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Old 14-07-2013, 17:43   #1
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Alternator Positive Wire Sizing

G'day Gurus,

This may have been covered somewhere but I've searched and can't find it.

I'm rewiring the charging system on our boat. It currently uses an old Victron diode system to separate the batteries. The house battery never sees more than 13.2V on charge and of course you can guess what happens to the house battery. It's only 120AH but our needs are very modest.

I'm replacing the Victron with a CTek D250S Dual and a SmartPass. I already have them to hand and I've seen them doing a good job elsewhere...so that's not the question

The boat is powered by a 3GM30 with the standard 55A alternator.

The sparky who wired the boat took the positive feed from the alternator across the boat for about 6 feet to a shunt, using 10 AWG wire. From the other side of the shunt it goes to the live side of the starting battery switch again using 10 AWG for about 2 feet. This is instead of a short run from the alternator to the battery +ve on the starter according to the Yanmar wiring diagram.

Unless I'm mistaken all charging current is travelling over that 10 AWG wire for a considerable distance.

I reckon that wire is much, much too small for the job at hand. For various reasons the location of the shunt is probably the best compromise. So what I intend to do is to beef up that wire substantially. Am I missing something here and is that 10 AWG wire enough or am I right in wanting to upsize it and if so what size would you recommend - I'd rather be over than under in size.

All battery cables (to the switches and to the starter etc) are bigger than 2 AWG - I would guess 0 AWG and I have made up all leads for the new interconnects using 2 AWG...except for the lead from the alternator.

I'd appreciate views on sizing please.

Also if anyone knows the size of that +ve stud on the standard Yanmar alternator I'd be grateful. I'm away from the boat during the week and forgot to check.
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Old 14-07-2013, 18:20   #2
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Re: Alternator Positive Wire Sizing

Yes, that is too small of wire. You will want 4awg or 6awg for a 3-4% voltage drop at the highest output. Since it is unlikely your standard alternator will be at 55A for any length of time, 6AWG will be fine. I have assumed your negative cable is approximately the same length as your current positive run.

Don't run the alternator +ve to the starter - going direct to the battery is the better way.

Kind of odd to have the shunt on the positive cable. Is there a meter connected to it?

The starter cable needs to be as large as you indicate because it carries a lot more current than your alternator cables.

Can't help you with the stud size because I don't have that alternator.

Mark
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Old 14-07-2013, 18:44   #3
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Re: Alternator Positive Wire Sizing

Here are three very good links with lots of info and check out Maine Sails site "Compass Marine", bottom link.

Basic Battery Wiring Diagrams
Electrical Systems 101

and

1/BOTH/2/OFF Switches Thoughts & Musings - SailboatOwners.com

Compass Marine "How To" Articles Photo Gallery by Compass Marine at pbase.com

And yes I would think 10 AWG is too small.
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Old 14-07-2013, 18:46   #4
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Re: Alternator Positive Wire Sizing

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Originally Posted by colemj View Post
[snip]

Kind of odd to have the shunt on the positive cable. Is there a meter connected to it?

[snip]
Mark
Thanks Mark,

yes there is a meter connected to it, and typically I never see more than 20A on the meter...however I'm hoping that might rise with a more efficient system.

Regards
Mike
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Old 14-07-2013, 19:15   #5
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Re: Alternator Positive Wire Sizing

Thanks for the links Glenn, I'd already read some of them but others are new and interesting.

Regards
Mike
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Old 16-07-2013, 00:55   #6
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Re: Alternator Positive Wire Sizing

the shunt on the alternator pos is fine. there is no way to do it on the neg. unless you managed to mount the alt in mid air not touching the engine... as the neg current goes though the mounting bracket.

10awg is way too small. even if it's only going a foot.

if you happen to have a roll of #2 (sounds like you do) then I would just use that, but it would defiantly be overkill. 6awg would give you 3% drop assuming neg is about the same distance.

stud is probably ether 1/4" or #10 possibly 5/16"... buying 3 is probably cheaper then 2 trips to the boat though if nobody else chimes in..

I would fuse the the alt wire as it leaves the battery switch. 80 or 90 amps.
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Old 16-07-2013, 01:03   #7
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Re: Alternator Positive Wire Sizing

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Originally Posted by smac999 View Post
the shunt on the alternator pos is fine. there is no way to do it on the neg. unless you managed to mount the alt in mid air not touching the engine... as the neg current goes though the mounting bracket.

