Fuse size

[Neshkoro]

I just bought and installed one of those fuse holders that mount on to the starter. It's installed and working. Does that seem like too big of a fuse for what its protecting? I guess 30 amps is better than having NO fuse. What's your thoughts?

[P.S.]

Depends on the load(s). If using all stock lights and just the single brake light, 20 or 25 amp fuse is fine. If using two tail lights and halogen headlights, 30 amp fuse minimum. If using all LED lights, a 15 amp fuse is fine.

For the ones about to type a reply saying 10 amps is enough fuse if using all LED lights, NO it's not. What if the motor is running 3 (amps on the coil), the lights are on, and you ahooga the horn?

[katy]

Originally Posted by P.S.

Depends on the load(s).For the ones about to type a reply saying 10 amps is enough fuse if using all LED lights, NO it's not. What if the motor is running 3 (amps on the coil), the lights are on, and you ahooga the horn?

The horn power comes right off the cutout, doesn't go through the fuse unless the engine is off.

[nkaminar]

Normally a fuse is sized to be 125% to 150% of the maximum current in the circuit. Leave the engine off and turn everything on and step on the brake and blow the horn and see how much amps are registered on the ammeter. Then size the fuse accordingly.

[Ruth]

Originally Posted by katy

The horn power comes right off the cutout, doesn't go through the fuse unless the engine is off.

Can't the same be said for the lights?

[Jim/GA]

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Originally Posted by katy

The horn power comes right off the cutout, doesn't go through the fuse unless the engine is off.

Well... "it depends".

If you are running a generator and you take your foot off the gas, step on the clutch (engine now at idle RPM), it is all coming from the battery.

Do this at night, with the lights on, step on the brake and blow the horn, and you are pulling it all from the battery. On an original 6 volt system, this will be a fair amount of amps. A 30 amp fuse is a good size (unless, as mentioned above, you are running LED headlights, tail lights, etc.).

Even with an alternator, at idle RPM you don't get a lot of amps output and you will still be taking a fair amount from the battery.

The 30 amp fuse protects you from the damage from a dead short, which will quickly exceed 30 amps out of the battery.

Common places for a dead short: headlight bucket connectors (not Ford's best design), horn power feed (hot all the time), and 1928-29 brake light switch (the repop ones are notorious for shorting out inside).

You know your vehicle best. If you can see less maximum draw from using LEDs, you can downsize the fuse a bit. But the 30 A will protect you from your biggest dead short risks.

[P.S.]

Originally Posted by Ruth

Can't the same be said for the lights?

Yes, unless the charging circuit is not charging. You don't want to blow the fuse because you're sitting at a stop light, the charging not kicked in.

[JayJay]

As Jim says, the fuse (or circuit breaker, in my case) protects from a dead short, which would be hundreds of amps. The increase in protection from say a 20A fuse will be minuscule compared to a 30A, while the risk of prematurely blowing as noted by many above would be significant. Why would you NOT run the 30A which has much experience of being quite adequate to the application? I can see where if you had installed a fuse block that protected branch circuits (e.g. lights, horn, dash) separately you might want to downsize but not the main.

I’ve had 30A fuses or breakers in all my cars and only blown one on the road, which I think was a fluke because I replaced it and went on my way.

[Rob Doe]

The fuse is chosen to adequately protect the lightest wires from catching fire, right???

[rotorwrench]

Rob Doe has it. The fuse is to protect the wire. Each different American wire gauge (stranded AWG) size has a corresponding fuse size to protect it. This link has a chart on page 505 of the PDF document that shows FAA recommendations.
https://www.faa.gov/documentlibrary/...-1b_w-chg1.pdf

A person can use a smaller rated fuse than those recommended but they should never use a larger rating than recommended or the wire may burn if shorted directly to ground. Smaller fuse size is more to protect the load than it is the wire that carries it. It's not uncommon in aviation to use smaller fuses fore electronic devices like radios, transponders, and navigation equipment.

This same document also has information about selecting wire for different electrical load applications. When a person looks at the chart, they can see that there are a lot of factors that are involved to select a proper gauge wire. Length and whether the wire is in bundles will make a dfifference. Resistence of the wire increases with length so that has an affect.

A 16-AWG wire is fully protected by either a 15-amp circuit breaker or a 10-amp glass fuse. A lot of folks use a 20-amp fuse on the model As that are equipped with a fuse but it could be a gamble if a dead short happens. A 15-amp would be a safer bet or just use a 15-amp CBF type short stop circuit breaker. The CBF types are made to connect to a fuse block. this is what they look like.
https://www.zoro.com/eaton-bussmann-...15/i/G2221003/

[2speed]

You also have to consider branch circuits with the wire size. The entire electrical system will get a larger fuse, than say a fuse that's just running your signal lights, with a smaller gauge wire.


You can also buy circuit breakers that will fit in place of the fuse. They will reset when they cool down and you won't be stranded in the dark.

