I need some help- circuit breaker question 6V vs 12V

[Chris]

I had a circuit breaker fail in my 1941 Ford. The car is original and still 6V. It burned up some of the wiring when I had a short so I rewired the whole car along with put in another circuit panel (original 1941 Ford). Got everything done last night and everything works but the headlights. I checked everything and one of the circuit breaker lugs has zero power at it. So it looks like this circuit breaker is bad. I am going to put new breakers in it, the question is- are they different from 12V to 6V? The book at the parts store list tons of breakers, but are either 12V or 24V. Nothing mentioned on 6V. I assumed with the correct amp rating it would not matter...but I don't know too much

Any help on this? I wanna get this ol car back on the road!!!!

[Jack E/NJ]

Chris>>>I assumed with the correct amp rating it would not matter>>>

Good assumption for systems below about 50volts DC.

Jack E/NJ

[Chris]

I just noticed Bob Drake sells new oroginal style CB's...I may just order one from him. Has anyone ever got one? Are they good quality?

[rotorwrench]

Amperage is Amperage whether it's 6-volt or 12-volt. The difference is in the wire it is protecting. 6-volt systems need some pretty big gauge wire to pass the same amount of current as 12 or 24-volt. 12-volt systems can use 18 gauge wire to pass the same current that 6-volt uses on 16 gauge and 24-volt can use 20 gauge. They would all use the same circuit protection since most breakers are rated up to at least 32-volt. Some are rated all the way up to 120-volt. They will trip if the current level rises above the rating regardless of the size of the wire same as a fuse will. Most are either 15 or 30 amp and are usually used on a buss where there are further fuses to protect individual circuits.

Kerby

[Mike51Merc]

Double the Voltage = Half the Current (AMPs)
Half the Voltage = Double the Current

Ohm's Law E = IR (look it up) tells us that devices that work at half the voltage pull twice the current, and vice versa. This is why 6 volt cars have heavier wires than 12 volt cars; they need twice the current (amps) to make the lights shine as brightly as they do on a 12 volt car.

What that means is that if a device with a fixed output (lightbulb candlepower, electric motor RPMs, etc.) pulls 14 amps at 6 volts, they will only need 7 amps at 12 volts to do the same job.

The bottom line is that on a 12 volt system, you'd want a circuit breaker (and/or fuses) with HALF the rating of your 6 volt ones.

Radio Shack carries circuit breakers that will do the job. They even look like the original ones.

[rotorwrench]

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Fuses and circuit breakers are sized for the wire being used. I use a chart published in the FAA Advisory Circular AC42.13-1B when I have to perform alterations to circuitry or add something new to the helicopters. The wire length has a factor in this but electrical engineers use a bigger wire to suit the amperage needs when the wire lengths get excessive. This is all figured in wire size & length charts both in bundles and in free air runs. Many times I'll use a smaller circuit protector so as to better protect the actual electrical component. You can use any size smaller that you want but you can't use a larger size than the chart allows for.

The old Fords mostly used a 35-amp system and the largest wires in the system with exception to the starter & battery cables are well protected with a 30 or 35-Amp breaker. If they call out a 30 Amp breaker and you can find one the same size, it will work. I see them all the time at the local truck stop. Same ones I use on my Harley Davidsons.

Kerby

[39portlander]

Chris, another source for 6 and 12V ?'s is Ron francis Wiring. www.ronfrancis.com If you don't have it, get the latest catolog/encyclopedia.

[Frank Miller]

Originally Posted by Mike51Merc

Double the Voltage = Half the Current (AMPs)
Half the Voltage = Double the Current

Ohm's Law E = IR (look it up) tells us that devices that work at half the voltage pull twice the current, and vice versa. This is why 6 volt cars have heavier wires than 12 volt cars; they need twice the current (amps) to make the lights shine as brightly as they do on a 12 volt car.

What that means is that if a device with a fixed output (lightbulb candlepower, electric motor RPMs, etc.) pulls 14 amps at 6 volts, they will only need 7 amps at 12 volts to do the same job.

The bottom line is that on a 12 volt system, you'd want a circuit breaker (and/or fuses) with HALF the rating of your 6 volt ones.

Radio Shack carries circuit breakers that will do the job. They even look like the original ones.

You're close. Volts x Current = watts. A 58 watt bulb requires 8amps at 6 volts. But 4 amps at 12 volts. The filament will have a higher resistance on the 12 volt bulb to limit the amps. Everyone else is right in that the circuit breakers only see current and do not care about voltage as there is no resistance until they surpass the current rating. At that point the bimetalic strip will curl from the heat and open the circuit. The two metqals have different expansion rates which cause it to curl.

[Chris]

Originally Posted by Frank Miller

You're close. Volts x Current = watts. A 58 watt bulb requires 8amps at 6 volts. But 4 amps at 12 volts. The filament will have a higher resistance on the 12 volt bulb to limit the amps. Everyone else is right in that the circuit breakers only see current and do not care about voltage as there is no resistance until they surpass the current rating. At that point the bimetalic strip will curl from the heat and open the circuit. The two metqals have different expansion rates which cause it to curl.

Thank you. Now my question is, if volts make no difference, how come my book list 12V breakers and 24 volt breakers??

[Jack E/NJ]

Oooo-oooh. I gotta answer for that. Because your modern book doesn't list any breakers for old cars & tractors equipped with 6 volt systems.

