Fuse location in IGN to Coil + wire

[kwalters]

Greetings,
After my FACET fuel pump burned up the other day I am in the process of mapping out the current state of my DC power wiring. After reviewing all the great information at the MMI site and discussion groups I can see that my wiring is not exactly standard. Things thought are working and I plan on fixing things bit by bit as I understand things better with an eye initially on proper fusing and avoiding fires.

One easy upgrade I see would be the addition of the 20 Amp fuse in what is supposed to be a 12 AWG purple wire from the IGN post on the ignition switch and the positive post on the coil. My first question has to do with the placement of this fuse holder. Unfortunately, this wire is not purple and does not run back to the ignition switch on my boat. Instead it runs a few feet to a buss bar that feeds three or four other circuits (the one I have traced out so far is the fuel pump, which is fused but with at 5 amp instead of the old 10 amp). Is there any reason I cannot simply place the fuse at the end of the wire where it attaches to the positive post on the coil? From the info at this site I gather the fuse is designed to protect everything else from a short in the primary side of the engine so to my mind this location should be fine.

I have the electronic ignition upgrade on the system with wires running from the distributor to both the positive and negative side of the coil. Also I have a mystery wire running from the positive post of the coil to the back of the alternator (not sure which post yet). The odd thing about this wire is that it starts at the coil at what looks like 12 AWG but within a foot or two drops down via a butt connector to a yellow wire what looks to be 16 or even 18 AWG! I can’t believe that this is what is supposed to be the 8 AWG line back to the IGN post carrying charging current. I need to examine the back of the alternator and confirm what other if any connections there are.

Ken

[roadnsky]

Instead it runs a few feet to a buss bar that feeds three or four other circuits (the one I have traced out so far is the fuel pump, which is fused but with at 5 amp instead of the old 10 amp).

The fuse should be a 5A instead of a 10A. Here's a previous thread posted by Don about this issue...

http://www.moyermarine.com/forums/showthread.php?t=1044&highlight=ignition+shorting

I have the electronic ignition upgrade on the system with wires running from the distributor to both the positive and negative side of the coil. Also I have a mystery wire running from the positive post of the coil to the back of the alternator (not sure which post yet). The odd thing about this wire is that it starts at the coil at what looks like 12 AWG but within a foot or two drops down via a butt connector to a yellow wire what looks to be 16 or even 18 AWG! I can’t believe that this is what is supposed to be the 8 AWG line back to the IGN post carrying charging current.

That mystery wire is the "Exciter" wire. It's so the alternator will be "excited" when the ignition switch is turned on.
It sounds like a repair job using a larger AWG wire for whatever reason. Normal size is 14.
Do you have the wiring diagram from Don? (I'm attaching it here)

[kwalters]

The fuse should be a 5A instead of a 10A. Here's a previous thread posted by Don about this issue...

http://www.moyermarine.com/forums/showthread.php?t=1044&highlight=ignition+shorting

That mystery wire is the "Exciter" wire. It's so the alternator will be "excited" when the ignition switch is turned on.
It sounds like a repair job using a larger AWG wire for whatever reason. Normal size is 14.
Do you have the wiring diagram from Don? (I'm attaching it here)

I have Don's drawing, thanks. So the Exciter wire should be 14 AWG and do you know if it goes to its own special post on the alternator? Also, what are your feelings on fuse location?

[roadnsky]

So the Exciter wire should be 14 AWG and do you know if it goes to its own special post on the alternator?

It does go to it's own post either on the ALT or sometimes it can come from the Voltage Regulator depending on your model.
I'm attaching the wiring instructions that MMI sends out for the 55A model they sell. It shows the EXCITER TERMINAL.
Do you know what Alternator you have? Got a pic?

Also, what are your feelings on fuse location?

If I understand you correctly, you're on the right track here...
"Is there any reason I cannot simply place the fuse at the end of the wire where it attaches to the positive post on the coil? From the info at this site I gather the fuse is designed to protect everything else from a short in the primary side of the engine so to my mind this location should be fine."
The object is to put protection between a possible short and further damage downstream.
Others with a higher degree of electrical wiring knowledge will eventually chime in here and I'd suggest going with their advice...
(Steve M, Rigsy? You guys out there?)

PS - If you really want to get your head spinning read this sometime when you're really bored... http://www.moyermarine.com/forums/sh...ernator+wiring

[roadnsky]

Just for more reference...
Here's a pic of my old 35A Motorola alternator.
Note the YELLOW 14AWG wire coming from the Voltage Regulator...

