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Old 27-02-2021, 14:04   #1
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Alternator output wire size question

Hi,

25 HP sailboat diesel.

I am upgrading from a 51 Amp alternator to a 72 amp alternator.
Currently with the 51 amp alt. the output wire goes about 8 inches to the starter + terminal with 8 AWG (then to the battery switch and battery banks with 2awg)

Do I need to increase the alternator output wire? Or is 8AWG good enough?

Thanks
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Old 27-02-2021, 14:35   #2
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Re: Alternator output wire size question

70 Amps needs a minimum of 6 gauge wire according to this chart. Your alternator won't be outputting 70A very often, but this is a bad place to cheap-out. A foot of 6awg wire won't break your budget.
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Old 27-02-2021, 14:38   #3
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Re: Alternator output wire size question

https://bulkwire.com/help/recommende...uge-calculator
#8 specs out fine, At least with these guys but it certainly would not hurt to use #6.
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Old 27-02-2021, 14:46   #4
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Re: Alternator output wire size question

It's in an engine room, so subtract 15 pct due to higher temps. 6 ga wire required per ABYC standards. 8 ga is good for 68 amps 6 ga is 102.
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Old 27-02-2021, 15:15   #5
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Re: Alternator output wire size question

You haven't mentioned anything about fusing. Starter circuits are often not fused or if so probably a lot higher rating than the contemplated jumper. Typically the alternator circuit is fused on the battery end at something like 125% of max output.


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Old 27-02-2021, 16:57   #6
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Re: Alternator output wire size question

No fusing on the starter circuit. There will be a MRBF fuse on the house bank.
Genuinedealz will make up a 6 ga. tinned wire with lugs for about $8 inc. shipping so I will go with that and sleep better. I just made a 8 ga one for the old Alt. which will go in the spare parts junk pile. :-)
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Old 27-02-2021, 17:06   #7
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Re: Alternator output wire size question

Not sure your quality of sleep is justified if I understand the situation correctly. If a short develops in the alternator, the current flowing from the battery back to the alternator might make your #6 jumper the fuse.


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Old 27-02-2021, 17:12   #8
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Re: Alternator output wire size question

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Not sure your quality of sleep is justified if I understand the situation correctly. If a short develops in the alternator, the current flowing from the battery back to the alternator might make your #6 jumper the fuse.
So would it best to fuse the #6 jumper or run a wire to a fuse to the battery and not use the starter wire as a buss bar?

Buster007: is the alternator feeding house and starter banks?
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Old 27-02-2021, 17:13   #9
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Re: Alternator output wire size question

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Not sure your quality of sleep is justified if I understand the situation correctly. If a short develops in the alternator, the current flowing from the battery back to the alternator might make your #6 jumper the fuse.


Frankly
Interesting point. This arrangement was issued by Catalina as a recall/retrofit about 20 years ago and thousands have done it. I haven't heard it mentioned before. Maybe they discount the possibility of an alternator short?
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Old 27-02-2021, 17:17   #10
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Re: Alternator output wire size question

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So would it best to fuse the #6 jumper or run a wire to a fuse to the battery and not use the starter wire as a buss bar?

Buster007: is the alternator feeding house and starter banks?
Yes. Maybe I wasn't clear.
The alternator currently has a #8 wire to the starter which picks up a #2 wire to the common on the battery switch. So the alternator will feed either the house or start battery ( or both) depending on the switch position.

old fashioned 1-2-all-off switch

the house bank positive will be fused. i understand no need to fuse the starter battery per ABYC
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Old 27-02-2021, 17:20   #11
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Re: Alternator output wire size question

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Yes. Maybe I wasn't clear.
The alternator currently has a #8 wire to the starter which picks up a #2 wire to the common on the battery switch. So the alternator will feed either the house or start battery ( or both) depending on the switch position.

old fashioned 1-2-all-off switch

the house bank positive will be fused. i understand no need to fuse the starter battery per ABYC

You mentioned it in your first post, I missed it. Thanks for the clarification.
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Old 27-02-2021, 17:20   #12
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Re: Alternator output wire size question

Quote:
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Not sure your quality of sleep is justified if I understand the situation correctly. If a short develops in the alternator, the current flowing from the battery back to the alternator might make your #6 jumper the fuse.


Frankly
no single boat ever has a fuse from the alt line to the starter post. it's 8"
nor does any car.

I only fuse when it's rerun with longer cable to the battery.
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Old 27-02-2021, 17:55   #13
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Re: Alternator output wire size question

Nothing technically incorrect with getting dual use out of the starter wiring (after starting the engine its' normal function is over). The issue I see is that if the jumper shorts to the engine block or a short develops in the alternator diodes/ wiring then the battery current will flow back into the short. If the battery switch is in the start position, then the current will be limited only by the starting battery internal resistance and wire/ connection resistance (might be several hundred amps). If the switch is in the house bank position, the current might be measured in thousands of amps. The MRBF fuse should clear things quickly but that #6 jumper is not going to be happy in either case.

These are all low probability events, but there was a reason you could see that kevlar belt flapping in the breeze on that P&W engine last week.

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Old 27-02-2021, 18:58   #14
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Re: Alternator output wire size question

Quote:
Originally Posted by Frankly View Post
Not sure your quality of sleep is justified if I understand the situation correctly. If a short develops in the alternator, the current flowing from the battery back to the alternator might make your #6 jumper the fuse.


Frankly
I have never seen a fuse on an alternator. If a short developed "in" an alternator the diodes would blow. Yes, It's quite common to fuse at the common on the battery switch.
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Old 27-02-2021, 19:03   #15
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Re: Alternator output wire size question

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I have never seen a fuse on an alternator. If a short developed "in" an alternator the diodes would blow. Yes, It's quite common to fuse at the common on the battery switch.


I ran our alternator output directly to the house bank. The wire is fused at the batteries to protect the wire in case the insulation chafes on the wire.
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