Wire size on alternator

[Maine Sail]

Originally Posted by Wotname

???
The negative side is usually the alternator case and the engine and then the BIG negative cable from engine to battery.

And often the negative side of the charging circuit has the most voltage drop due to the alt current passing through multiple poor conducting metals, rust, corrosion, gasket material etc. before it picks up the main engine negative cable.. Also many factory alternators are isolated ground, such as many of the Hitachi's, so they pass through quite a bit of very small GA wire before even getting to larger neg cable..

If one desires any sort of charging performance, where the alternator is not seeing absorption voltage prematurely, even a 3% voltage drop, without properly sensing the battery terminals, is too much voltage drop. Stack a 3% + drop on top of an alternator that cuts back the voltage limit, based on alt temp, and it can literally destroy your charging performance.

I wonder if anyone could tell me how much milk I have in my fridge. I am away from my fridge and don't know..

[Sailmonkey]

Originally Posted by Maine Sail

I wonder if anyone could tell me how much milk I have in my fridge. I am away from my fridge and don't know..

Sqrt 7^34 deciliters


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[colemj]

Originally Posted by Boatyarddog

I'm not sure where you got that r

As stated by others, I used round trip for the calculation, so your 6' example is electrically 12'.

Mark

[deblen]

Depends on max current per table.

https://www.bluesea.com/resources/1437

[Sailorman Ed]

And often the negative side of the charging circuit has the most voltage drop due to the alt current passing through multiple poor conducting metals, rust, corrosion, gasket material etc. before it picks up the main engine negative cable.. Also many factory alternators are isolated ground, such as many of the Hitachi's, so they pass through quite a bit of very small GA wire before even getting to larger neg cable..

So we should run a separate ground wire from the alt case back to the battery or only to the engine battery cable?

if anyone could tell me how much milk I have in my fridge

This is easy, "not enough"

[Wotname]

Originally Posted by a64pilot

Silly question, usually the wire from the Alt does run to the battery bank,
But, three ft or less away from the Alt. is usually the starter solenoid, that has a HUGE wire that also goes to the bank. So why not just take your Alt wire to the solenoid?

I believe my stock Yanmar wiring harness had a 10Ga wire, though it may be an 8

The smaller stock Yanmar harness does indeed utilize the HUGE wire on the solenoid.

[Wotname]

Originally Posted by Maine Sail

And often the negative side of the charging circuit has the most voltage drop due to the alt current passing through multiple poor conducting metals, rust, corrosion, gasket material etc. before it picks up the main engine negative cable.. Also many factory alternators are isolated ground, such as many of the Hitachi's, so they pass through quite a bit of very small GA wire before even getting to larger neg cable..

If one desires any sort of charging performance, where the alternator is not seeing absorption voltage prematurely, even a 3% voltage drop, without properly sensing the battery terminals, is too much voltage drop. Stack a 3% + drop on top of an alternator that cuts back the voltage limit, based on alt temp, and it can literally destroy your charging performance.

I wonder if anyone could tell me how much milk I have in my fridge. I am away from my fridge and don't know..

Ahh yes, I forget that many don't their engine in a pristine state .

BTW, what's milk

[delmarrey]

Most manufactures recommend #8 wire for under 2' of length, and #6 for under 12' for an 80 amp alt. If your going to an isolator/combiner you'll need to read their instructions, for it matters.

[a64pilot]

Originally Posted by Wotname

The smaller stock Yanmar harness does indeed utilize the HUGE wire on the solenoid.

Didn't know that, I never traced the thing, just cut it off

[a64pilot]

Originally Posted by Sailorman Ed

So we should run a separate ground wire from the alt case back to the battery or only to the engine battery cable?



This is easy, "not enough"

Biggest take away I got from his post, was to ensure your sensing the voltage of the battery bank directly, and while he didn't say this, if you have a good regulator, be sure your sensing both battery bank temp and alt temp, after you shelve the stock alt of course

[twinboat]

Stock engines run to alternator output to the starter post.

It is a common practice to run a new wire to the house bank. That's the bank you want to charge first. Then you can use a combiner to the start battery.

[hellosailor]

With a 60A alternator in most cars, they use a 10-SAE charging wire, last time I checked. BUT. There may be two wires to the batteries, one for charging and the other for voltage sensing, and while they are often both 10-SAE the sense wire can be lighter without any problem.


10-SAE is about equal to 12-AWG, and assuming you are not interested in "how cheap can we build a boat?" you really want to measure the length of the wiring run, use a voltage drop chart, and select a cable that will have zero voltage drop. Ideally.


Except, the pursuit of "zero" will probably mean a cable too thick to attach to the alternator or some similar problem. And you don't need to aim for zero. The more voltage drop that is in that cable, the more voltage the alternator has to produce in order to charge your batteries. So the cheap factory wire means the alternator will be running hotter than it needs to, and wasting power (and engine power, fuel) while a heavier cable will let the alternator run cooler, or charge the batteries faster.


You compromise with your wallet and what's available and eventually say "That's good enough for me", at whatever point you feel meets your personal needs.

[Lars_L]

Calculate the size of the wire:

Resistivity of copper ρ = 17.2 nΩm = 0.0172 Ω mm²/m
Total length between battery and generator (both ways) l = 2.0 m
System voltage 12 V
Allowed voltage drop 3%
U = 3% * 12 V = 0.36 V
Dimensioned current I = 60 A
Max resistance R = U / I = 0,36 V / 60 A = 0.006 Ω
Cable area A = ρ * l / R = 0.0172 Ω mm²/m * 2.0 m / 0.006 Ω = 5.73 mm²


A simpler way that’s easier to remember is:

Area = length one way * current / 10
1.0 m * 60 A / 10 = 6.0 mm²

[Boatyarddog]

Originally Posted by Wotname

???

The negative side is usually the alternator case and the engine and then the BIG negative cable from engine to battery.

Exactly! And a 60amp charging unit rarely peaks.
If your alternator is larger, you need to charge rapidly, the run is long or you have a lot of loads, then a larger size conductor would be good.


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[Boatyarddog]

Originally Posted by Sailmonkey

Round trip.


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The calculations include 2 conduits.
Most boats run more than 1 negative conduit, or should be.


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