300 amp X 12 volt Circuit breaker

[lal Naja]

I've been thinking about installing a circuit breaker connected close to the battery and I've found this on ebay.

http://cgi.ebay.com/ws/eBayISAPI.dll...:X:RTQ:US:1123

The seller claims it will handle a starter motor and my understanding is a starter can draw 300 amps. Have any of you installed a breaker on your battery's positive cable?

Appreciate your thoughts.

Arthur

[StewbieC]

Something else to go wrong.

I wouldn't fit one!

[VintageUpgrades]

I wouldn't bother fusing the starter. I am also not a fan of circuit breakers for 12 volt use. I had a few bad experiences in my car audio days of breakers not breaking when they were supposed to. Put a good maxi-fuse or mega-fuse on your power input.

[Excaliber]

If the starter on your Cobra draws anywhere near 300 amps you've got some serious problems. Using a circuit breaker for that kind of protection is like using a band aid for cancer.

[lineslinger]

What Ernie said, 300 Amps? Most homes only offer 200 amp service.

[lal Naja]

Originally Posted by Excaliber

If the starter on your Cobra draws anywhere near 300 amps you've got some serious problems. Using a circuit breaker for that kind of protection is like using a band aid for cancer.

Regarding the amperage draw for a starter. I've read a number of articlesl/forums on the subject and if there is any truth to what is written, as in most forums, amperage can be even higher than 300amps depending on conditions. Below is an excerpt from one if the many forums that are out there.

"A starter motor from a normal 2 Litre car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin.

Under winter conditions, this current can easily double when the oil is thick.

Diesel engines have a very high compression ratio e.g. 22:1 and require more powerfull starter motors. On average they draw between 300 and 500 Amps for average sized car engines 2 to 3 litre while on truck engines the current could easily reach 1000 Amps at initial turn over."

[lal Naja]

Originally Posted by lineslinger

What Ernie said, 300 Amps? Most homes only offer 200 amp service.

House voltage and 12 volts although related as current are really quite different. 300 amps at 12 volts is 3,600 watts. 200 amp house service at 240 volt is 48,000 watts.

And back to having a breaker on a 12 volt supply. This is again something that I've read while doing research. A custom car builder somewhere out there had placed such a breaker at his battery. I guess he felt compelled to prepare for catastrophic electrical failure and did not want his cherished car to spontaniously vaporise.

So thanks to all for your input. Keep em coming.

Arthur

[lal Naja]

Originally Posted by VintageUpgrades

I wouldn't bother fusing the starter. I am also not a fan of circuit breakers for 12 volt use. I had a few bad experiences in my car audio days of breakers not breaking when they were supposed to. Put a good maxi-fuse or mega-fuse on your power input.

Are you referring to a fuse on the power input of an amplifier?

If so, I do not have a sound system on my Cobra. I was planning one and had the power amp installed, but someone got to my box of goodies and stole the head unit, speakers, subs, crossovers etc. So I gave up and decided to listen to my pipes, or to my headset intercom.

Arthur

[Excaliber]

"A starter motor from a normal 2 Litre car draws between 60 and 200 Amps when turning over the engine when the oil is warm and thin.

I'm calling BS on a 200 amp starter draw on a 2 litre car, even when cold. Such an occurance would be catastrophic and indicative of serious problems with the car, (see the band aid scenario above).

So, you running a DEISEL in that Cobra and expect to draw a 1,000 amps at 36 to 48 volts anytime soon? Puhlease, lets keep it real around here. The above examples are for the most part "fantasy", hypothetical, technically correct under extreme conditions only.

Technically it wouldn't hurt to have a 300 amp circuit breaker on your car, hypothetically you might even need it sometime. Like, when hell is about to freeze over.

[Ronbo]

A lot of factors effect current draw. Number of cylinders plus compression ratio being the big factors.

Also since it's an inductive load, in-rush current will be quite high (400~600A in the case of a high compression, large displacement V8) untill the magnetic fields build to full capacity. Not to mention 80~90 lbs of metal (crank, rods, pistons). In the case of starters there are two current spikes; the field developement and the gear engagement.

Once everything is up and spinning the current will drop down quite a bit, gear reduction starters also run less in-rush but for a longer period of time. (ultimately it still takes the same power to do the job)

Unless you've done a terrible job running your battery cable to the engine, this is pretty much a fix for a non-existant problem. If you already have a cut-off switch, this is totally unneeded.

Ernie, Remember those awfull Olds diesel 350's? The car needed two batteries to crank the engine. My buddie's '68 Eldorado had a factory battery that looked like it was from a locomotive engine. (of course that 500 engine could probably pull a small train)

[VintageUpgrades]

Originally Posted by lal Naja

Are you referring to a fuse on the power input of an amplifier?

If so, I do not have a sound system on my Cobra. I was planning one and had the power amp installed, but someone got to my box of goodies and stole the head unit, speakers, subs, crossovers etc. So I gave up and decided to listen to my pipes, or to my headset intercom.

Arthur

I am referring to fusing the main power cable feeding the car's accessories; ususally connected at the battery or starter solenoid. It's purpose is to blow in case something pinches or cuts this wire.

[Excaliber]

I am referring to fusing the main power cable feeding the car's accessories;

ERA's have that, standard equipment from the factory. I would ASSUME that most of the brand name builders also routinely provide such protection? Might be a fusible link, circuit breaker, big ass fuse, etc.

