Recommended Breaker Size for Ignition Switch ?

[btrayfors]

Originally Posted by sdowney717

IF you fuse the starting motor circuit.

What happens if the starter motor lugs,
The oil is really cold and the motor really stiff.
Battery voltage is low and engine is cranking slowly.
The engine hydrolocks or cant spin at all for whatever reason.

Will that fuse blow if those things occur?
I know that if the starter motor is under great load, the current flow goes way up. I can imagine a nuisance if the fuse blows.
I have actually fuel hydrolocked gas motors couple of times.

All good and valid points.

I don't know the answers definitively, but would offer the following.

1. If you're going to use a fuse, size it pretty large (mine is 400A for a Perkins 4-108) and use a fuse which can take quite a bit more current than its rating for quite some time before blowing (like an ANL).

2. In over five years of operation since I installed mine, I've done numerous cold starts without problem. I've tried to start with low voltage (failing start battery -- slow cranking). No problem. I've tried to start with a nearly dead start battery. No problem.

3. I don't know what the LRA rating is for my starter motor, but believe you're right: if it were truly locked it would draw a heck of a lot. Ditto if the engine itself were hydrolocked or otherwise impaired from cranking.

My believe is that IF you keep your engine in decent condition, including the starting circuitry, connections, etc.....then there should be no problem blowing fuses in the start circuit.

Also, if you press the starter button and the engine doesn't turn over as it should you should note immediately that something is wrong, and should investigate quickly. Holding down the starter button with a locked-up engine isn't a good idea anyway and, most probably, would be likely to draw a tremendous amount of current.

In such a case, you'd have to ask yourself if the blown fuse wasn't a good thing to keep you from further damaging the starter, solenoid or engine itself :-)

Bill

[goboatingnow]

Can I just clear up an issue. Fusing to the size of the line is just a " practice". Fusing plays all sorts of roles. It quite acceptable to fuse to the appliance or the sum of the appliances. ( once all are within the line capacity )

Where I have certain devices that I want to target protection I regularly size the circuit breaker to the load rather then the wire ( which could be 10 times higher ) I have lots of 5 amp breakers on 20 amp wire.

Dave

[FlyingCloud1937]

Size all cable to the potential load.

load = Volt Amps

load = ampere

load=Start-up

load= V-drop


Fuse to Protect the Cable

Over Current Fault Protection is not the same as CABLE protection, Load protection is not Cable or fault protection.




Lloyd

[sdowney717]

A dead short will blow a breaker if the wire can carry enough current. Otherwise, the wire becomes a heater and can get hot enough to burn.

Also a wire can be undersized enough that the load or a partial short can overheat the wire without blowing the breaker. How hot that gets depends on what load the wire feeds. The load can also have a partial failure, so it might then overheat itself and the wire and not blow the breaker.

One time I had a 25 foot wire run of 8 gauge wire to a pump which I accidently shorted the ends together. The copper ends welded, the wire got red hot and burned the insulation, made a lot of smoke and since it was not breakered, I had to turn off the main battery switch. Bad enough that I was there and the cause. That wire now has a 40 amp breaker.

[Allan S]

My 9.9 Johnson Evinrude Yaghtwin has no breaker, so I installed a 30 amper. Kept blowing it so now its bypassed.

[FlyingCloud1937]

Heat isn't only a DC issue. I re-wired a 1996 48 ft Tolly Craft. The factory AC was attempting to burn the boat down.

Tolly used a poor choice for the AC breakers in the panel. The fault of these breaker is the push on connector instead of screw terminals.

These were 15 amp breakers, but the UL rating is for sustained ampere is 7 amps.

Breakers don't protect against heat.

Lloyd

[MarkSF]

Originally Posted by Target9000

It was my understanding that you fuse to the wire, not to the devices being fed by the wire. I've read that the real purpose of the fuse is to protect the wire from overheating and catching fire/melting. The wire is what should be correctly sized to the things it feeds. So wouldn't the more appropriate thing to do be to size the wires to these devices correctly and fuse the wires according to the very standardized tables that show the acceptable fuse ratings?

I'm new to 12v DC, so go easy if I'm misguided here.

The info I have is that the breaker protects the wiring, and the fuse protects the device. Good practice is wire guage matched to the breaker, eg 16 gauge with a 15 amp breaker, or thicker with a bigger breaker, and so on. Then a smaller inline fuse may be used with stereo, radio, etc, which is placed close to the device.

This is all per Don Casey's Sailboat Maintenance book, which goes into plenty of detail. Worth buying if you don't have it!

[MarkSF]

Originally Posted by goboatingnow

Can I just clear up an issue. Fusing to the size of the line is just a " practice". Fusing plays all sorts of roles. It quite acceptable to fuse to the appliance or the sum of the appliances. ( once all are within the line capacity )

Where I have certain devices that I want to target protection I regularly size the circuit breaker to the load rather then the wire ( which could be 10 times higher ) I have lots of 5 amp breakers on 20 amp wire.

