fuses vs. breakers for DC engine wiring applications

[kwalters]

Greetings,
I was planning on adding a 40AMP ATC style fuse to the red wire going from the starter solenoid to the BATT post of the Ignition switch (after reviewing the DC power schematic and locating the 8AWG fuse holder for sale at this site).

Is there any reason I should not just use a push button circuit breaker for this application? It appears the breaker would be cheaper initially and there would be no fuses to keep around. I am guessing there is a reason fuses are commonly used/recommended, but am wondering what the reason might be.
Ken

[ArtJ]

RE fuses

A few years ago I added a fuse to my starter circuit. I believe it was at least
a 70 amp fuse, possibly as much as 100A.

Not sure if the breaker would
be appropriate or not in this application, like you, I have only seen
fuses recommended, possibly because it would necessitate checking
the starter (as opposed to a quick reset) and maybe avert increased
damage. Also I think most fuses are time related and the relay may not
necessarily be. I wonder if relay would be more or less prone to sparks,
given that we have gasoline engines.

By the way, I had a starter partiallly shot out this fall, the only symptom
was the ammeter going t -30amps and the fuse did not blow. Possibly
I should go with a smaller fuse, but initially starter current could spike high whichmay be the reason for the higher amperage fuse. I think it was selected
from a table and recommendation in the West Marine catalog.

Above are just some thoughts, not recommendations one way or the other.

[ndutton]

An important factor is how accurate you want the over-amperage circuit interruption to be. I've seen thermal circuit breakers fresh off the shelf hold loads 25% over their rating for prolonged periods of time. Magnetic breakers may be more accurate but I don't have enough troubleshooting experience with them to say for certain.

Fuses come in a variety of time-delay values as well with the number of internal elements being the controlling factor. The ATC type you're considering is a single element fuse (I believe) and therefore should give you the most precise interruption point.

[Raymond]

I have also decided that it is time to add overload protection to the starting circuit. West marine has a fuse that attaches directly to the battery post - no wire between the battery and the fuse. How does one properly size the fuse? How much does the starter draw?

Raymond

[rigspelt]

How much does the starter draw?

Careful to buy circuit protectors that are ignition-protected if they are used in the engine space around fuel. (I had to return some after special ordering).

Re your question about starter draw: The rule of thumb is that the A4 starter usually draws considerable less than 200 amps, maybe on the order of 135 amps. I think MMI recommends a 200 amp fuse. See

http://www.moyermarine.com/forums/showthread.php?t=1044

http://www.moyermarine.com/forums/showthread.php?t=2722.

Fuses are simpler devices. I had to chase a gremlin once: a wise friend said to swap out the expensive circuit breaker, and that indeed was the problem. That was a few years ago though -- technology is changing. http://bluesea.com/files/resources/r...t_Breakers.pdf

Very helpful: http://beta.circuitwizard.bluesea.com/

[kwalters]

I think I will stick with fuses as seems to be recommended by Don based on the drawings and parts available at MMI. Just to be clear though, I am fusing the wire between the starter solenoid and the BATT terminal on the ignition switch, not the starter cable from the battery to the starter. I had not really thought about that one, but probably not a bad idea.
-- Ken

[rigspelt]

Just to be clear though, I am fusing the wire between the starter solenoid and the BATT terminal on the ignition switch,

I see, you mean the starter solenoid draw, not the starter motor. I had a heck of a time trying to pin down that number in my researching last year when I was rewiring. The little solenoid has a short spike draw, not continuous. I ended up thinking that a 20 amp fuse was OK -- hopefully got that right.
Update: on 12 g wire dedicated to only the starter solenoid.

[roadnsky]

I ended up thinking that a 20 amp fuse was OK -- hopefully got that right.

Don's drawing recommends a 40 amp...

