Some Wiring Questions

[HOTFLASH]

I have redone some of my wiring and their connections, and have learned a lot along the way. I have a few questions to ask of the Afourian community. I am sorry that I do not have a way to do a diagram and attach it.

Most of my wiring is set up as shown in the MM General Distribution of Engine DC Powers with a couple other goodies like an oil pressure/temp alarm. I have 2 group 27 batteries going to an Off-1-Both-2 switch, and the original 35A Motorola alternator. Note that my alternator output to solenoid connection is direct and #8 red. (Or is it solenoid to alternator?)

The PO had one wire running from the the alternator output to the ignition switch BATT terminal. I have no ammeter. Is this right? What gauge should it be? Should it be fused?

The PO had a wire from alternator output to the power bus. Is this OK? How best to run power to the power bus? What gauge should it be? Should it be fused?

Should the cables coming from the batteries be fused and what size?

I would appreciate learning what others think about these queries.

[Dave Neptune]

Hotflash, Neil will probably post one of his "masterpeice's" of A-4 wiring.
The stock alt like mine has 2 wires, one the exciter goes to the coil and is a small probably #14ga and the big one off the terminal on the back of the alt is the output and should be #8ga to handle the output of the alt. Where and how you have the output hooked up depends on your layout and guages.

Dave Neptune

[ndutton]

With apologies, I'm way late for work this morning and do not have enough time to address the questions properly. I'm sure during the course of the day there will be others with excellent responses.

Briefly though, choosing a properly sized wire factors in total load, length of run and proximity to other wires over the length of the run. Fuses are for your safety and the protection of the connected systems and are sized according to the wire gauge or connected load, whichever is less.

Before I can respond to the power buss connection question I need to know how your battery switch is connected.

[HOTFLASH]

Neil. So you are the guru--I hope I get your ear. I do have a basic understanding of the principles of gauge and fusing. I have short runs, 3-6 feet. I was just asking what the usual practice or rule of thumb was regarding gauge and fuses in the specific situations I enumerated.

Because they do not conform to any diagram I have found on this site, including the famous MM one, I am mostly curious about the wire running between ignition "BATT" terminal and the alternator output terminal, and the one between alternator output and the main accessory power bus. Are there better ways of doing this?

To respond to your question re battery switch/cable--I have only one 3 ft #2 cable coming out of the battery switch, and that goes to the starter solenoid.

Did this tell you enough?

And Thanks!
Mary

[ndutton]

Your alternator output wire @ #8 is sufficient for anything the Motorola 35A alternator will ever put out. It's connected to the solenoid terminal only because the battery cable is too and that's how the charge gets back to the battery.

As for the alternator output to ignition switch wire, it's not the alternator output that matters but that it is in common with the battery on your boat. This wire could just as easily be connected to the solenoid at the other end of the aforementioned #8 red wire.

There are textbooks written on how to determine the proper gauge of a wire such as in this application and I could type for an hour or two discussing amperage, insulation grade, allowable % voltage drop, derating due to bundling and who knows what else. I won't bore you or other readers with the details unless requested so here's my cookbook answer:

if your installation is typical and based on your reported short harness run, #14 or #12 wire should be fine. Yes, it should be fused and without detailed information of the combined loads it will carry, a suitable fuse for #14 wire is 15 amps, 20 amps for #12.

Assuming your power buss is for house loads I'm not a fan of your wire from the alternator output to the power buss, would rather see the buss powered from the battery through the battery switch and let the alternator replenish the battery as necessary. You should be able to disconnect the power buss easily in an emergency. The wire gauge and fuse size is determined by the maximum combined load off the power buss. I can't begin to guess what that might be on your boat but for reference, my system is designed for 50 amps and #6 wire. I've never seen more than 20 amps under normal circumstances on my main distribution panel ammeter.

Your #2 battery cable is a size larger than most and I like it. I would fuse it not exceeding the wire rating. 130 amps is suitable for #4 wire inside the engine space according to the ABYC and should work fine for your system.

I hope I've addressed your specific questions satisfactorily. If you or anybody is interested in a primer on power circuit design I'm up for it, just say the word.

[HOTFLASH]

Neil,

Thanks for your consultation. I will wire #8 (orange, right?) the power feed to the power bus from the battery switch, through a fuse.

"Is for the alternator output to ignition switch wire, it's not the alternator output that matters but that it is in common with the battery on your boat. This wire could just as easily be connected to the solenoid at the other end of the aforementioned #8 red wire"

What color wire should this be ?

Mary

[ndutton]

Originally Posted by HOTFLASH

What color wire should this be ?

Red.

Notice that the Moyer wiring diagram calls this wire out as #8 and fused @ 40 amps. This differs from my assessment of yours because the Moyer diagram has an ammeter in the engine instrument panel and all the charging amps go through it. Not so in your case.

edit:
I just checked my engine harness drawing for wire gauge. The wire we're talking about is #12 red on my boat and my harness is three times the length of yours (speaks to voltage drop). I'm having zero issues.

Oh yeah, and you got my ear with the first post. We got yer back!

[HOTFLASH]

Neil,

Made the changes today, but do not have the fuse yet for the battery switch to power bus run. Everything seemed to work fine. A PO had stranded untinned wire (but mostly with the correct colors) and uninsulated terminal connections for many runs. I have changed all the critical ones to tinned/insulated, and will work my way to all of them.

Thank you for your assistance!

Mary

[ndutton]

And another Atomic 4 is delivered into greater reliability via a P.O. exorcism.

Nicely done Mary.

[sastanley]

Mary, nice work...it takes a while...just keep plugging away at them, one by one.

In addition to some interesting ways to regulate the alternator (some sort of automotive one-wire regulator which cross referenced to a Studebaker, which broke, so he wired some sort of resistor and a 3-way switch into the charging circuit), the P.O. of my boat also wired the bilge pump with lamp cord.