Alternator Fuse (yes, you need one)

[simplyconnected]

...The 63 amp alternator seems to be more than adequate right now, even with a power top, power windows and seat...

The convertible top can only operate when the car is not in gear, power windows are only operated on occasion, and the power seat... I think you get my drift.

There are two major drains on an alt;
* Hard, cold starting after sitting all night on a winter morning. Usually, it's still dark, the heater/defroster motor is going, and the headlights and tail lights are on. In the north, ice and snow or frost cover the windshield, so the wipers are on. Some folks turn on the radio to hear the news/weather/time on their way to work (now we have heated seats)... This is when fan belts squeal from slipping badly because the gen/alt is outputting at capacity.

* The other time is when the electric fan runs for long times on hot days, or when the A/C compressor clutch is engaged. The electric fan runs when the heater is in 'DEFROST' because the A/C tries to dry the air. So, car makers replaced a constantly running mechanical fan with a more efficient fan that runs only when needed.

In the above scenarios, the demand on electric power is huge, even with the car traveling. The only solution is a very large alt; one that will have enough capacity to charge the battery AND supply the load. A perpetually low (or dead) battery won't last long.

So John, as long as your demand is as low as it was when new, 35-amps is plenty. Even so, Ford conserved electrical demand by shutting off the parking lights when you turn on your squarebird's headlights. It's an on-coming motorcyclist's nightmare if one of your headlights is burned out. - Dave

[jopizz]

Dave,

Thanks for letting me know. The 63 amp alternator seems to be more than adequate right now, even with a power top, power windows and seat. Of course it was Autozone's cheapest one so I'm sure it won't last forever. I'll probably upgrade when it goes.

John

[simplyconnected]

John, you're good to go. #4 AWG is great! You could safely run a 200-amp alt with that wire. And if you ever do, simply upgrade the fuse.

Your 100-amp fuse is perfect for the sixty amp alt. I would also add one of those for my ground wire (to the bell housing).

Many old school mechanics may consider this monster-size cable to be unnecessary. Remember, generators have no diodes, therefore no danger of unloading the battery through the armature.

Notice, new cars have fuses and relays up front, near the battery. One of those fuses protects against diode failure.

[Dakota Boy]

200-Amp "ANL fuse/fuse holder" purchased from a car stereo/audio store:






This is the wiring diagram I used when I relocated my battery to the trunk. I did use the big wire too.

[jopizz]

Dave,

I have a standard GM 63 amp alternator on my '59 using a 4 gauge starter cable. The smallest mega fuse I could find was 100 amp so that's what I have. Does that sound alright.

John

[Joe Johnston]

Thanks for the info and picture. Makes a nice neat installation and I'm sure with all those confounded alternator splash shields I can find a hiding place of a fuse holder in the 63. No problem with the GM unit on the 57 either!

[simplyconnected]

Joe, you have great questions. Certainly, a fuse or breaker is better than none. You have one of the smallest alternators around which is still far better than a generator because it charges at idle speeds.

Alternators have no danger of outputting more than they should. So, fusing for the rated output wouldn't serve much purpose because it isn't really a fault. Alternators can not be 'strained' like motors. Usually, they fail because bearings wear out, brushes wear down (yes, they have two brushes), or diodes fail (either open or short).

So we are back to fusing for the benefit of the wire. The wire must be larger than the rated continuous output of the alt and the fuse must guard against overcurrent from the battery unloading through a shorted diode.

In your instance, #10 AWG THHN copper stranded wire will safely carry 35-amps. A 40-amp manually resetable breaker would work nicely. In my experience, I have never seen a fuse fail but I've seen plenty of faulty breakers. Whichever you choose, make sure your connections cannot come loose. 30-amps and above should have screw terminals.

'Multiplication factor' is the same as 'duty cycle'. Many factors are involved including ambient temps (under the hood), copper diameter and length, insulation type, how much current and for how long, and the type of fuse or breaker used.

