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Old 07-01-2020, 02:54 PM   #1
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Alternator wire size

I recently replaced my alternator and while doing so I noticed the wire looked a little light for the 120 amp alternator. I confirmed with the prior owner that the 120 amp alternator and 6 awg wire is original from the factory. I estimated the positive wire is about 15 ft and the negative is about 5 ft to the starter which from there continues another 10 feet on 2/0 cable. If I use my dc wire app for a 30 foot dc loop it shows I should be using 2/0 for the entire length with a 2.4% voltage drop. If I use a 10% voltage drop it shows to use 4 awg. I have a Balmar external regulator so could limit output to 90-100 amps which would sneak the 6 awg wire within limits.

The alternator is a 120 amp Bosch so not sure of the specs but suppose once warmed up itís only putting out 90 amps or so. The alternator Wire didnít have a fuse on it as my vintage boat didnít require one at time of build but I plan to add one. Iíd like to put a 150 amp fuse to avoid nuisance trips but then need to step up to 2 awg wire.

All inputs appreciated.
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Old 07-01-2020, 03:35 PM   #2
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The ampacity of #6 is exactly 120 amps so you are marginally ok there as I suspect you will never see 120 amps out of a normal internally regulated alternator.

Figuring 20' of #6 and 10' of 2/0 gives you 1.0 volt plus 0.1 volt or a 1.1 volt drop overall, which is too much.

I would go with #2 instead of the #6. That will give you a total of 0.5V drop which is reasonable for an alternator output.

Realize that an internally regulated alternator will recharge batteries rather slowly since the voltage is usually set to something in the upper 13s.

David
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Old 07-01-2020, 04:21 PM   #3
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The ampacity of #6 is exactly 120 amps so you are marginally ok there as I suspect you will never see 120 amps out of a normal internally regulated alternator.

Figuring 20' of #6 and 10' of 2/0 gives you 1.0 volt plus 0.1 volt or a 1.1 volt drop overall, which is too much.

I would go with #2 instead of the #6. That will give you a total of 0.5V drop which is reasonable for an alternator output.

Realize that an internally regulated alternator will recharge batteries rather slowly since the voltage is usually set to something in the upper 13s.

David

David, thanks for the reply. From the dc wire app I have I knew that #6 was a little light for 120 amps. I agree I doubt I will ever see 120 amps or not at least for a very long period, even with my external Balmar regulator. I guess when figuring the wire size I should use 5 feet for the negative to the starter as from there back its 2/0.

So with AWG 2 my app is showing this has an ampacity of 178 amps so if I use a 150 amp breaker on this line I should be ok? Previously I had the alternator going to a battery isolator but when I measured the voltage drop it was close to 1v. On a tangent to this post but I'm wiring the alternator directly to my house bank and will keep the starter battery topped up with an ACR.

Thanks
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Old 07-01-2020, 04:33 PM   #4
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First, I would increase the wire size above the minimum. Second, I would consider it's proximity to temperature (wire size recommendations are based on bundling and ambient temperature). Depending on routing around the engine, they can get pretty hot. My 220 amp alternator with external regulator, has 4/0 cables going less than 6' from the alternator to the distribution panels. While I had the cable and only had to buy the connectors, the cost seemed relatively trivial compared to the peace of mind. For your application, I would think 2/0 would be fine.

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Old 07-01-2020, 05:50 PM   #5
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The wire should be sized not to exceed a 3% voltage drop, if you actually want any performance out of the alternator. Even a 3% drop at 14.4V means just 13.97V at the batteries and the regulator will enter absorption before the bank even gets there..
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Old 07-01-2020, 05:59 PM   #6
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Thanks for the inputs. I think 2 gauge would have been enough but while ordering on Amazon they had 1/0 which could be delivered by Friday so I went with this. Should limit my voltage drop to less than 2%. I can't believe the original builder went with 6 gauge, totally inadequate in my view.
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Old 07-01-2020, 06:35 PM   #7
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Ahh, you have a Balmar external regulator. Wire its external voltage sense to the battery, not the alternator output. That will assure that the battery gets the correct charging voltage, irrespective of voltage drop in the wire or isolator.

So in that case, any wire size larger than #6 is just improving efficiency, not the charging voltage profile. I would still go with #2 and a 150 amp breaker or fuse protecting it.

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Old 07-01-2020, 07:08 PM   #8
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Ahh, you have a Balmar external regulator. Wire its external voltage sense to the battery, not the alternator output. That will assure that the battery gets the correct charging voltage, irrespective of voltage drop in the wire or isolator.

So in that case, any wire size larger than #6 is just improving efficiency, not the charging voltage profile. I would still go with #2 and a 150 amp breaker or fuse protecting it.