[Ruth]

Originally Posted by 2speed

You also have to consider branch circuits with the wire size. The entire electrical system will get a larger fuse, than say a fuse that's just running your signal lights, with a smaller gauge wire.


You can also buy circuit breakers that will fit in place of the fuse. They will reset when they cool down and you won't be stranded in the dark.

This is what I use. They will not reset until the problem is rectified. They are directional also, so ensure you install correctly.

[P.S.]

Originally Posted by Rob Doe

The fuse is chosen to adequately protect the lightest wires from catching fire, right???

Yes. And to expand upon what others have said, most Model A wiring harnesses are made from 14 ga. wire. In my location, code dictates that 14 ga. wire will handle up to 15 amps safely (at room temperature or <140 degrees F).

Current is a rating that includes a time constant. So, let's say a headlight bucket connector shorted and because the wire is 14 ga. the wire conductor would not fail until current reached (lets say) over 50 amps. However, even if the current was only 30 amps, the wire would get hotter and hotter, and with enough time, could become hot enough to burn the insulation and start a fire. Then there is wire length to consider, but let's stick to shorter lengths, like those found in a Model A wiring harness. Then, we could go on and on talking about branch circuits, like the Model A.

So, generally speaking, yes. Fuse size is chosen based on protecting the wire.

[nkaminar]

Everyone is correct that there is a maximum amount of current that a wire can carry. And that depends on whether the wire is sitting in air or buried in a conduit or otherwise has restricted air circulation. It also depends on the kind of insulation that the wire has. A 6 volt system will, in general, carry twice as much current for a given wattage than a 12 volt system. But a short to ground has the potential to carry whatever the battery will supply, something like 200 amps. In my car there is a 30 amp main fuse and then individual fuses for branch circuits. For instance, the fuse for the electronic tachometer is only 1 amp. The rest of the branch fuses are sized to protect the devices that are connected to them but not more than what the wires will carry.

[Hitman]

Originally Posted by P.S.

Yes. And to expand upon what others have said, most Model A wiring harnesses are made from 14 ga. wire. In my location, code dictates that 14 ga. wire will handle up to 15 amps safely (at room temperature or <140 degrees F).

Current is a rating that includes a time constant. So, let's say a headlight bucket connector shorted and because the wire is 14 ga. the wire conductor would not fail until current reached (lets say) over 50 amps. However, even if the current was only 30 amps, the wire would get hotter and hotter, and with enough time, could become hot enough to burn the insulation and start a fire. Then there is wire length to consider, but let's stick to shorter lengths, like those found in a Model A wiring harness. Then, we could go on and on talking about branch circuits, like the Model A.

So, generally speaking, yes. Fuse size is chosen based on protecting the wire.

For a house, this is correct. Automotive applications don’t have a “code”.

[Hitman]

Originally Posted by nkaminar

Everyone is correct that there is a maximum amount of current that a wire can carry. And that depends on whether the wire is sitting in air or buried in a conduit or otherwise has restricted air circulation. It also depends on the kind of insulation that the wire has. A 6 volt system will, in general, carry twice as much current for a given wattage than a 12 volt system. But a short to ground has the potential to carry whatever the battery will supply, something like 200 amps. In my car there is a 30 amp main fuse and then individual fuses for branch circuits. For instance, the fuse for the electronic tachometer is only 1 amp. The rest of the branch fuses are sized to protect the devices that are connected to them but not more than what the wires will carry.

This is the true answer. You can estimate draw and load, but at the end of the day a shorted (grounded) wire will carry the supply load at a max load since the ground can handle it.

Go with 30 amps and not over think anything. Henry didn’t, that’s why it’s not a judging standard question.

[P.S.]

Originally Posted by Hitman

For a house, this is correct. Automotive applications don’t have a “code”.

Yes. Was just using that example.

[rotorwrench]

Henry had intelligent people designing his systems but the era that the model Ts and As were manufactured in left a lot to be desired on a safety standpoint. Fuses were not common until the early 30s and they first opted for circuit breakers. The Powerhouse generators were capable of putting out 20 amps where the later Autolite types were not quite as capable at around 15-amps but were generally never adjusted to that kind of output. The adjustable field strength 3rd or 5th brush had to be manually adjusted either by a dealer or by the operator. This alone left a lot of room for overcharging the battery and other operator induced problems. If maintenance was performed by competent mechanics that followed Ford recommendations then problems would have been minimal but human nature tends to throw that thought out the window.

I've read a few documents that stated that Ford used 16-gauge electrical wire which would carry the loads as originally designed. The running lights are the largest continuous load. They had their own wire from the generator and were within a 10-amp range as originally designed with only a single tail light. The brake light is considered an intermittent load so it's a lesser draw on the system. How many model As in the modern era still only have one tail/brake light assembly? Add higher candle power headlamp bulbs and what does that do? A person can see that Ford was not at fault in their design with exception of a wire protective fuse. It's the later generations that change the system by adding more or higher loads.