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JamesR

Your ideas on fuses and wiring

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I'm asking this in general discussion because I'm looking for people's "school of thought" rather than specific knowledge.

 

I'm just about done completely rewiring my '54 Ford wagon with a 6 volt retro kit from Ron Francis. It's gone fairly well so far, but for some reason the new fuse box in the kit did not have a designated fuse for the horn circuit, even though there was  a feed from the fuse box for the horn. I called them and they said, "yeah, you could put a 30A fuse in the horn circuit," so I ordered one. They offered no explanation as to why the kit is without one.

 

I wonder why the kit didn't provide a fuse for the horn in the panel. Is there some reason you wouldn't fuse a horn circuit? Maybe not having a horn due to a blown fuse is deemed more dangerous (in traffic situations) than not having the circuit protected by a fuse, but I couldn't say. The horn is also a very low use item, and not used from much more than a few seconds at a time.

 

THE STARTER: With that in mind, I realized that the original wiring had a circuit breaker for the starter circuit, or at least the starter relay circuit. I took all of that out with the original wiring. I asked the Ron Francis company (by email) why their kit didn't have any apparent fuse for this circuit, and they said something like, "the fuse panel in your kit provides protection for those circuits," but I don't see where. The instructions list all the circuits protected by the fuse panel, and the starter circuit in not among them. And all of the other circuits on the panel are accounted for. Again they offered to sell me a fuse for the starter circuit, and suggested a 60 to 80 amp  fuse, but doesn't that seem high for a relay circuit?

 

Ron Francis has been great so far, and I recommend them, but I'm a little confused on these items. Thanks for your input.

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This should be interesting, with all electrical questions there will be two different answers, and you will be left with deciding which one to believe. 

 

Bob 

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There is only one fuse in my Pontiac and it is for the lights.  Horn and starter are not fused, probably because of the high draw of them.

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Horns have tremendous inrush current. On most older cars they are unfused. Likely to prevent nuisance fuse blowing.

 

The starter itself is never fused. Relays might be. There could be fuselinks somewhere for things that are otherwise unfused.

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Bloo said it. The instantaneous in rush current for a horn is large and could blow a normal sized fuse. There are "spring oriented fuses" to address this problem but they are usually used in AC circuits, not 6 Volt DC, so the answer is use a large size fuse, like a 20 Amp or 25 Amp fuse. The in rush current would be say 12-15 amps and settle at 6-8 amps., well below the 20 amp fuse size. As for what package to use, use the black phenolic (plastic) in line style screw together fuse holder, one that uses common size glass fuses. Suggest taking the time to solder the wire to the tip in the fuse holder. Makes for a nicer installation. You could use the blade type fuse and fuse holder but its not period correct. Suppose where it's located and if visible and your desire for appearance.

Edited by Friartuck (see edit history)
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Let's start with the horn. Most companies like Ron Francis, Painless and American Autowire use a common power feed for multiple accessories that use constant power. Such as horns, dome light, courtesy light things like that. The fuse panel might be marked BAT or B+. If you have fuses marked in the way pull and check for power at your BAT terminal on your horn relay. If you lose power then the circuit is protected even though it is not listed. As for the starter which circuit do you want to protect? The ignition to solenoid or the motor? The solenoid circuit would only require a small maybe 10A, many small industrial equipment companies do this but you very seldom see it with car companies. The starter motor itself that is a different story. General rule of thumb is 1A of starter draw per cubic inch of engine size. Ford back in the early 90's used this setup it did not work great. They used an ANL type fuse at about 175A. If you jump started the car it would blow. I even saw them blow if the wipers were frozen to the windshield and you tried to operate them. The only way to get a fuse setup for that kind of amperage is thru a modern car audio company. Those loud obnoxious stereo systems you hear before you see them can have a fuse setup over 400A.

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7 hours ago, Bloo said:

Horns have tremendous inrush current. On most older cars they are unfused. Likely to prevent nuisance fuse blowing.