Seriously though, just about any breakers made for dc systems below about 50v should work OK in a 6 volt system...even a 1 volt system.

Jack E/NJ

[Jack E/NJ]

Frank Miller>>>A 58 watt bulb requires 8amps at 6 volts.>>>

Sorry Frank, yer prolly not gonna be an arithmetician when you grow up. 8^)

Jack E/NJ

[Mike51Merc]

Originally Posted by Frank Miller

You're close. Volts x Current = watts. A 58 watt bulb requires 8amps at 6 volts. But 4 amps at 12 volts. The filament will have a higher resistance on the 12 volt bulb to limit the amps. Everyone else is right in that the circuit breakers only see current and do not care about voltage as there is no resistance until they surpass the current rating. At that point the bimetalic strip will curl from the heat and open the circuit. The two metqals have different expansion rates which cause it to curl.

Frank, you're theoretically correct (but arithmetically flawed). I was treating the devices as constants because I didn't want to further complicate matters.

[fourtydlx]

Any of you elec. wizards know how to lower volts out of a12 v battery maintainer to put on 6v battery? thanks

[47COE]

Originally Posted by Chris

Thank you. Now my question is, if volts make no difference, how come my book list 12V breakers and 24 volt breakers??

A breaker or switch needs to open fast enough that it doesn't draw an arc. Too slow and an arc can start that doesn't go out. The higher the voltage, the more likely that is to happen.

AC is easier because it drops to 0 volts 60 times per second (or 50 times per second in much of the world) and any arc that might have started will go out as the AC voltage passes through 0.

So you will see switches rated at 250 volts AC and 30 volts DC.

A breaker rated at 24 DC volts will work at all lower voltages as well. But might have problems with arcing of the contacts at voltages higher than 24 volts.

You don't see 6 volt rated breakers because they have all been designed to operate at higher voltages. Not much market for a breaker rated at only 6 volts.

Tom

[Jack E/NJ]

Fourtydlx>>>Any of you elec. wizards know how to lower volts out of a12 v battery maintainer to put on 6v battery? thanks >>>

Not a wizard but a 12v maintainer (aka trickle charger) should be able to handle a 6v battery though it might get a bit warmer than with a 12v battery. These chargers are usually amp-limited and can even handle a dead short.

Jack E/NJ

[George/Maine]

With all the street lights today don,t realy see the need for a circuit breaker,put a fuse and if it blows find out why.You realy could burn wires with a circuit breaker,if it were to high amp rating.

[Frank Miller]

Damn typo I meant 48watts. Thanks for the correction.

[rotorwrench]

Fourtydlx
I wouldn't recommend using a dropping resistor for a 12-volt charger to use with a 6-volt battery. You lose too much with the resistor and the charger can't figure it out. A rectifier or wheatstone bridge circuit might work but I'll tell you how I do it. My 6-volt charger maintainer shuts off too soon to charge a battery the way I like so I just charge it with another 6-volt battery. I hook them in series and charge both at a 12-volt rate. I realize this may seem extravagant but I have more than one 6-volt vehicle so it's not for me.

Kerby

[mshmodela]

V=IR, or I=V/R. 30A = 12V/R , divided both side by 2.

30A / 2 = 12V /(R x 2) = 15A = 6V / R where R is the resistance of the breaker. So a 30A, 12V breaker would pop with only 15A at 6V.

[JSeery]

The original breakers would have been 15A and 30A (one of each).

[Drbrown]

Circuit breakers are an extremely important part of a car. If you buy a replacement circuit breaker, I recommend you be sure to get a Made in USA unit, either NOS or used from a trusted source, via forum classified ads (Fordbarn, V8 Club, HAMB) or eBay etc.

I'm a non-conformist when it comes to stock electrical systems in older cars. The majority of forum members here don't experience or haven't experienced the problems I have ..... (1) Unprotected by a circuit breaker or fuse, my '47 ignition circuit shorted and melted the wiring insulation, and (2) my stock dual-circuit breaker serving both headlight/parking light systems overheated internally a few times and shut down the lighting unexpectedly - in the dark .... no evidence of shorts in the system.

I rewired using over-sized wire for a 12 volt system, converted the system to blade-type fuse boards serving every device, and incorporated relays for the ignition circuit, headlights and fog lights. At the cost of sacrificing some of the car's hidden vintage character , the benefit is safety, lower overall electrical system resistance, easier starting, brighter lighting and easy roadside repairs.

[JSeery]

This is a later model CB panel, but it is the same basic arrangement. All of the earlier wiring diagrams I have seen (41 and up) show the two CBs, but are not clear on the amperage ratings or which one connects to what.

[rotorwrench]

Originally Posted by mshmodela

V=IR, or I=V/R. 30A = 12V/R , divided both side by 2.

30A / 2 = 12V /(R x 2) = 15A = 6V / R where R is the resistance of the breaker. So a 30A, 12V breaker would pop with only 15A at 6V.

Amps are current draw or flow and volts are the carrier in a circuit. The theory applies to a circuit but the circuit protective device is largely only affected by current that flows through it. Voltage only affects it when it is a lot higher. If you look at most circuit protective devices, they have the current and maximum voltage printed on them. Cheap automatic reset breakers made in third world counties may not though.

In aviation, we use the same manual reset type breakers for both 12 and 24-volt systems since they are both below 32-volts DC. There is no difference.

This is an old thread.