In-Line Fuse

I have a 1951 Farmall H and I am currently installing a new alternator and installing all new wiring. Would it be a good idea to install an in-line fuse on the wiring harness that I am building? If so, where would be the best place to install it...between the battery and the wire leading to the starter switch? If the fuse blew here it would isolate everything. My tech sheet for my alternator was tested at 39 Amps so what would be a good fuse size to install? The Farmall never had a in-line fuse before and was hot all the time...not too safe. Johnny

You obviously cant use a fuze in the heavy cable starting motor circuit but if your little heart desires, sure you can use others in the low amperage circuits.

To protect the wiring that feeds hot batetry voltage OUT of the battery to low amperage tractor loads, Id fuze it at or near its source which may be where the big battery cable attaches to a starter switch. (A wire from there up to ammeters Supply terminal feeds loads like lights n ignition via the series ammeter PLUS carries charging current back into the battery also via the ammeter) The fuze would go right there (at switch where big cable attaches) that way the wiring from there up to the ammeter etc is protected.

The fuze is sized to protect the wiring (its ampacity), so if youre using say 10 gauge wire from the starter switch up to the ammeter I'd use a 30 amp fuze.

BUT NOTE if the alternator can produce say 50 amps, that wire were talking about above (from starter switch to ammeters Supply terminal and from ammeters Load terminal to alternators output) should by rights have the ampacity to carry such amperage
WHICH MEANS YOU SHOULD USE BIGGER WIRE (maybe 10 or 8 gauge depending on alternator) BOTH from alternator to ammeter and from ammeter to starter switch AND THAT ALSO MEANS YOU NEED A BIGGER FUZE SAY 40 OR 50 AMPS !!!!!!!!!!!

NOTE theres also an issue in that if the alternators sensing voltage wiring circuitry becomes open, the alternator may overcharge so one needs to use caution when and where an alternator (at least it voltage sensing circuitry) is fuzed !!!!!!!!!!!!!!!!!!!!!!!!!

ALSO if you are really concerned you could place a 15 or 20 amp fuze (depending on if you use 12 or 14 gauge hook up lighting n ignition wiring) at the ammeters Load terminal and out of it feed the lights and ignition.


Soooooooooo I would at least use a fuze at the ammeters Load terminal output (say 15 or 20 amp depending on 12 or 14 gauge wiring) to protect the wiring to Lights and Ignition etc.......

BUTTTTTTTT if you wanna try to fuze n protect the alternators charging circuit, i.e. from alternator to ammeter and from ammeter to big battery cable at starter switch, youre talking bigger wire (8 or 10 gauge) which requires a bigger fuze (40 or 50 maps etc depending on alternator used) and a matching 40 or 50 amp fuze.



If it were me Id just fuze n protect the ammeters Load side circuitry (to lights n ignition) with a 15 or 20 amp fuze right on the ammeters Load terminal,,,,,,,,,,Id use 8 or 10 gauge wiring from alternator to ammeters Load terminal and from ammeters Supply terminal to the battery via the starter switch conenction,,,,,,,,,,,not sure I would fuze the charging circuitry (alternator to ammeter n ammeter to starter switch) unless wired such that the voltage sensing circuit cant get open (due to a blown fuze) and allow the alternator to overcharge ??????????????????/


I SURE LOOK FORWARD TO HOW MY SPARKY FRIENDS BOB M AND JIM WOULD FUZE AN ALTERNATOR !!!!!!!!!!!

John T (NOT any alternator expert, more a generator guy)

Good advice below from John T. Expanding just a bit:

1 - DON'T use a fuse in the alternator output circuit (alternator BAT to ammeter to battery + post). Reason: Should the fuse ever let go while the alternator is charging it'll likely ruin the alternator.

Note that OEM manufacturers typically do not fuse this circuit for this reason (also for cost...)

2 - For your 39 amp alternator use a minimum of #10 wire for the circuit above (#8 is preferred.)

3 - Assuming only normal electrical loads on your H (ie. just lights and ignition using #14 wire) use a 20 amp fuse where the lights/ignition switches get power - ie. at the the ammeter post wired to the alternator. This will locate the fuse conveniently in the ammeter box - just like the tractor was wired orginally.

Hope this helps!

Yo Bob, Glad you agree on it could be a problem trying to fuze the alternator.

MY BEST GUESS IS:

1) If the main output became open looks like the alternator would increase n max out current trying to charge that infinite resistance.......

2) If the voltage sense circuit to the alternators lil side No 1 terminal became open it senses theres low voltage n will again increase its output ......

After my initial hunches and your post and thinking the above 2 are correct, I DONT THINK ID FUZE IT EITHER NOW

John T (hope to see you in New York in July, dont let a thing like your daughters wedding stand in the way lol)

Hey John T - That's pretty much it.

Unlike a generator, a full-fielded alternator and with an open-circuited output can develop well in excess of 100 volts - sufficient to destroy the regulator the output diodes. So breaking the output circuit under load can literally "let out the smoke" from the alternator's guts. (BTW I have first hand experience with this type of failure!)

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Still hoping to make the NY JD Expo - and maybe even heckling you from the back of the tent this time(!)

Johnny,

If you want some protection in your charging circuit, do what the OEMs do. Put a "fusible link" in the system where the small wires hook to the hot battery cable (starter switch or solenoid). A fusible link is a piece of copper wire with hi temp insulation four gauges smaller (4 numbers larger) than the circuit wire it is supposed to protect. Use a #14 fusible link to protect #10 circuit wire All the big 3 use them, most good auto parts houses will have them.

Just wonder what would be wrong with having the sensing wire (#2 terminal) connected to the battery side of the appropriate fuse link. JimN

Could I install two 20 amp in-line fuse's coming off my ammeter...one fuse running to the ignition switch and the other 20 amp fuse running to my lights? Or could I pigtail the ignition switch and the lights together on one 20 amp fuse? There is not much room on the ammeter trying to put three wires on the ammeter terminal...espically with the alternator BAT wire. Any suggestions would be most helpful?