Fusing my ignition circuit.

[TomY]

Hi Maine. A couple seasons ago, a poorly bundled 12 gauge wire came loose above my engine and hit the only hot spot on the exhaust. The insulation melted, and we had a dead short.

The wires insulation ignited and burned from the alternator back to the engine panel. Seeing smoke, I turned the ignition switch off which stopped the burning. We were lucky as the wire caught nothing else on fire.

This season I had a bad connection in the ignition circuit and replaced some wiring(my boat is 51 years old). In doing that, I decided to install an inline fuse to attempt protecting the wires in the ignition circuit.

It's a pretty simple circuit; Short cable from starting battery to ON/OFF switch. Short cable to starter terminal. 10 gauge wire on same starter terminal goes to glow plug solenoid, then to ignition panel Amp gauge. Then the 10 ga. goes out of the Amp meter to a post on the dual output alternator. Off the inlet to the Amp gauge, a 12 gauge wire feeds the ignition panel guages, switches, via the keyed ignition switch.

I cut an inline fuse holder into the 10 gauge line just after the starter post and used a 30 amp fuse. That worked fine until I hit the glow plug solenoid. In about 5 seconds, the load from the glow plugs blows the 30 amp. So I took the inline fuse holder out and am now back to square one. But all the wires are safely bundled. Still, I would like to protect those wires after that experience.

I know about the battery terminal fuses now available and will install one(I'm not worried about the battery cables, short and enclosed in safe spaces. But I don't see how a 300 amp fuse is going to protect that 10 gauge wire which I presume was installed for the higher amperage of the glow plugs and alternator.

Then the remaining circuits off the ignition panel are all 12 gauge, including the wire(power to alternator from a switch in my panel) that once caused the fire due to a dead short.

I can't see the 300 amp fuse protected those down stream wires. Maybe it will?

Or is the only solution to fuse each wire size as it steps down? It seems counter intuitive(I have been wiring houses for decades as a design builder)to be over sizing fuses to wire size, especially if the glow plugs amperage exceeds the 30 amps a 10 gauge wire is safely rated for.

Thanks.

 

[Maine Sail]

Hi Maine. A couple seasons ago, a poorly bundled 12 gauge wire came loose above my engine and hit the only hot spot on the exhaust. The insulation melted, and we had a dead short.

The wires insulation ignited and burned from the alternator back to the engine panel. Seeing smoke, I turned the ignition switch off which stopped the burning. We were lucky as the wire caught nothing else on fire.

This season I had a bad connection in the ignition circuit and replaced some wiring(my boat is 51 years old). In doing that, I decided to install an inline fuse to attempt protecting the wires in the ignition circuit.

Ouch! Lucky you did not lose Christmas!!

It's a pretty simple circuit; Short cable from starting battery to ON/OFF switch. Short cable to starter terminal. 10 gauge wire on same starter terminal goes to glow plug solenoid, then to ignition panel Amp gauge. Then the 10 ga. goes out of the Amp meter to a post on the dual output alternator. Off the inlet to the Amp gauge, a 12 gauge wire feeds the ignition panel guages, switches, via the keyed ignition switch.

Simple is good but any time you have a new size wire you need to protect that wire starting at its "source". So the 10GA wire to the solenoid should have an in-line fuse right after the starter post. The start battery circuit should have a fuse ideally within 7" of the battery to protect the starter wire. If you tell me the gauge we can size a fuse for your diesel.

GET RID OF THE AMMETER CIRCUIT if your alt output runs through it before it hits the batteries! The easy solution is to replace the ammeter gauge with a simple volt meter. Then all you do is jump the alt B+/output over to the starter post with a 6GA jumper or better yet run the alt directly to the HOUSE battery bank and fuse that wire within 7" of the battery bank.

If you tell me more about your charging circuit we can probably get you more alternator output and a safer system too.

I cut an inline fuse holder into the 10 gauge line just after the starter post and used a 30 amp fuse. That worked fine until I hit the glow plug solenoid. In about 5 seconds, the load from the glow plugs blows the 30 amp. So I took the inline fuse holder out and am now back to square one. But all the wires are safely bundled. Still, I would like to protect those wires after that experience.