[kwalters]

I see, you mean the starter solenoid draw, not the starter motor. I had a heck of a time trying to pin down that number in my researching last year when I was rewiring. The little solenoid has a short spike draw, not continuous. I ended up thinking that a 20 amp fuse was OK -- hopefully got that right.

On my boat at least this feed that Don suggests be protected by a 40AMP fuse provides all ignition switched DC power to boat via the engine control panel – fuel pump, coil, blower, bilge, AND the starter solenoid. It is not the yellow starter solenoid wire “ST” from the engine control panel which at least in the MMI drawing has no fuse of its own (though it would not hurt anything to have one).
There are 3 wires to the starter - one from the battery carrying cranking current to the starter, one that goes on the same large post as the cranking current cable, the wire in question, which feeds the BATT terminal on the ignition switch, and the wire from the start button to the “S” terminal on the starter solenoid. This small gauge wire (12) to the "s" terminal carries very little in the way of amps by design. Probably just a couple at most and a 20 amp fuse is offering little or no protection from fire - 20 amps is a lot of juice to put through a 12 awg wire, no?

[ndutton]

Well, no.

12 gauge wire is rated for 20 amps so no worries. The intent of the fuse is to protect the wiring downstream so for instance, putting a 40 amp fuse on a 12 gauge circuit does not protect the wire at its rated capacity.

[kwalters]

OK, thanks for clarifying that. Probably why I am not an electrician! So putting a fuse on the yellow wire going to "S" from the "ST" button would protect the solenoid, right? What are your thoughts on fusing the solenoid and the size of the fuse to protect the circuit?
--Ken

[ndutton]

OK, you asked for it.

First, the disclaimer. Regarding the proper size fuse to protect any of our engine systems, I’m not going there. This is a subject for Don Moyer or others on this forum that know a lot more about it than me. Further, the following comments are from a pure electrical point of view and do not necessarily mesh with ABYC standards.

If we assume the engine related wiring is professionally engineered (i.e., from the factory), we can make certain inferences from the wire sizes. Smaller wire means smaller current carrying ability for circuits of smaller loads. In your example, the starter solenoid circuit is probably wired with 16 gauge wire. Checking a wire ampacity chart (readily available online) shows the maximum ampacity for 16 gauge wire is 10 amps. Therefore the maximum allowable fuse is 10 amps. A smaller fuse is acceptable provided it meets the solenoid’s needs and current characteristics (initial spike or whatever) but a larger fuse is not. If a larger fuse is desired you’ll need to replace the wire with a suitable gauge. In an example of combined circuits, fusing the engine control supply at 40 amps, even if the supply wire is of sufficient gauge, does not protect the smaller wires connected to it downstream. If you have a short on your 16 gauge solenoid circuit and the only fuse is a 40 amp at the ign source, that little 16 ga wire will get 4 times its rating before the fuse blows. The proper way to go is to add smaller fuses ahead of smaller wires.

Fudge factors:
Voltage drop due to the internal resistance of the wire is affected by wire gauge, circuit load and wire length, the latter being the common determinant. Calculated voltage drop should not exceed 3%. In my experience this is a non-issue in our boats because due to limited boat size (how big a boat can an A4 push?) our circuits don’t approach the wire lengths that produce a drop. In addition, good practice dictates that we do not load our circuits beyond 80% of their maximum ampacity. By following this practice, you’ll never have wiring or fuse problems designed in.

In the solenoid circuit example, this would mean the circuit wired with 16 gauge wire (assumed) and properly fused at a maximum of 10 amps carries a maximum load of 8 amps (80% of 10 amps). As another example, if the designed load on a circuit like cabin lighting was 18 amps, I would recommend wiring the circuit with 10 gauge wire (30 amp rating) rather than 12 gauge (20 amp rating) to achieve the 80% load factor (12 gauge: 80% x 20 amps = 16 amps, insufficient for the 18 amp design load) and still fuse it at 20 amps.

Ok, I’ve blathered on longer than I intended. Hope this helps.