Dave

Yes, it's hard to imagine damage occurring due to putting a SMALLER breaker in. But a breaker rated higher that the wiring IS dangerous.

[evm1024]

As noted the breaker should trip at a current less than the rating of the wire to keep the wire from starting a fire. Having a much smaller breaker than the rating of the wire is not a problem and could be done to minimize the voltage drop.

I recently had a starter motor lock up on a perkins 4-236. I am rewiring the boat as part of the restoration and had not gotten to the starter cables. In any case the stall current was able to melt the insulation in the #2 (not 2/0) starter ground cable and fill the engine room with smoke. Because I checked what was going on rather than keep pushing the starter button I avoided a fire.

The up shot is that I now have finished the starter circuit rewire with 2/0 and there is a 250 amp ANL within 7" of the starter battery. So far I have not popped the ANL and actually I don't expect to blow this fuse as that it should pass 750 amps for around 8 seconds before blowing.

Here is the info on ANL: Detailed Specifications for ANL Fuses*—*Blue Sea Systems

I should add that the Perkins starts in less than a second so far in all conditions.

regards< Ethan

[btrayfors]

Originally Posted by evm1024

As noted the breaker should trip at a current less than the rating of the wire to keep the wire from starting a fire. Having a much smaller breaker than the rating of the wire is not a problem and could be done to minimize the voltage drop.

I recently had a starter motor lock up on a perkins 4-236. I am rewiring the boat as part of the restoration and had not gotten to the starter cables. In any case the stall current was able to melt the insulation in the #2 (not 2/0) starter ground cable and fill the engine room with smoke. Because I checked what was going on rather than keep pushing the starter button I avoided a fire.

The up shot is that I now have finished the starter circuit rewire with 2/0 and there is a 250 amp ANL within 7" of the starter battery. So far I have not popped the ANL and actually I don't expect to blow this fuse as that it should pass 750 amps for around 8 seconds before blowing.

Here is the info on ANL: Detailed Specifications for ANL Fuses*&mdash;*Blue Sea Systems

I should add that the Perkins starts in less than a second so far in all conditions.

regards< Ethan

Ethan,

I agree with your reasoning re: smaller breakers.

And, with the use of ANL fuses in the start circuit. However, I believe that a 250A ANL for a Perkins 4-236 is cutting it too close. I have a 400A ANL in my start circuit for a Perkins 4-108 and it's worked well for over 5 years. However, when I first put one it -- I can't remember the exact size...maybe a 200 or 250....I managed to blow it during testing, so went with the 400A.

BTW, my 4-108 normally starts fast, too. Barely a couple of turns before it fires off.

Further to this thought: the Perkins starters are about 2KW I believe, or 1500 watt draw or about 120A when running. LRA (locked rotor amperage or startup amperage) is much higher, perhaps as high as 1,000 amps for a millisecond, then tapering back to 400-500A for a few seconds, then down to 150A or less. I haven't had the chance to do the actual measurements yet, but hope to get to it this year. Still, my reading and my experience blowing the lesser ANL fuse leads me to believe that these figures are at least ballpark.

FWIW,

Bill

[evm1024]

Hi,

It was interesting that the 250 worked. I was expecting it to fail. I plan on replacing it at some point soon. I did get a small stock of 250s for $4 each and have not spent the $ for a larger breaker.

I was thinking a 350 or perhaps 400 for long term use. Sorry if it implied that I was recommending a 250 in the starter circuit. Just showing that starters are intermittent loads and that you do not need to size the fuse for continuous loads.

Regards, Ethan

[goboatingnow]

Originally Posted by MarkSF

The info I have is that the breaker protects the wiring, and the fuse protects the device. Good practice is wire guage matched to the breaker, eg 16 gauge with a 15 amp breaker, or thicker with a bigger breaker, and so on. Then a smaller inline fuse may be used with stereo, radio, etc, which is placed close to the device.

This is all per Don Casey's Sailboat Maintenance book, which goes into plenty of detail. Worth buying if you don't have it!

No circuit breakers are designed for device protection as well. They can easily replace equipment fuses ( see E.T.A) various types of circuit breakers have particular characteristics that suit device protection. Fuses are used because they are cheap.

I replaced all my inline fuses with thermal CBs.

Yes of course you never use breakers rated above the safe current carrying capacity of the line. Thats just common sense.

Dave

[sdowney717]

here is a self resetting breaker goes into a fuse holder.
Littelfuse CBF10 Circuit Breaker, Fuse Clip Series, 10A, 12V

I like these breakers. I put some of these into my old style lectrasan control box.

With fuses your just wasting money. I have blown plenty when trouble shooting electrical stuff and they are a nuisance and breakers are sweet.