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Old 15-12-2013, 15:06   #1
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Battery Cable Size

I need to make battery cables for a 25 hp skiff. Cables need to be about 17 feet long, so figure 35 ft round trip. I'm told by the engine mfg. that the load would be no more than 30 amps.

Do I size for 10% voltage drop, or 3%? I will say that reading the 10% chart indicates #10 wire, which is smaller than what I would use. I was hoping to use #6 (routing issues), but reading the 3% chart indicates that would be good only for a maximum run of about 30 feet. There will be a disconnect switch, so that with connectors would add a small loss.

Thanks for sharing any knowledge or experience!
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Old 15-12-2013, 15:15   #2
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Re: Battery Cable Size

35' round trip use #4 tinned copper
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Old 15-12-2013, 19:26   #3
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Re: Battery Cable Size

Is the 30 amps from the alternator? What are you powering? Some loads prefer 3%, others are less picky. #6 is fine. That would be a 3.45% drop @ 30 amps given your parameters. With #6 wire, you are still under 3% voltage drop at 26 amps. It is only if you pushing >26 you will be over 3%. There is nothing unsafe about pushing 30 amps over a #6 wire. Just make sure you are using an appropriate size fuse (120 Amps or less for #6).

I agree if you are just buying wire with no constraints, #4 would be the best, but I see no issue using #6 if you have a preference to do so.
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Old 15-12-2013, 20:06   #4
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Re: Battery Cable Size

Sorry I didn't make it clear, but this is an outboard motor, and the 30 amp load would be from the starter.

I'm sure knotnow has the right answer,but I was curious if someone might have successfully stretched the boundary a bit.
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Old 16-12-2013, 08:31   #5
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Re: Battery Cable Size

All rotating DC equipment works best at there design voltage. As recommended above; AWG 4 Boat Cable will provide a voltage drop <3% and your starter will thank you by doing its job.
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Old 16-12-2013, 10:29   #6
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Re: Battery Cable Size

Another option is using a small PC680 Odyssey AGM battery at 15 lbs for starting and at that weight you can probably mount the battery right at the stern for a very short cable run. I have done this with a 427 CID engine in my hot rod, and those PC680 have no problems starting an engine that big, for your 25 hp outboard would be easy for this battery. 15 lbs sure beats 60 lbs.
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Old 16-12-2013, 10:53   #7
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Re: Battery Cable Size

Have you checked out genuinedealz.com? I just ordered battery cables from them, using the custom configurator. Their prices are so good, I used the savings to go up a cable size from what I was originally planning.

Can I also suggest having a fuse near to the battery, should be within a foot or so. ABYC standards now require it. Blue Sea Systems sell a fuse holder that goes right onto the battery terminal. (I'm also retrofitting those at the same time as upgrading the cables)
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Old 16-12-2013, 11:22   #8
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Are you sure ABYC requires fuses for start battery service? Almost zero boats use fuses in start circuits.
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Old 16-12-2013, 11:55   #9
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Re: Battery Cable Size

First of all it will take more than 30 amps to start that motor. I was curious so I just measured the starting amps for my 650 cc motorcycle engine- 100 amps. Your 25 hp outboard will be near that size.

#4 wire at that current and 35' round trip gives a 7.5% voltage drop. Probably ok.

ABYC does not require a fuse between the battery and the starter. But in your case with such a long run, it might not be a bad idea. Lots of distance to chafe and cause a fault to ground. #4 can safely carry 160 amps outside of the engine room, which you don't really have on an outboard; the wire will be at ambient temperature. So I would put a 150 amp fuse near the battery to be double safe.

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Old 16-12-2013, 15:09   #10
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Re: Battery Cable Size

Quote:
Originally Posted by djmarchand View Post
ABYC does not require a fuse between the battery and the starter.
David
This thread is very informative :

Fuses Needed to Comply with ABYC Rules

It seems if you have one battery, and no isolator switch, you don't have to have a fuse. It would still be a good idea, and the Blue Sea Systems battery fuse holder is so easy to add.
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Old 16-12-2013, 15:31   #11
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A 17 foot 0 AWG cable with 12v 30A will give you a voltage drop of 0.8% or 100mv. I suggest you try and get your battery closer to the engine. Your starters going to draw more than 30 Amps. Also starters hate low voltage and will burn up the comutator due to arching from high currents. To be safe and reliability I would go for 0 awg
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Old 16-12-2013, 16:31   #12
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Re: Battery Cable Size

gg-
When we're done with battery cables, we can move on to sex and politics, right?

The difference between a fully charged "12" volt battery, and a dead one, is just one volt. A brand new "12" volt battery will be 12.6-12.7 volts generally, depending on manufacturer. So if you start at 12.6 volts and your cables cause a 5% power drop...
that's now only 11.97 volts going to the starter. If the battery wasn't fully charged, if it was two years old, if you were starting out at 12.3 volts...it is now 11.685 volts, which is pretty close to a nominally dead battery.

Now a starter motor may pull the battery voltage down and run happily on even 10.5 volts, but the question is how reliably?

So while everyone has their own opinions, and usually experiences to match, part of the answer to your question is as Dirty Harry said: "How lucky do you feel today, punk?"

I'd rather buy the damned expensive oversized fully tinned cables, and make sure they are properly swaged at the fittings, than pay for a tow boat or lose a good day on the water waiting for a jump start. Run the numbers, consult your wallet, pick your pleasure.

You may want to mailorder the cables from www.genuinedealz.com, because they'll only charge about a buck per fitting to do proper crimping of the terminals, and they're generally competitive on everything else about cables. Don't skimp on the crimping.
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Old 17-12-2013, 18:38   #13
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Re: Battery Cable Size

Thanks; lot of good info here.

I accidentally lost a long reply to your messages last night, and didn't have the energy to replicate it. Not sure if I do now either, so this might be the abridged version.

Deck officer, I've got that covered! I'm now using the 3 year old Odyssey 625 (I think it is) from my 30 hp motor . But I do want to keep the stern area clear. I'm impressed that the 680 will start a 7000 cc motor!


A fuse sounds like a good idea, and the fuse holder I see might improve the connections.

Djmarchand, the 30 amps figure I quoted was from the manufacturer, so I'd think it was reasonable. Haven't tried pulling the 25, but my 30 is not hard to pull. On the other hand, I can vaguely remember the effort of starting a 90 or 125 cc motorcycle, and extrapolate from there. Difference in horsepower or compression perhaps. But I can't say for sure what the number is.

Hellosailor, gosh, if I have to call for a tow or jump for a 25 hp, there better be a doctor nearby

Guess I'll back off a bit on my attempt to downsize
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