David
I have a ARS 5 regulator. I assume itís getting the battery voltage from terminal 2, is that correct? Previously I had my alternator + terminal connected to a battery isolator so canít think it would get the voltage from this connection. I will check to ensure terminal 2 is coming from my house battery and not my start battery. With 1/0 cables I should be getting minimal voltage drop. Thanks for your help. You are a wealth of information.
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Old 07-03-2020, 05:25 AM   #9
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A fuse in the Alt output blowing will usually wipe out the diodes in the alt.


The better quality rotary switches will have a field disconnect built in so someone going to OFF on the rotary doesn't wipe out the alt diodes if it is operating..
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Old 07-03-2020, 08:02 AM   #10
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A fuse in the Alt output blowing will usually wipe out the diodes in the alt.


The better quality rotary switches will have a field disconnect built in so someone going to OFF on the rotary doesn't wipe out the alt diodes if it is operating..
Isnít the purpose of fusing the alternator to avoid a fire on a dead short of the stator? I understand if the fuse blows the diodes will be blown but that probably means the alternator was fried from something else anyways. Replacing an alternator doesnít worry me but I donít want to replace my boat.
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Old 07-07-2020, 03:41 PM   #11
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A fuse in the Alt output blowing will usually wipe out the diodes in the alt.
Any fuse in an alternator output wire should be sized at 150% of the alternators rating. This means there is no way the alternators output can cause the fuse to blow. If that fuse blows, it needed to to protect the vessel.

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The better quality rotary switches will have a field disconnect built in so someone going to OFF on the rotary doesn't wipe out the alt diodes if it is operating..

And nearly every field disconnect switch we come across does not have the AFD circuit even wired.

The vast majority of the few we come across that are using it, have it wired incorrectly, so as to offer no protection at all.

I just worked on a boat in May where the diodes had been blown the previous fall. The owner proudly proclaimed he had used an AFD switch and that should have prevented it. "That switch is defective!". I politely had to explain to him that his alternator "excite wire" is just to get it going. Once started that wire can be disconnected and the alt will keep on chugging right along.
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Old 07-07-2020, 03:55 PM   #12
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I have a ARS 5 regulator. I assume itís getting the battery voltage from terminal 2, is that correct? Previously I had my alternator + terminal connected to a battery isolator so canít think it would get the voltage from this connection. I will check to ensure terminal 2 is coming from my house battery and not my start battery. With 1/0 cables I should be getting minimal voltage drop. Thanks for your help. You are a wealth of information.

Voltage sensing on the Balmar ARS-5 is done through regulator B+/terminal #2 and Regulator B-/Terminal #1. Both of these wires should be as close to the banks + & - terminals as is possible for the application. Ideally, any performance alternator would be directly wired to the house bank and then the alternator can accurately sense the terminal voltage.

Do yourself a big favor and get rid of the diode isolator, if that is what you have. They cause anywhere between 0.6V and 1.1V of drop, depending upon current, in the alt B+ feed. This drop is stacked on top of every termination, switch, fuse etc. and the wire drop. There are much better products available today, for charging multiple banks, that don't negatively impact charging performance like a diode isolator does..

In regards to voltage sensing, keep in mind that the ARS-5 regulator B+ also has to carry the field load so sensing is not as accurate as it is with the MC-614 which has a dedicated + v-sense wire that carries no current. Regulator B- carries very little current, mA's at best, as field - is directly connected to the large gauge alternator B-.

This article below goes into detail on voltage sensing issues and how to properly wire Balmar regulators for accurate voltage sensing.

https://marinehowto.com/alternators-voltage-sensing/
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Old 07-07-2020, 04:57 PM   #13
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Voltage sensing on the Balmar ARS-5 is done through regulator B+/terminal #2 and Regulator B-/Terminal #1. Both of these wires should be as close to the banks + & - terminals as is possible for the application. Ideally, any performance alternator would be directly wired to the house bank and then the alternator can accurately sense the terminal voltage.

Do yourself a big favor and get rid of the diode isolator, if that is what you have. They cause anywhere between 0.6V and 1.1V of drop, depending upon current, in the alt B+ feed. This drop is stacked on top of every termination, switch, fuse etc. and the wire drop. There are much better products available today, for charging multiple banks, that don't negatively impact charging performance like a diode isolator does..

In regards to voltage sensing, keep in mind that the ARS-5 regulator B+ also has to carry the field load so sensing is not as accurate as it is with the MC-614 which has a dedicated + v-sense wire that carries no current. Regulator B- carries very little current, mA's at best, as field - is directly connected to the large gauge alternator B-.

This article below goes into detail on voltage sensing issues and how to properly wire Balmar regulators for accurate voltage sensing.

https://marinehowto.com/alternators-voltage-sensing/
Thanks, yes I replaced the diode isolator with a blue sea acr based on the article at the above referenced site. I fused my Bosch 120 amp alternator at 150 amps but am now worried this is too low. Iíll up the fuse to 175a as the 1/0 cable can easily handle this load and I wonít unintentionally fry my alternator from the fuse unnecessarily blowing.
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