10awg is way too small. even if it's only going a foot.

if you happen to have a roll of #2 (sounds like you do) then I would just use that, but it would defiantly be overkill. 6awg would give you 3% drop assuming neg is about the same distance.

stud is probably ether 1/4" or #10 possibly 5/16"... buying 3 is probably cheaper then 2 trips to the boat though if nobody else chimes in..

I would fuse the the alt wire as it leaves the battery switch. 80 or 90 amps.
Thanks smac

I do have some 2AWG left over so I might use that - even if it's a bit ridiculous. Strange as it seems it's hard to get hold of good tinned stuff in a wide range of sizes in Aus.

I think the Yanmar is all metric and so it's likely to be either M8 or M6 - it looked small - that's roughly 5/16 or a "small" 1/4 - really 15/64.

I've got both of them on hand...I'm just stuffed if it's something silly like M5 or M4!!

It would be good to know for sure. If I had a brain I'd have checked before I left the boat.
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Old 17-07-2013, 11:02   #8
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Re: Alternator Positive Wire Sizing

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Originally Posted by smac999 View Post
...I would fuse the the alt wire as it leaves the battery switch. 80 or 90 amps.
Shouldn't this fuse be as close to the battery as possible? Within 7 inches?
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Old 17-07-2013, 18:10   #9
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Re: Alternator Positive Wire Sizing

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Shouldn't this fuse be as close to the battery as possible? Within 7 inches?
I'd be interested in the answer from the OP as I've been turning this over in my mind. However, I _think_ the answer is that for the purposes of this run of wiring the battery switch _is_ the battery. At least it's the high current source end of this cable run and so would protect the shunt and the run to the alternator.

Interested to see what the OP says.
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Old 17-07-2013, 20:48   #10
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If the #10 wire hasn't been getting hot then it will handle the load. The condition of the old insulation will tell you if it has been overheating.

Use #8 if you are worried but the insulation is the priority.


#2 will be to stiff and a lug will be hard to come by in a small stud size.
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Old 17-07-2013, 23:38   #11
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Re: Alternator Positive Wire Sizing

I see that some people are saying that #10 wire is way too small. I'm curious how that is being calculated. Local electrical code where I live allows 50 amps of house current through plain old #10 THHN. Higher currents are allowed through #10 with high temperature insulation.

I'll have to go over to my office to look up voltage drops over long distances in order to get official numbers to work with, but off the top of my head, I'm pretty sure that a 10 foot run doesn't rate a requirement for oversizing the conductor in any of the books that I have. Also since this is the alternator wire, is the voltage regulator not able to compensate for any small voltage drop that may occur over the length of that wire? Is a small voltage drop a big problem because this is only a 12v system? I'm not trying to disagree. I'm trying to understand the reason why #10 is a problem.

As I see it, at worst, #10 might be very slightly undersized if the temperature rating of the wire is low & in that case, #8 might be technically required. I mean, this is just the alternator wire that will never carry more than 55 amps, right? This isn't a wire that is going to carry full load battery amperage to a starting motor, right? Starting motors can draw hundreds of amps. They need bigger wires. The alternator? A big wire there too? Really? Am I missing something?

Also, I seem to remember that old automotive manuals said that you should not fuse the charging wire off of the alternator, but rather run it straight to the battery. I seem to remember that if that connection were broken for any reason, it would instantly fry the voltage regulator on the alternator. Maybe that was then & this is now. Maybe modern voltage regulators are now immune to that problem. I don't know. I think that I would check on that possibility before making a change though.

Diodes will always give you a noticeable voltage drop. I can't remember for sure off the top of my head what the actual number is, but I think it's like 0.7vdc per series connection. The number doesn't get bigger with increased current loading, like resistive voltage drops do.
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Old 18-07-2013, 00:07   #12
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Re: Alternator Positive Wire Sizing

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Shouldn't this fuse be as close to the battery as possible? Within 7 inches?

if the wire was connected to the battery then yes. but it's connected to the battery switch. so that is the source of power for that size wire. the wire from the battery to the switch would be bigger. (and carry hundreds of amps for the starter) under abyc the that battery to switch to starter cable doesn't need to be fused. but I always do. but it'll be in the hundreds of amps. at the switch the large battery cable goes down in size to the alternator cable, and that is where the smaller fuse for the alternator would go. protecting the smaller cable, where the smaller cable starts
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Old 18-07-2013, 02:29   #13
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Re: Alternator Positive Wire Sizing

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Originally Posted by pbiJim View Post
I see that some people are saying that #10 wire is way too small. I'm curious how that is being calculated. Local electrical code where I live allows 50 amps of house current through plain old #10 THHN. Higher currents are allowed through #10 with high temperature insulation.

