Slant Six Forum

fuse or circuit breaker
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Author:  DadTruck [ Fri Sep 04, 2020 6:44 pm ]
Post subject:  fuse or circuit breaker

I plan to install protection for the main power cable on the rear mounted battery in the 68 Barracuda. I already have a battery disconnect switch installed that will shut down a running motor when tripped.
A potential circuit breaker and a potential fuse are linked below.
To get the amperage draw, I have a Clamp Meter that reads up to 400 A DC on the high range, plan is to read the current draw at the battery on a cold motor start ( summer time in indiana cold, probably 80F ambient) , then allow the motor to get running hot and re check. This new motor in the 68B is very sensitive to ignition advance on hot starts,,, I could dial in some ignition advance during cranking and really tax the starter on a hot re start.

a couple of questions
1) which would be preferable, a fuse or a circuit breaker for protection of a rear mounted battery?
2) is the amperage draw test mentioned above an accurate method to size the circuit breaker or fuse?

I also plan to add protection to the power out cable on the alternator. I am using a 90 Amp Nippenso alternator. Under hood I am leaning to using a fuse. I'll set the fuse to the cable size, I need to re check that.

potential circuit breaker ... H4QAvD_BwE

potential fuse ... bp/5841888

Author:  DadTruck [ Sun Sep 06, 2020 2:17 pm ]
Post subject:  Re: fuse or circuit breaker

did a test today with a DC clamp amp meter. I disabled the ignition, then cranked the motor and read the meter at the battery.
The meter flashed 200, 185, 205, 200, 185,,,,
That was on a motor at ambient temp (80F) and had not been started in a few days.

I hooked up the ignition and let the engine run to normal operating temp, it idled in the garage so it did not really see a heat soak.
After disabling the ignition and again checking back at the battery, the meter flashed 205, 225, 250, 200, 203,,,

Author:  Joshie225 [ Sun Sep 06, 2020 10:08 pm ]
Post subject:  Re: fuse or circuit breaker

There are plenty of cars with batteries in the trunk. Do those, like the '97 BMW M3 I used to own, have any circuit protection on the main cable? I didn't think so, but that's just a lazy assumption. If you really don't want that cable hot all the time why not use a starter relay like a Ford?

Author:  GTS225 [ Mon Sep 07, 2020 6:06 am ]
Post subject:  Re: fuse or circuit breaker

Joshie's right. Even NHRA allows such a setup. Go ahead and put your master switch back there, but right after that, (downstream), mount a constant duty solenoid that can be de-energized by a toggle up front.
Hmmmm.....realization're worried about the primary cable abrading somewhere between the master and it's termination in the engine bay, and grounding with no protection for the cable or battery. Joshie's also right about that. Almost no cars have such a protective device. You might be hard pressed to locate a 250 amp breaker for DC loads. If you can't, there's nothing saying you couldn't parallel twin 125's to share the startup amperage load. ('Course, a breaker makes it very tempting to reset as many times as needed to get the car back home.)


Author:  DadTruck [ Mon Sep 07, 2020 4:43 pm ]
Post subject:  Re: fuse or circuit breaker

why not use a starter relay like a Ford?

A Ford starter solenoid is an option.
Remembering what uncle E used to say 'make everything as simple as possible, but no simpler'...

With that in mind, either using a Ford Solenoid or a Circuit Breaker requires inserting an electro mechanical device in to the vehicle wiring.
Its not like one option requires nothing and the other one requires a huge amount of effort. They both require a little bit re configuration.
The Solenoid is probably a bit cheaper than a high amp DC breaker, but the Solenoid requires more wiring and connections than a single breaker.
The schematic attached below shows a second constant duty relay required along with the Ford Solenoid, so perhaps the Ford Solenoid option requires that two devices be added.
Even with the Ford Solenoid there is still a hot wire going from the rear mounted master disconnect to the under hood Mopar starter relay, so its not like a Ford Solenoid provides protection to all of the rear to front circuits.

The Circuit Breaker when tripped will knock everything off line to the battery at the main battery connection. And I already have a rear mounted main power disconnect that when
switched off kills all power except the car alarm. For me that is a simple system.

High Amp DC breakers are available. ... 6AQAvD_BwE

Ford Solenoid.png
Ford Solenoid.png [ 189.16 KiB | Viewed 343 times ]

Author:  emsvitil [ Mon Sep 07, 2020 5:31 pm ]
Post subject:  Re: fuse or circuit breaker

The master disconnect should be right after the battery.

The way you have it, a stuck solenoid can still supply power.

Author:  DadTruck [ Mon Sep 07, 2020 6:17 pm ]
Post subject:  Re: fuse or circuit breaker

That is not my wiring diagram. It is one that gets a lot of use over at FABO.

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