Fuse pic ...

Sometimes a pic can be worth 5 or 6 words. This is the setup on my 130 amp 3G conversion. Alt was $25 on E-Pay. Mike

[Joe Johnston]

Theory question

Thanks for the explaination, but a little knowledge leads to more questions:

1) Can the fuse be matched to the alt output? My 63 has the original 30 (or 35) amp alt and mechanical regulator. All works well, no problems. Would a 40 amp fuse be capable of handling the load?

2) Would a manually resettable circuit breaker of equal amperage be as good as a fuse?

At under $20 its cheap insurance from a fire, I think it is a great idea to install a fuse in the system and plan on doing it. I understand a large fuse will blow also, but am just curious as to the fuses amperage being so much larger than the old 30 amp alt, or for newer cars, why there is no mention of a multiplication factor to choose the correct fuse.

You can probably tell I am electron challenged.

[simplyconnected]

I've noticed you mentioning adding a fuse to the alt output wire several times in the forum. What do you recommend? Are there any with terminals that can just be attached to the alt post? I think this is something I should do to both my cars but would like it as inconspicuous as possible for the 63.

Thanks in advance
Joe

The nitty-gritty: Think to the future. There is no such thing as a wire that is too large, and future alternators go up in output. I would start with a #6-AWG stranded copper wire and fuse it for 150-amps. If your alt outputs more than 200-amps, increase the wire size to #4-AWG, and increase the fuse to 250-amps. Wires this heavy are stiff, so put a nice loop between the engine and the body. Always solidly mount the wire to framework (like alternator bracket) and mounted body loops or tie-downs so the end lugs don’t bear both vibration and the support of the wire.

Most folks think fuses (or breakers) are there to protect people. Ultimately they are, but more specifically they protect wires from causing fires. Alternators have diodes that are always powered. At any time, one could short to ground, causing the battery to unload through the alternator's battery wire (the big one). This doesn't happen often, but it could happen while the car is safely put to bed with nobody around.

Rule #1: THE POWER SUPPLY MUST MEET (OR EXCEED) THE DEMAND.

Rule #2: THE WIRE MUST SAFELY CARRY FULL-LOAD CURRENT.

'Safely' means, the wire needs to be de-rated because of heat rise caused by current flow and/or engine compartment temps.

The fuse must be sized to protect the WIRE, but duty cycle needs to be considered. Most wire can sustain 400% of constant current limit for a second. Fuses blow in far less than a second so we can go up about 125% in fuse rating. OEM battery cables are a good example of duty cycle. Notice, all the considerations when choosing wire. All wire insulation has a temperature rating (which is the wire’s ability to shed heat). If the copper size is large enough, the wire will produce very little heat. 60˚C THHN wire is available at Home Depot. It works just fine and is relatively inexpensive.

Alternators come in rated sizes that continue to increase because automotive systems add more accessories. The big jump came with Electric Fans. Before EF’s 75-amp alternators were large enough. After EF’s, 100-amps were bare minimum but I have seen 400-amp alternators. We never want to drain the battery with any accessory while the engine is running. Battery power must always be there to start the engine.

Littelfuse - MEGA Fuse is a perfect fuse choice and widely available at local and online stores. The price range is wide too:
OReilly’s charges - $6.99 for the fuse (UPC: 79458016826) and $9.99 for the holder (UPC: 79458022100)
Delcity.com charges - $3.97 for the fuse and $8.97 for the holder plus shipping.

This fuse really belongs mounted on the fender apron, connected to the Starter Solenoid, in parallel with the battery cable. This connection offers another stud for connecting existing (yellow) power wires. I suppose the alternator wire could be bolted directly to the fuse, but I would not put any undue stress or vibration on a fuse lug. That’s why a two-post holder with a cover is available.

I invite all discussion, cheaper prices, and concerns regarding the need for this fuse. - Dave