Good thought process but many of these glow plug circuits can draw 40+A. A 10 GA wire in an engine space can take a 50A fuse @ 100%. See the table below for max ampacity.

If you blow a 50A fuse you may need to go to 8GA or 6GA wire. Remember the fuse is only there to protect the wire for catastrophic faults. You can also go to 150% of the table below but I usually try to size for 100% ampacity and generally prefer to up the wire rather than up the fuse beyond 100%.

I know about the battery terminal fuses now available and will install one(I'm not worried about the battery cables, short and enclosed in safe spaces. But I don't see how a 300 amp fuse is going to protect that 10 gauge wire which I presume was installed for the higher amperage of the glow plugs and alternator.

A 300A fuse should not be used to protect 10GA wire.

Then the remaining circuits off the ignition panel are all 12 gauge, including the wire(power to alternator from a switch in my panel) that once caused the fire due to a dead short.

Two words, more "WIRE TIES"

Everything should be fused as close to its "source", which is always the battery bank or in the case of the starter post where the wire jumps down to a smaller size from a larger already protected wire.

I can't see the 300 amp fuse protected those down stream wires. Maybe it will?

A 10GA wire in a dead short "may" pop the fuse but not likely before the jacket has severely melted or possibly ignited. Be sure to always use 105C rated wire. A lot of the wire in older boats was 60C or 75C rated.. It ignites at lower temps...

Or is the only solution to fuse each wire size as it steps down?

See you already knew the answer..

It seems counter intuitive(I have been wiring houses for decades as a design builder)to be over sizing fuses to wire size, especially if the glow plugs amperage exceeds the 30 amps a 10 gauge wire is safely rated for.

There is a differnce between max "continuous" current and max short circuit current or max ampacity for sizing over current protection. A 10GA wire outside an engine space, not bundled with other wires, can technically, under ABYC E-11, be fused at 150% of 60A which is a 90A fuse, or, inside an engine space at 50A @ 100% or 75A to hit 150%... In a dead short with over current protection the duration is not long enough to melt the wire and this is why max ampacity is considerably higher than when sizing wire for continuous current loads and voltage drop. 30A continuous vs. up to 90A max ampacity...

You simply need a larger fuse or heavy duty breaker in the glow circuit.. Going to 6GA wire can make a world of difference in the speed of the glow....

Slap a DC clamp meter around your glow circuit wire and measure it then you'll know....

 

[TomY]

Thanks. For now, I'm going to re-check my bundling and add more ties if needed. Once the season starts, I don't do too much unless it's broken.

I'll start looking for an inline fuse holder and 50A fuse. The standard type(I had a few) have lighter wire(12 or 14 ga.) so I guess a trip to Hamilton is needed. I do want to fuse the glow plug- alternator circuit, and then fuse the 12 ga. wiring at the ignition panel where it starts(I can add a fuse holder to the panel) And I'll protect the battery cable with a terminal fuse, I don't know the size but that switch also has cables to windlass panel, and it would be nice to protect the cables leading to the windlass breaker(which covers the cables to the bow) Then I'll have all the wires protected.

I see what you mean by shortening the alternator and battery and going to larger wire. But the Amp gauge is part of the old panel and I'd like to keep it(I'm used to reading it). Can you tell me off hand how you would wire that instead of it being inline between alternator and battery?

I have a plan now, thanks again Maine.

 

[Maine Sail]

I see what you mean by shortening the alternator and battery and going to larger wire. But the Amp gauge is part of the old panel and I'd like to keep it(I'm used to reading it). Can you tell me off hand how you would wire that instead of it being inline between alternator and battery?

I have a plan now, thanks again Maine.

I'd use a real battery monitor with a real shunt. If I showed you a pic of the actual shunt in one of those cheap dash mounted ammeters, and a 500A battery monitor shunt, you'd probably shart yourself.... When I get home I'll throw some pics up....

 

[Stu Jackson]

I see what you mean by shortening the alternator and battery and going to larger wire. But the Amp gauge is part of the old panel and I'd like to keep it(I'm used to reading it). Can you tell me off hand how you would wire that instead of it being inline between alternator and battery?