[lal Naja]

Originally Posted by Excaliber

ERA's have that, standard equipment from the factory. I would ASSUME that most of the brand name builders also routinely provide such protection? Might be a fusible link, circuit breaker, big ass fuse, etc.

Thanks, Can you, or anyone else suggest, a good source as to where to review/locate a suitable fusable link, Circuit breaker or big ass fuse. Web link would be great.

I do like the safety factor in just the event that somehow the main cable from the starter to battery in the trunk got pinched. I'm taking every precaution to make sure this will not happen. I'm even running the battery cable through rubber hose at every place where it comes near or in contact with the frame.

Thanks guys appreciate all the comments.

Arthur

[lal Naja]

Originally Posted by VintageUpgrades

I am referring to fusing the main power cable feeding the car's accessories; ususally connected at the battery or starter solenoid. It's purpose is to blow in case something pinches or cuts this wire.

Thanks for the confirmation! Can you suggest a source where I may research such a BAF, that's the acronym for the 'big ass fuse' that has been suggested.

Thanks!

Arthur

[Excaliber]

Here's a link to circuit breakers virtually identical to what ERA uses.

http://www.wiringproducts.com/contents/en-us/d60.html

It mounts on the firewall, right next to the starter solenoid. A reasonably heavy wire attaches to the same post on the solenoid that the main battery + cable attaches to. The other end of that "feed" wire goes to one side on the circuit breaker. ALL power to anything (but the starter) passes through this circuit breaker. Add up your worst case scenario for a amp load to the car and choose a circuit breaker that is equal to or greater than that amount (far less than 300).

As you have a Chebby engine your solenoid is likely mounted on the starter. This will require a much longer wire and subseguently a much heavier wire due to the length to reach from the starter + plus battery terminal connection to the newly mounted circuit breaker. Ford really did have a better idea in that regard,,,

It is all but impossible to provide a protection circuit for the starter, any starter. It simply draws to much power, it NEEDS to draw massive power. Enough power to "burn up the starter" if that what it takes.

So what happens if there is a "short" to ground by the battery plus cable? The damage will be limited STRICTLY to the + cable and the battery. No other systems will be damaged (generally speaking). Where and how the + cable makes contact with the "ground" will determine the amount of potential amp draw from the battery. The + cable MAY get "red hot", melt the insulation, discharge the battery to the point of it exploding. The cable will either burn through, or the battery will blow up first. It is highly unlikely you could achieve a strong enough "short" for the cable to carry any significant amperage in the first place.

Such a scenario might be a "wreck", where the battery comes out of it's holding clamps. The + cable may come in contact with the headers which would melt the thick insulation. That could provide a pretty good discharge rate, all though, that may be the least of your problems if your in a wreck that bad.

Let's say the insulation "rubs through" on the body, wears away the insulation. Thats a piss poor connection, it won't draw big amps, less than 50 I'd bet! A 50 amp circuit breaker, for protection against this scenario will not be big enough to run the starter however! You would need a 300 amp for that, which won't keep the car from burning down. A typical "short" will draw far less than 300 amps, but enough to possibly blow up the battery or catch the cable on fire. In such a case the only thing you could say is, "Life sucks sometimes".

[Ronbo]

Had a "mishap" many years ago where I was changing out a solenoid on my Chevelle. After re-mounting the header I failed to notice the battery cable touching the header tube.

All was well until the headers heated up and burned through the insulation. I heard a hissing sound followed by lots of white smoke. After scrambling to get a 5/16 wench to undo the cable, I cranked on the neg side terminal (which was very hot at the time) and the whole thing pulled out of the battery. (melted the casing)

Casualties were the battery and both cables insulation fried off. And of course the obligatory ribbing from my buddies.

I pay better attention now though.

As far as exploding batteries, in the case of lead acid types, over charging and fire are more likely to cause an explosion than rapid discharging (Granted rapid discharging can certainly make other stuff explode / catch fire).

We had a kid that worked at the paging company I was an engineer for who had hydraulics installed on his POS Geo. One night the hydraulic system shorted out and lit the interior off. The batteries then exploded one by one in the resulting inferno. None of them exploded from the short.

[lal Naja]

Thanks for the link to the BAF & the enlightenment.

Arthur

[Wayne Maybury]

The battery in my Cobra is in the trunk, right behind the passenger seat. I have a cutoff switch mounted on the bulkhead just behind that seat. The "+" lead between the battery and the switch is very short. I can easily turn the switch off if anything were to go wrong. In addition, I always turn it off when I am not using the car so it acts as an anti-theft device.

I have 2 big circuit breakers on my sailboat, one for the windlass and the other for the inverter. I have never had any problems with either of them.

Wayne

[patrickt]

Originally Posted by Ronbo

A lot of factors effect current draw. Number of cylinders plus compression ratio being the big factors.

Also since it's an inductive load, in-rush current will be quite high (400~600A in the case of a high compression, large displacement V8)...

Originally Posted by Excaliber

I'm calling BS on a 200 amp starter draw on a 2 litre car, even when cold..

OK, let's keep everyone honest. Since I have nothing better to do, I hooked up my ammeter to the starter circuit and took current readings. I have the old style, big-ass ACDelco OEM starter. It drew 210 amps, while cranking, on my decently charged battery. But since my ammeter can also capture the peak in-rush current, I checked that as well. And as they say, read 'em and weep:

[Excaliber]

So, your confessing that your motor is really a 2 litre?