[High Hopes]

Hi Ken,

I suspect the yellow wire is the exciter connection which draws very little current and it only does this when the engine is starting. Your "small" wire is fine.

As far as fusing goes, add fuses close to the source - on the battery wire that goes to the ignition switch is a good place. Remember that everything downstream from the fuse is protected. Nothing upstream from the fuse is. So wires leading to fuses are not protected. Wires can cause as many shorts as the things they power. I would make sure the entire ignition circuit is fused already - I suspect it is.

I use some additional fuses, but not to protect each device. One big fuse really protects all things quite well. Rather, I added fuses to facilitate troubleshooting. If I only have one fuse and it blows, I don't know where the problem is. If I have a fuse for the blower, another for the cockpit gauges, and another for ignition, then I have less things to weed through if one of those fuses blows.

Also, if a gauge lamp shorts or if a wire to the blower shorts, the engine still runs. So there is a benefit from having some things on another circuit.

I hope this helps. - SM

[Don Moyer]

Very useful and well stated Steve, thank you.

Don

[rigspelt]

The "source"

add fuses close to the source

I was going to make this point too, but I've been held back in my amateur mind thinking about what "source" means. AYBC E-11 2008: "11.10.1.1.1 Overcurrent Protection Device Location - Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point at which the conductor is connected to the source of power measured along the conductor" (with some exceptions). Their accompanying diagram shows various scenarios, but does not label the battery terminal that they consider to be the "source" (no plus or minus signs).

What twists me up is that while we think of the red wires attached to positive battery terminals as being "positive" and "power", as if the electricity flows from positive to negative, it actually flows from negative to positive (Calder page 121 3rd edition). Source in that sense, it seems to me, would be the negative end of a red wire, furthest from the positive post of the battery terminal. But, that's not what we should think of as "source" for fuse protection.

Nigel Calder (Boatowner's Mechanical and Electrical Manual 3rd editiion) clarifies that a fuse goes on the end of the positive wires closest to the positive post of the battery, and protects the circuit beyond that point, further down the positive wires away from the battery and toward the appliance being supplied with electricity (see figures on pages 184 and 187). See page 188 in his book for a text discussion. Charlie Wing's book concurs (pages 92 and 103, Boatowner's Illustrated Electrical Handbook 2nd edition).

While I still can't get my head around the physics of why the wires running from the positive post are considered downstream and protect circuits further along, there it is. Calder (p 121), writing about DC circuits and using the usual convention we all now work with: "Hot (positive) wires carry current through switches to all lights, instruments, etc; these are then grounded...".

So, I put my fuses and breakers in the positive wires closest to where they connect to the wires that are connected to the positive terminal of the battery (their source). I hope I got that right.

[High Hopes]

Sounds right to me Rigsy. Wow, lots of reading there.

Conventional current flow is always from positive to negative just because we say so - we need a convention in doing the math and when we connect things together. Because electrons are negative, we use a minus sign when describing their charge. So charge sign is different from current sign. Because the charge carriers in electricity are negative, the charge carriers flow in the opposite direction from conventional current. If electrons were positive charges, they would flow from + to - .

I was taught that Ben Franklin named the polarities the way he did because lightning comes down from the sky? But when physicists learned more, we realized that the charge carries in electric circuits were actually negative bits of charge. But they still kept the conventional, and now established, polarity names.

So you can think of electrons sort of waiting around for someplace to go. You can fuse the positive terminal to limit the current pull capacity there or you can fuse the abundance of electrons at the negative terminal and limit the outflow push from there. It doesn't matter, except for convention, because there is no charge flow unless you have a connection. So no electrons leak out and no power can be draw without complete circuit.

Just to mess with your mind a little more, all telephone central offices use Red and Black color coding for their wiring, however, Red is a negative voltage (-48V), but Black is still the ground.

Best,

Steve

[kwalters]

Thanks everyone! I now have a good understanding of what needs to be done. What a fantastic resource this is.

[roadnsky]

Man, I just knew if we threw out some bait we'd get those two (Steve & Rigs) going again...

[High Hopes]

I can't help it. I love this stuff.

[roadnsky]

I can't help it. I love this stuff.

We do too...

[rigspelt]

Remember that everything downstream from the fuse is protected. Nothing upstream from the fuse is. So wires leading to fuses are not protected. Wires can cause as many shorts as the things they power.

Which is why, as wires get smaller in branching circuits downstream of the bigger wire/bigger fuse, they may also need appropriately smaller fuses, to protect the smaller wire in the branch. That's why the electric fuel pump branch wire has a fuse in it, for example.