30 Amp fuse voltage.

The Master Cylinder

The Master Cylinder

Well-known member
A friend blew her fuse today on her '31 Tudor. While looking at the fuse I noticed it said 30 amp, 120 volts? I never realized fuses were rated in volts. Will a 120v fuse work properly in a 6 volt system? How about a 30 amp, 12 volt circuit breaker in 6 volt system?

Thanks
 
A fuse is rated to protect the conductor wire in a circuit. A 30-amp fuse is protection for a larger wire size than the any of the 16-guage wires in a model A electrical system. The voltage rating is a never exceed voltage for that particular fuse. The 6-volt system wire minimum guage is 16 awg. Larger gauges can be used to carry higher amperage. The starting system in the highest amp component in the vehicle so it uses 0 to 2 guage cables with no fuse. Too many amps for a fuse when starting. Some folks use larger cables to insure reliability.

If it's blowing a fuse then there is a short in the system. A 15-amp fuse will be better protection as long as the system is short free. I'd inspect all the terminals for looseness of signs of shorting such a damaged insulation.
 
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GPierce said:
The voltage is the maximum safe voltage the fuse is designed for
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OK That makes sense.

rotorwrench said:
A fuse is rated to protect the conductor wire in a circuit. A 30-amp fuse is protection for a larger wire size than the any of the 16-guage wires in a model A electrical system. The voltage rating is a never exceed voltage for that particular fuse.

If it's blowing a fuse then there is a short in the system. A 15-amp fuse will be better protection as long as the system is short free. I'd inspect all the terminals for looseness of signs of shorting such a damaged insulation.
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Oh, yes! She had issues. I replaced everything inside her headlight buckets. Surprisingly the rest of the wiring is in good shape.

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The Master Cylinder said:
I wonder why they put the voltage of the fuse? Just makes it confusing.
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That's so one doesn't use a lower voltage rated fuse in a higher voltage system. Think about it, a lower voltage rated fuse can explode in a higher voltage system.
 
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An old friend of mine was an engineer at PG&E, the California utility company. The high voltage transmission lines, 50,000 volts or more, are protected with fuses. The are a much different design that the 120 volt fuses we use in our cars. Plus, they are isolated from grounded metal by several feet of air. He was trained on how to remove the fuses. He used a long fiberglass pole, rubber insulating gloves, and a full protective suit in case of flash. He had to jerk the fuse out of its holder with the pole. A long arc followed the fuse when it was removed. The training was done in a lab that had generators to produce the high voltage.
 
That photo of the light socket makes me want to ask. I have a 28, and it still has those butt splices that are just kinda stuck together. I sure would like to change that over to something. I CAN force the male part of the headlamp assemble where the conduit attaches out, and replace it....but with what??? Anyone know what I am talking about, and have an idea?
 
The old original bayonet connection on the headlights was reliable but is not reproduced in the same configuration. The old bayonet terminals and the bakelite separator/insulator were good. The repro terminals can work but the plastic insulator and the bayonet lock collar are not the best reproductions.
 
Ever since the starter mounted fuse holder came onto the market, I have been confronted by many Model A owners complaining of blown fuses. In most cases, a fuse less than 30A was found to be unable to withstand the peak current of a stock Model A. Through trial & error, the 30A fuse has become the standard for the Model A, and this is why dealers market the fuse holder with a 30A fuse.
However, I did find that a more exotic "slow blow fuse" rated at slightly less then 30A would work, but these were scarce & more expensive.
 
A short stop circuit breaker is a good alternative. If a person can start isolating suspected circuits, they can likely get home without replacing a bunch of fuses. Ford started using circuit breakers a few years after the model A era.
 
A circuit breaker buzzes which reduces the current when a short circuit occurs, but it does not disconnect the battery. A fuse is a safer device in my opinion.
 
Bob Bidonde said:
A circuit breaker buzzes which reduces the current when a short circuit occurs, but it does not disconnect the battery. A fuse is a safer device in my opinion.
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Uh? Surely a the fundamental raison d'etre of a circuit breaker is to break the circuit! If it's not breaking the circuit when triggered it's either not a circuit breaker or it's broken!
 
I do not like semi modern cars that use a sacrificial wire as a fuse. When is burning up a wire safe?

I also use the 30 amp circuit breaker that fits into the standard fuse holder. It has never given me any problems. Although I still carry extra fuses.
 
nkaminar said:
I do not like semi modern cars that use a sacrificial wire as a fuse. When is burning up a wire safe?

I also use the 30 amp circuit breaker that fits into the standard fuse holder. It has never given me any problems. Although I still carry extra fuses.
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I agree, Fusible Links are a safety hazard...

Picture from Ron Francis Wiring Catalog.

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