 

The starter itself is never fused. Relays might be. There could be fuselinks somewhere for things that are otherwise unfused.

Surge current is the term  for the start up current.

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2 hours ago, certjeff1 said:

The fuse panel might be marked BAT or B+. If you have fuses marked in the way pull and check for power at your BAT terminal on your horn relay. If you lose power then the circuit is protected even though it is not listed.

Great idea. I do have fuses identified as battery accessory and ignition accessory on the panel, but I thought they were exclusively dedicated to feed wires that were labeled the same...however, the feed wires might just be a way to put additional items on an existing circuit. I hadn't considered that. I wish the instructions had been a bit more clear. I'll probably try your test, but - judging from the other comments - it sounds like unfused horn circuits are common on old cars. I'm pretty sure my car had no fused horn circuit originally. (Excluding the overdrive, the entire car had only 2 fuses and 2 circuit breakers! Maybe that's why it tried to catch on fire last year!) Still, I never had a problem with the horn circuit. And with a horn, a blown fuse is would be more than a nuisance in a traffic situation, so I may opt for no fuse for the horn.

 

Quote

As for the starter which circuit do you want to protect?The ignition to solenoid or the motor? The solenoid circuit would only require a small maybe 10A, many small industrial equipment companies do this but you very seldom see it with car companies.

 Below I've shown how the original circuit was done from the factory, per the manual. It appears to me from the diagram that the starter solenoid (relay) circuit was protected by the circuit breaker on the original panel. Having said that, the charge indicator (ammeter in this context of this diagram) appears to have been protected by the same circuit breaker. (EDIT: Now that I look at the diagram again, I may have interpreted it wrong. What do you think? IS the starter relay circuit fused or protected by circuit breaker?) I've been told that ammeters, at least on old cars, are considered a potential  hazard by some people. With that in mind, the main intention of the circuit breaker may have been to protect the ammeter...but I can't say for sure. My car has no ammeter, but an idiot light to indicate proper charging. It's on a different circuit, so it that would amend this wiring diagram.

 

THANKS THANKS THANKS to everyone taking the time to explain this stuff to me! Greatly appreciated, and you've helped enormously so far.

 

 

Scan_20190221.png

Edited by JamesR (see edit history)

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12 hours ago, 1937hd45 said:

with all electrical questions there will be two different answers

 

Well, you got your pulses and your minuses. to consider.

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5 hours ago, 60FlatTop said:

 

Well, you got your pulses and your minuses. to consider.

I like to think of myself as positive and looking at the bright side of things, I'll go to my grave with all things electrical a total mystery. Understanding it  is a God given gift, something than can not be taught. Bob 

Edited by 1937hd45 (see edit history)
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Bob You stated what I also feel - absolutely perfectly -  " all things electrical a total mystery"

Walt

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An important thing to know a about a fuse is it is intended to protect the wire going to a device, not the device it powers. The fuse should be sized according to the size of the wire so it blows before the wire going to the device overheats.

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1 minute ago, Ronnie said:

An important thing to know a about a fuse is it is intended to protect the wire going to a device, not the device it powers. The fuse should be sized according to the size of the wire so it blows before the wire going to the device overheats.

OK, If you believe the power comes from the battery, the fire will fry everything from the battery to the fuse, correct? 

 

Bob 

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39 minutes ago, 1937hd45 said:

OK, If you believe the power comes from the battery, the fire will fry everything from the battery to the fuse, correct? 

 

Bob 

That's incorrect, Bob. The electricity going through the wire is happening at a speed that approaches instantaneous. The overload that the fuse protects against is measured in amps, and happens through the whole circuit essentially at the same time. The fuse isn't a barrier. It's basically a non-reusable off switch to prevent fires. Having said that, the wire of an overloaded circuit that isn't protected by a fuse or breaker won't necessarily catch on fire over it's entire length at the exact same time I don't think.

