path to the coil is of similar resistance to the ground short (parallel paths).
I'm not saying its perfect or complete. Its on par with the fusible link.
This seems to be why most years there's no fusible link on the alternator output wire. That was the question-statement I was answering.
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If the alternator produces a fault that "is" a short at the output terminal, this will either pop the link or burn down the harness. That is why I've cautioned guys who do a bypass by adding a wire from the alternator direct to the battery---add a link or fuse--because that new wire bypasses any protection if the alternator does short
 
67Dart273 said:
Rob I assume you are upgrading the wire size? I would go with either at least no8 or maybe a pair or 3 parallel no10's for the charging line. Then you can figure the ampacity of that wiring and fuse it by the wire size. In other words you want to get the wire size substantially larger than the total current the thing will see, and then it can be fused with some reserve.
Click to expand...
I actually was just going to add the fuse into the stock wire, "whatever" gauge that is. You don't think it's big enough I guess?
 
67Dart273 said:
Not quite true. A shorted stator or diode can result in pretty much a dead short at the alternator output stud. That, in part is exactly why the factory fuse link needed to be there, but it is damn poor protection, BECAUSE the wiring was undersized for what it should have been, and the fuse link was "oversized" (or on the ragged edge) by the amount of current in the circuit.

Steve up there in Canada HAD a shorted alternator once that was driving him nuts. I experienced this in the early 70's with a loaner car with mine in the body shop. A diode FELL OUT into the alternator and ended up causing a short that MELTED THE ENTIRE underhood harness---and then---the link blew.
Click to expand...

"and then the link blew"

Well, wasn't THAT just helpful? LOL
 
If you don't have a big alternator rated for more than whatever the wire is, 12-10 wire, I'd run maybe a 40A fuse. I doubt 30 would be enough. Bear in mind heat---engine heat---will de-rate the fuse value.

That IS the whole problem---wire was never big enough. And on the cars with the flat spade bulkhead connectors, neither are the connector pins
 
67Dart273 said:
If you don't have a big alternator rated for more than whatever the wire is, 12-10 wire, I'd run maybe a 40A fuse. I doubt 30 would be enough. Bear in mind heat---engine heat---will de-rate the fuse value.

That IS the whole problem---wire was never big enough. And on the cars with the flat spade bulkhead connectors, neither are the connector pins
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Ok. Thanks. It's super easy to replace anyway. You think an 8ga would do it? Keep in mind it's a 55 amp alternator and the rest of the electric system will remain stock with no other additions. I thought about A/C, but I'm not doing that.
 
Yall say "size the fuse accordingly" like I fuckin know how to do that. LMAO
 
What's the smallest wire you want to protect?
Its like a house wiring. Use 15 amp breakers on the 14 gage wire, but 20 amp breakers are allowed on the 12 gage wire.
And like a house, just cause there's a fuse doesn't mean all problems will be prevented.
 
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