I have a plan now, thanks again Maine.

Reading the ammeter is no different than reading a voltmeter. The purpose of checking the ammeter is to assure that the current is flowing, i.e., the alternator is working. You do the same reading the voltmeter, since the voltage will go up to 14+V when the alternator starts.

One of the important issues discussed in our untiring efforts to dissuade skippers from using ammeters in their cockpit panels is included in all the discussions about wiring harnesses. Some are discussed here and if you poke around on that topic, you see links to Maine Sail's excellent corroborating article. http://c34.org/bbs/index.php/topic,5078.30.html

I caution you to implement MS's excellent advice. Running an alternator output charging wire all the way to cockpit and back to your batteries, however close they may be, creates the very fusing issues you mention, just because of all the different wire sizes.

Here are some other corroborating wiring methods that may help you implement MS's suggestions:

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: http://c34.org/bbs/index.php/topic,6604.0.html

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowners.com/showthread.php?t=137615

 

[TomY]

Reading the ammeter is no different than reading a voltmeter. The purpose of checking the ammeter is to assure that the current is flowing, i.e., the alternator is working. You do the same reading the voltmeter, since the voltage will go up to 14+V when the alternator starts.

One of the important issues discussed in our untiring efforts to dissuade skippers from using ammeters in their cockpit panels is included in all the discussions about wiring harnesses. Some are discussed here and if you poke around on that topic, you see links to Maine Sail's excellent corroborating article. http://c34.org/bbs/index.php/topic,5078.30.html

I caution you to implement MS's excellent advice. Running an alternator output charging wire all the way to cockpit and back to your batteries, however close they may be, creates the very fusing issues you mention, just because of all the different wire sizes.

Here are some other corroborating wiring methods that may help you implement MS's suggestions:

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: http://c34.org/bbs/index.php/topic,6604.0.html

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowners.com/showthread.php?t=137615

Thanks Stu. As sailing season is underway, I'll re check my wire ties and I'm sure I'll be ok(and look for an inline fuse as mentioned above). I'm very careful about that(wires tied) now.

My start battery is isolated from the bank and the wiring, even through the old A meter to the alternator is very short(charging is no problem). I can see replacing the original meters with voltage meters will allow me to run directly from alt. to bank and starting battery and make fusing to protect wires a simpler process.

But that will be a project as I'll then have to adapt to the old panel in the cockpit(I like to keep the old boat as built when I can) to new meters, etc.

The reason I even got into this is I had a problem energizing the glow plugs last weekend. After some searching, I found a large voltage drop between starter lug(that picks up short start battery cable) and the glow plug solenoid. I cleaned connections several times, and still got a dropped voltage(the 10 gauge wire was less than 12" long). Finally I just cut it out and crimped up a new cable(without a spliced connector, cleaned up to install an inline fuse), and off we went.

Yesterday I got out to the boat and had time to look at the old wire. I expected a difference in resistance but was surprised to find no continuity at all through the short wire. Some tugging on the short tail, and the wire parted right at the plastic splice block.

 

[Stu Jackson]

Great. Ain't wiring fun & games?

Good luck, have a great season.

 

[SailingHarry]

Maine, as always, has absolutely unassailable information. He's highly qualified, and highly conservative -- the kind of guy you want in your corner.

I would make one small suggestion away from conservative here, and while less than perfect, it's still a huge improvement for your old girl. That solenoid wire is clearly undersized (as Maine points out, you loose volts on that small wire at those currents). Up it to an 8, or maybe a 6 (at that length, cost isn't a factor -- what do you have on the shelf?).

Here's my bit of heresy. I'd suggest that your risk on that short wire (you said under a foot) is very low -- remember, you've lasted several decades with the old one. I would be very tempted to run the new one without a fuse, as adding a fuse in a very short wire is kind of messy. Then I'd add a proper fuse where you drop down a size from the solenoid.