Edited by JamesR (see edit history)

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3 minutes ago, JamesR said:

That's incorrect, Bob. The electricity going through the wire is happening at a speed that approaches instantaneous. The overload that the fuse protects against is measured in amps, and happens through the whole circuit essentially at the same time. The fuse isn't a barrier. It's basically to prevent fires. Having said that, the wire of an overloaded circuit that isn't protected by a fuse or breaker won't necessarily catch on fire over it's entire length at the exact same time I don't think.

I know were this is going to wind up, but if the power IS NOT coming from the battery were is it coming from? 

 

Bob 

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57 minutes ago, 1937hd45 said:

OK, If you believe the power comes from the battery, the fire will fry everything from the battery to the fuse, correct? 

 

Bob 

 

Only if there is a short to ground before the fuse. When setting up a fusing system, the fuse should be as near to the power source (the battery) as possible. Often a fuselink is placed after the battery to protect the whole system.

 

As Ronnie said, the fuse is there to protect the wire not the device.

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2 minutes ago, 1937hd45 said:

I know were this is going to wind up, but if the power IS NOT coming from the battery were is it coming from? 

 

Bob 

Caveat: I'm rewiring my old car, and used to work at a consulting engineering firm as a tech, but I'm not an engineer and will probably get my self into trouble trying to explain principles I barely understand myself but:

I don't think anyone said that power is not coming from the battery. The battery is a power source, as is a generator. A car typically needs far more electrical power to function than a standard battery can supply, so a generator is needed (called an alternator in later cars because of the alternating current is uses.)  The generator in my car charges the battery, and has it's voltage and it's current moderated by a "voltage regulator" (also called a  "generator regulator"  in my car) so that changes in voltage and current produced by the generator won't harm the battery or other electrical devices like light bulbs.

 

Sometimes an analogy is drawn between a lighting circuit and water going through a pipe to illustrate some characteristics of electricity, but that  analogy breaks down on many other levels. Electricity isn't really like something flowing downstream or through a pipe, so the idea of a fuse being a barrier that protects everything behind it but not in front of it isn't quite accurate. As maok said, there are also faults that blow a fuse, and that's a different thing than a circuit over load or wire being undersized.

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Ronnie makes a very good point that I overlooked, protecting the wire itself. So I'll revise my early post to a 12-15 Amp in line fuse assuming 14 gauge wire is used which would be realistic for a 6 Volt harness.

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3 hours ago, 1937hd45 said:

OK, If you believe the power comes from the battery, the fire will fry everything from the battery to the fuse, correct? 

 

Bob 

 

Wiring varies from year to year and between brands of vehicles but below is a simplified diagram of how most cars are wired.

 

The battery cable going to the starter is large enough that it can handle the full amperage of the battery when the starter is engaged without overheating under normal conditions. The smaller wiring throughout the rest of the car can't handle that much amperage so they must be protected by fuses and/or breakers inside the fuse box(es). The fusible link you see in the photo protects the large wire going to the fuse box and other wiring. It works like a fuse but it isn't intended to be replaced like a normal fuse and it shouldn't blow unless there is a major short in the wiring. Some cars have multiple fusible links to protect wires feeding different circuits.

 

simple circuit.jpg

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5 hours ago, JamesR said:

Sometimes an analogy is drawn between a lighting circuit and water going through a pipe to illustrate some characteristics of electricity, but that  analogy breaks down on many other levels. Electricity isn't really like something flowing downstream or through a pipe, so the idea of a fuse being a barrier that protects everything behind it but not in front of it isn't quite accurate. 

 

Actually the water analogy works very well, including check valves (diodes), pumps (batteries or other power sources), small pipe (resistors), heat exchangers (loads), Bernoulli (Kirchoff), etc, etc.

 

At work I used the reverse analogy when describing cooling water systems to the electrical engineers on a project! "Draw the circuit" I would tell them when they couldn't understand water flow. It worked.

