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Old 09-05-2019, 15:41   #1
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Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Hello

My new to me Beneteau 393 has 3 Firefly AGM batteries. One is currently used as the engine starting battery and the other two are the house bank. Since I am in need of much more capacity I decided to buy a less expensive AGM starting battery and add that third Firefly to the house bank.

My concern is the jumpers on the house bank are only 6AWG. They are about 24" long so voltage drop doesn't seem to be a big deal in this case maybe? I honestly expected those to be at least 2 or 4 AWG (and slightly shorter as well but the batteries sit end to end not side to side).

Should I replace them with a larger gauge jumper or can I just get another pair of 4 AWG jumpers in the same length as the existing jumpers?

Thanks

Kirk
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Old 09-05-2019, 17:08   #2
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

It really depends what loads are on the boat. And how the wires are. Ie each batt have jumpers feeding a bus. (1/3 current in each cable). Or batteries jumpered together. In which case 2 jumpers have 1/3 current and 2 jumpers have 2/3 current.

If you have a bowthruster they are way too small. Even for a windlass they are light. If you have nothing but a few lights. Then It’s fine.

If you can Parallel the house to the engine for starting. Then likly too small.

What size wires come off the bank.
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Old 09-05-2019, 17:26   #3
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

yes.. what are the loads?
other high loads
electric winches.
inverter - 12v to 120vac
alternator - gauge the bank and length?


-dkenny64
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Old 09-05-2019, 17:32   #4
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Go for as big a wire that you can handle or fit, and youíll never have a problem with it.
Wire can only be too small, canít be too big.
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Old 09-05-2019, 18:03   #5
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Should have mentioned the house bank is wired in series using the 6/0 AWG. The 2 leads from those are 2/0 I think.

I don’t have a thruster but do have a windlass that only works when the motor is running. I suspect it could run of the engine battery? Might be why they have the big Firefly AGM on that one. I’ll have to take another look to confirm that (just came to mind).

Yeah maybe I’ll have to get the tools and items I’ll need to make new 2/0 Jumpers.
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Old 10-05-2019, 09:04   #6
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Re go for as big a wire as possible. And keep them as short as possible
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Old 10-05-2019, 11:56   #7
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Quote:
Originally Posted by a64pilot View Post
Go for as big a wire that you can handle or fit, and youíll never have a problem with it.
Wire can only be too small, canít be too big.

PLUS ONE!


Error on the side of caution. I think 6ga and 4ga are way to small. I went with 2/0 just to be safe. These batteries can produce 800 amps if shorted.


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Old 10-05-2019, 14:59   #8
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

I've never seen 6/0 AWG.
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Old 11-05-2019, 04:50   #9
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

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PLUS ONE!
Error on the side of caution. I think 6ga and 4ga are way to small. I went with 2/0 just to be safe. These batteries can produce 800 amps if shorted.
Jim

Well, the cable size is independant of the Amps the batts could produce. It should be sized to accomodate the sum of the consumers at the other end. Then a fuse or breaker close to the batt protects those cables (and you plus your family and your boat) "if shorted", so they don't melt and then catch fire.

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Old 11-05-2019, 12:25   #10
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Jim-
"These batteries can produce 800 amps if shorted."
Heck, even a Group 24 or Group 27 SLI car battery can produce 3000 AMPS into a crowbar short. Way more than 800 amps. And you don't have to get much larger to pull even higher amperage. Treat battery cables near the batteries with fear and respect, because they bite hard. And the bigger you cables are--the better they can bite.

I would argue that you can have cables that are too large. At a certain point, especially if you are using locally available crimps and such, you find out you can't accommodate the hardware in the crimper, or the lugs take up too much room at the terminals...Sometimes the larger lugs won't fit the primary fuse holder. So by all means, just check that you don't go "bigger" into problems. And if is is a choice between bigger and tinner wire, go for the tinned marine wire.
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Old 11-05-2019, 14:06   #11
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

It's all about numbers. "Feelings" and "thinkings" are nice, but they really are not the way to decide what to do with something complex like battery wiring.

Start by looking up the "ampacity" of the wire involved. It depends on the temperature rating of the wire's insulation, but a good conservative number for 6AWG wire is 50 AMPS. If the MOST you can draw using a reasonable combination of power loads is 50 AMPS then the 6AWG wire will be good. Of course, 6AWG wire should then be fused so it can not draw more than 50 AMPS even in a short circuit. Battery terminal fuses like these are a good solution for protecting battery cables "at the source."

People are suggesting 2/0 AWG wire, which is rated at 175 amps. If your boat can draw near that much, do use it. Otherwise, it is a waste of money. Bigger cables aren't better, they do the same job as smaller ones--if the smaller ones are sized correctly.

Finally, be sure that you actually have your batteries connected together correctly. It is slightly more complex, and much more important than most people think.
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Old 11-05-2019, 17:37   #12
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

I just rewired my boat. I went with 2/0 interconnects because my windlass connects close to battery bus and that uses 1/0. I ran 2 AWG to my panel feed but there isn't much in the way of heavy draws, maybe autopilot is the biggest. For my start bank, everything is 2/0 from the engine to switch and back to ground. The engine was wired with 2awg and I will say it starts a little easier.

My parallel switch can start engine from house at which point I would be using 2AWG but my engine was starting off that before this spring so I figured it was good enough.

I went by the book and fused everything. Marinehowto was an awesome source of information about crimpers and schematics. My boat now makes sense and works very well after the rewire.
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Old 17-05-2019, 16:39   #13
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Re: Correct Gauge Jumper Wires (for Firefly AGM's x 3)

Thanks for the replies. Iíve been busy preparing for my trip south (Seattle to Portland).
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