I know, it's not the "right" way to do it. My boat is 32 years old, and I struggle constantly with correcting and upgrading things that weren't done to today's standards. I've made huge improvements, but even my improvements aren't up to standards. My battery cable goes well over the standard of 7" before a fuse -- I probably have close to 10". But, hey, I have a fuse now, which it didn't for the first 30+ years. And you'll have that long 12ga wire fused as well as the harness, even if you don't fuse the short wire to to the solenoid.

Harry

 

[Stu Jackson]

The other thing to do is to install a glow plug solenoid: http://www.c34.org/wiki/index.php?title=Faster,_More_Efficient_Glow_Plug_Heating_with_a_Solenoid

Fuse the wiring, it's sooo much safer, regardless of length.

 

[SailingHarry]

The other thing to do is to install a glow plug solenoid: http://www.c34.org/wiki/index.php?title=Faster,_More_Efficient_Glow_Plug_Heating_with_a_Solenoid

Fuse the wiring, it's sooo much safer, regardless of length.

Stu,

I agree with you on the value of a fuse. But, let's consider this application. He's got a stud on a starter that he needs to connect to. A MRBF isn't really appropriate here. So he needs to find an inline fuse with #8 wire and a 50A fuse. Good luck. Then he needs to splice it into a 1 foot chunk of wire. Now has an inline fuse dangling in the middle of a 1 foot chunk of wire, in a hard-to-reach location, that would have smoothly fed from the starter to the relay. And so now instead of a 1 foot unfused wire, he has only 6 inches (the first 6 inches from the starter to the fuse) unfused, and the last 6 inches to the relay are fused.

I'm not sure that the practical implementation of a very noble goal really works here. Or if it does, I'm not sure HOW to make it work. He really ought to put in a breaker panel by the battery, and run a new line from there. But even that is messy (somehow, you need to get from the battery to the 50A fuse or breaker with a wire (that is probably unfused -- OK as long as he keeps it under 7"), and then he probably HAS to go to #6, and ...)

Harry

 

[Stu Jackson]

Harry, the simple answer is to buy two feet of wire and make room for the fuse.

 

[TomY]

This is great info and I appreciate it. I may implement Harrys idea for now. The fire I had was in fact was in the 12 gauge panel wiring past this solenoid circuit. The loosely bundled wire was the On/Off switch I installed for my Balmar alternator. I can protect all that 12 gauge wire with a fuse holder now unused in the panel. I have rebundled wires as well as covered the dangerous area of exhaust in wrapping.

I went to my Hamilton catalogue and found what Harry also mentioned. The only "inline" fuse holder I saw is an upsized Maxi(or something) auto type fuse. It's too big and heavy by the looks to go unsupported so I would think I'd have to run more wire to a nearby bulkhead. Hmmm.... And then I have the vibration situation, as well as a couple feet of unprotected wiring, for the bulkhead mounted fuse.

I do in fact have a solenoid for the glow plugs and it is the 10 gauge wire that carries the glow plugs feed which is from 1 foot+ of 10 gauge from the starter lug. But that 10 gauge then goes through the old Amp gauge and to Alternator. But these are short and protected runs. But I'll address these and perhaps include replacing the Amp gauge with Volt so I can get them out of the direct line to Alt.

Fusing the 12 gauge after the 10gauge would protect the glow plug switch circuit from the panel as well as the instrument gauges, panel light, etc and would have protected the Alt. switch wire that shorted on the now protected exhaust.

One other experience I had was coming through the Cape Cod Canal several years ago. I noticed the Amp gauge to the start battery was reading well above what was normal. Nearly at the Northern end(I left from Onset), I smelled burning plastic and suddenly the Amp gauge went to normal...

I was alone but lifted the manhole cover in my cockpit while I steered and that awful smell of electronics/plastic lead me to the glow plug solenoid, which had melted. The push button switch had not released when I started the engine now an hour ago.

I replaced the switch(after hot wiring my way back to Maine) and now pay attention to that. This shows some things though. I guess the wire is up to the load of the glow plugs. The glow plugs on the Nissan SD 22 are robust(I'm now on another SD22)! And a Ford type solenoid is a slow burn fuse?

I"ll continue making this a better situation and I do appreciate all the excellent advice I've had so far.