 

BTW, the fuse usually protects devices and wiring downstream. I do agree it is not a barrier (like a dam), but a valve that closes when too much flow is going through it. A one time use valve. An emergency sluice gate.👍

 

5 hours ago, JamesR said:

As maok said, there are also faults that blow a fuse, and that's a different thing than a circuit over load or wire being undersized.

 

A fuse can only blow from overcurrent in most circuits. The exception is operating the fuse in too high temperature environment, but that is not an issue in a vehicle, unless it is on fire!😉 The time/current tables are available for all fuses. A fuse can fail from vibration breaking the element, usually seen as a fuse that looks good, since the break is hard to see. Use a DMM/ohmmeter to check fuses and make sure they are good. A wire being undersized will not blow a fuse. The wire might melt or cause a fire is  the issue of current exceeding the capacity of the wire. Only faults should blow a fuse. It should not blow from no fault, and if it blows (except the vibration thing), there has been a fault that caused excessive current to flow. 

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4 hours ago, Frank DuVal said:

 

Actually the water analogy works very well,

 

Thanks Frank. I enjoy learning about this stuff as I rewire my car. I have a question about what you said...there's probably a logic to it, but I'm not getting it.

 

You said: "A fuse can only blow from overcurrent in most circuits." Then you said: " Only faults should blow a fuse." Can you explain that to me? I take fault to mean "short", as a hot conductor touching a ground. And overcurrent to mean too many amps. Maybe my vocabulary definitions are off. Thanks for any explanation you can provide. 

 

Back to my rewiring job: my rebuilt 6 volt generator will arrive on Monday, and I believe it is set up from the rebuilder for positive ground and I want to change it to negative ground, which is how I got the car. (The car came from the factory as a positive ground, but the folks at Ron Francis have been recommending staying with negative ground.) The instructions I've found to polarize the generator to a negative ground is to a) hook the battery up as a negative ground in the car, then b) disconnect wire at field terminal on the voltage regulator and spark to battery terminal on the  voltage regulator a couple of times.  I've found this in reference to old Fords enough that it seems plausible. Does that sound correct to you? Other makes seem to have different methods, for example older English cars.

 

 

Edited by JamesR (see edit history)

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Why change the car from positive ground to negative ground? It is still 6 volt, right?  The ammeter will read backwards until the wires going to it are swapped. Did Ron Francis label the ammeter wires for negative ground? The heater motor (if it has one...), if it is permanent magnet, will spin backwards (I do not know if it is that type). The series wound starter motor does not care abut polarity. And don't forget the polarity of the ignition coil.

 

Polarizing the generator is different on Circuit "B" generators than Circuit "A" generators. GM uses mostly Circuit "A" generators, and you would leave the field connected. This year Ford probably a circuit "B" generator, and you need to follow the directions for the right year Ford. 

 

A fault is a condition of wrong. Device fails to operate.

 

A fault can be an open circuit, in which case the fuse will not blow, but the device does not work, hence a fault. I call this a "Long" as people use the phrase "it has a short" when a lamp blinks on and off while moving the cord.  A short would be sparks and blowing a fuse or circuit breaker, obviously not the case with a lamp blinking without sparks! A fault can also be a wire touching something (chassis/wire/etc) of opposite polarity, which is a short. A fault can also be a motor with bad bushings drawing too much current, more than the ampacity of the fuse, blowing it.

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2 hours ago, Frank DuVal said:

Why change the car from positive ground to negative ground? It is still 6 volt, right?

 

I'm actually not changing the car...it was changed from original positive ground to negative before I got it 15 years ago. I'm just changing the new generator to negative ground, or at least making sure it's negative ground.  (Yes, it is 6 volt.) I've rewired the whole car (except the overdrive, which I'll do later this year.) The folks at Ron Francis strongly recommended that the car remain negative ground, as that's how they set up the kit they sold me - coil wiring, etc. (it's a universal kit.) Since the rebuilt generator is designated for a car that was originally pos. ground, I'm assuming that's how it comes. Also, I have a couple of other old cars, and keeping them all negative ground makes life simpler for a space cadet like me. The original heater motor never turned backwards with neg. ground, but I've changed out the heater motor for a new one. I checked it and it runs the correct direction with negative ground, too.