I'll have to go over to my office to look up voltage drops over long distances in order to get official numbers to work with, but off the top of my head, I'm pretty sure that a 10 foot run doesn't rate a requirement for oversizing the conductor in any of the books that I have. Also since this is the alternator wire, is the voltage regulator not able to compensate for any small voltage drop that may occur over the length of that wire? Is a small voltage drop a big problem because this is only a 12v system? I'm not trying to disagree. I'm trying to understand the reason why #10 is a problem.

As I see it, at worst, #10 might be very slightly undersized if the temperature rating of the wire is low & in that case, #8 might be technically required. I mean, this is just the alternator wire that will never carry more than 55 amps, right? This isn't a wire that is going to carry full load battery amperage to a starting motor, right? Starting motors can draw hundreds of amps. They need bigger wires. The alternator? A big wire there too? Really? Am I missing something?

Also, I seem to remember that old automotive manuals said that you should not fuse the charging wire off of the alternator, but rather run it straight to the battery. I seem to remember that if that connection were broken for any reason, it would instantly fry the voltage regulator on the alternator. Maybe that was then & this is now. Maybe modern voltage regulators are now immune to that problem. I don't know. I think that I would check on that possibility before making a change though.

Diodes will always give you a noticeable voltage drop. I can't remember for sure off the top of my head what the actual number is, but I think it's like 0.7vdc per series connection. The number doesn't get bigger with increased current loading, like resistive voltage drops do.
When charging batteries hitting the proper target voltage is critical. With "self sensed" alternators especially, this means large gauge wire to attain the lowest possible voltage drop.. It is all about minimizing voltage drop not how much ampacity the wire can physically handle..

I always hear "3%" tossed about for charging but a 3% drop at 14.4V results in only 14.0V at the batteries.. Now add a diode on top of the 3% and your are really getting poor charging performance and suffering from chronic under charging..

I regularly wire high capacity alts with 1/0 or 2/0 wire...Most marine alts can still be fried by a load disconnect. A few auto style alts use avalanche diodes but I don't know any marine units that do. Still if an alt is direct wired to the bank it should ideally be fused for the ampacity of the wire and as close to the battery as possible..
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Old 18-07-2013, 03:53   #14
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Re: Alternator Positive Wire Sizing

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I always hear "3%" tossed about for charging but a 3% drop at 14.4V results in only 14.0V at the batteries.. Now add a diode on top of the 3% and your are really getting poor charging performance and suffering from chronic under charging..
Voltage drop causes longer charging times, not undercharging per say.

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Old 18-07-2013, 06:27   #15
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Re: Alternator Positive Wire Sizing

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Voltage drop causes longer charging times, not undercharging per say.

dave

On a sailboat if you are only pushing to 13.4V or so, which means drastically longer & slower charging times, you will eventually wind up at "chronic under charging". This is especially true with the average engine time people "think" will charge the batteries on sailboats..... You simply do not have the luxury of time to run the motor for 24+ hours at 13.4V to get them to near full..

I am the one having to try and remedy this situation almost daily. Yesterday was an improper installation of a high dollar alternator system where the owner was still seeing just 13.1V at the batteries with just 25A flowing. There was a 1.3V volt of drop at just 25A. The positive & negative wires were grossly undersized and there was an isolator in the circuit with the alt sensing itself. When I got there his batteries were at 11.8V (rested) and this after an admitted 1 hour motor just the day before which the owner implied meant his bank was near full. He had trouble staring his motor that morning on two group 31 batteries...... His analog volt meter was also out of whack and reading on the high side...

I find this sort of installation simply unacceptable for the 2.5K he paid the yard to "fix" his charging issues, but I see it all the time. I quite often see in excess of 1V of drop in charging circuits. This results in batteries so sulfated in just a couple of years that they are well beyond saving. Getting the voltage to a proper level can drastically improve the situation and greatly increase battery life. The vast majority of boats I work on are mooring sailed also so there is no shore power charging during the season. An improperly installed & wired and set up alternator & circuit can lead to destroyed batteries if fairly short order under these circumstances...

When I left his boat yesterday his alt was pumping out 113A (cold alt) under heat gun/inverter load (artificial load for testing) and achieved his 14.6V bulk 1/absorption 1 voltage at the batteries with 32A flowing. After heating up the alt settled in at 78-83A for as long as I left the heat gun on.. He also required a new set of Trojan's because the two year old bank, installed with the new alternator, was completely knackered...

It is sad that this sort of installation occurs but I find it more the norm than the exception.
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