 

 

Quote

This year Ford probably a circuit "B" generator

That corresponds with the consensus of my online research. I think I'm pretty much set for my new generator tomorrow.

 

As far as my original questions, at this point I think I'll forgo installing the fuses I asked about. Haven't decided against it...just giving it some more thought.

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Fuses and fusible links are designed to protect the wiring from any type of circuit fault caused by amperage overload. But over the years I have seen both fail to protect the wiring. Most recently we had a 1971 Dodge Dart Swinger that the fusible link from the starter relay to the fuse panel fried, but by the time in burned thru it damaged the bulkhead connector beyond reuse. We found the voltage regulator was full fielding the alternator causing the amperage overload. Many years back we had a late 80's Ford Crown Victoria that a local NAPA store had supplied 6 alternators for. When they gave us a call. We said we would supply an alternator on one that if ours failed we wanted the car. We made that stipulation since we could not open any of the NAPA alternators without voiding the warranty. Ours did fail, we got the car, opened the alternator and found the number 1 positive diode burned. This is caused by load dump. After repairing the alternator and running tests we found that the AC compressor was drawing 75A, without ever blowing and fuses or burning any fusible links. We compared another Crown Vic and it was only drawing 15a.

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6 hours ago, certjeff1 said:

Fuses and fusible links are designed to protect the wiring from any type of circuit fault caused by amperage overload. But over the years I have seen both fail to protect the wiring. Most recently we had a 1971 Dodge Dart Swinger that the fusible link from the starter relay to the fuse panel fried, but by the time in burned thru it damaged the bulkhead connector beyond reuse. We found the voltage regulator was full fielding the alternator causing the amperage overload. Many years back we had a late 80's Ford Crown Victoria that a local NAPA store had supplied 6 alternators for. When they gave us a call. We said we would supply an alternator on one that if ours failed we wanted the car. We made that stipulation since we could not open any of the NAPA alternators without voiding the warranty. Ours did fail, we got the car, opened the alternator and found the number 1 positive diode burned. This is caused by load dump.

 

 

Thanks for relating your experiences and your help. . I started off this thread with the statement that I was looking for "schools of thought," or people's personal philosophy with regards to automotive wiring practices. I find that covers a significant percentage of conversations about car wiring. In other words, while there are many definite do's and don'ts  regarding wiring, a whole lot of the conversation involves opinions or perspectives...sometimes in the guise of "fact." For example, the Ron Francis company strongly advises against fusible links. I don't know the exact particulars of their objections to them, but they seem to think they aren't very safe. Other people like them and use them successfully. I think they don't like inline glass fuses, either. Some folks at RF also don't think ammeters are safe, as the entire electrical load of the car is carried across this delicate device. It's all interesting. My Ford had an inline glass fuse for the heater motor for 65 years and never a problem, but I'm inclined to follow their recommendations as I'm pretty much a novice, even at my advanced age. My car has an idiot light instead of an ammeter, anyway. I'm glad I'm not making major electrical alterations to my car, though other people do quite well at that.

Quote

After repairing the alternator and running tests we found that the AC compressor was drawing 75A, without ever blowing and fuses or burning any fusible links. We compared another Crown Vic and it was only drawing 15a.

 

I'm also glad that my Ford is about as electrically simple as a functional and practical car can be. It's a Mainline...it has no options, not even a radio...not even an electric windshield wiper...not even a switch that turns on  the interior lights when you open the door!! I'm leaving it that way. I just hope that my crimps and solders are good and that I properly coordinated the wiring kit instructions with the factory specs and shop manual. We'll see.

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