6 gauge,12' run,need breaker?

I have a 25hp Yamaha electric start. I have one battery in the stern close to the outboard and am installing an additional battery in the bow with a 4 positon battery selector switch. The bow battery is 12' from the stern battery and I have 6 gauge stranded wire for the 12' run. The bow battery will be used for the 38 lb trolling motor and manually controled bilge pump.The stern battery (also a deep cycle marine) will be the starter battery. If the bow battery dies I will be able to use the stern battery for a while and still be able to start the engine. My conundrum is this. I was advised by MinnKota to put a 40 amp breaker in the 12' run. If I do that, and then later have to start the outboard with the bow battery I am sure it will trip the breaker. Is there any way to do what I want to do and still protect the trolling motor?

Re: 6 gauge,12' run,need breaker?

Think about this. If you are worried about the engine tripping a 40A breaker, shouldn't you also be worried about the 6 gauge wire being big enough to carry starting current as well? When you combine two batteries on a dual battery switch all of the cables between the batteries and the switch and switch and the engine must be the same size or larger than the red and black engine battery cables. Yes, you should have a breaker in the lead going to the troller. You actually have three runs of wire to deal with. 1) The POS terminal of the stern battery needs to run to the BAT 1 terminal on the switch which is 12 feet away up front. 2) The engine POS battery cable does not remain connected to the START battery -- it goes to the COM terminal on the switch. 3) The NEG terminal of the troller battery needs to be connected to the NEG terminal of the START battery (BAT 1). The POS terminal of the AUX (troller) battery goes to the BAT 2 terminal on the switch but that is just a short distance. Are you aware that most jumper cables are about 12 feet or more in length and would cost far less than the switch and associated wiring which you would likely never have to use. There is a much greater chance of running the troller battery dead than there is the starting battery. And if you thought the engine would be able to charge the troller battery via the switch that would be a reasonable assumption but you would have to run wide open for half a day to put any significant charge on that battery. And isn't your 25 HP motor able to be rope started as well so the battery (either of them) would not be required at all. The battery is used to spin the starter. The engine doesn't need to the battery to actually run.

Re: 6 gauge,12' run,need breaker?

Thank you Silvertip for you response. I have drawn a schematic of the system I would like to install. Do you think that 6 gauge stranded wire will carry the load from battery #2 in case I ever have to use it to start the motor? Normally I will always start with battery #1. The motor only has electric start. The 40amp breaker for the trolling motor will be placed as shown. Does the schematic look like it will work? Many thanks.....

Re: 6 gauge,12' run,need breaker?

Schematically it will work. What you seem to be ignoring is the current draw of the starter motor. I don't have that number but you need to determine what it is. Maximum cranking amps is normally indicated the service manual for your engine. But here is the main issue. You show 6 gauge wire from the STARTING battery to the switch as well as from the TROLLER battery to the switch. Look at the size of the two battery cables that come from the engine. I don't think those are 6 gauge cables. Those cables are the size they are because of the amount of current they must carry. Therefore, since you intend to start the engine most of the time from BAT 1 and in an emergency brom BAT 2, all of those cables need to be the same size as the engine battery cables. There is also a thing called resistance which all wire has some. the longer the distance, the more resistance the poses to the flow of electricity. You don't have a 12 foot run -- you have a 24 foot run (12 down and 12 back). That much length can present a fair amount of resistance to a wire that is already marginal for the load, thus creating heat (lots of it in a short time). You have now been informed of the requirements. But you need to determine maximum starter draw and size all cables according. Just so you know, 6 gauge wire in various Ampacity Charts shows anywhere from 55 to 75 amps depending on temperature. I'm also betting if you remove the cover at the top of the engine you will find a rope start groove in the engine flywheel.

Re: 6 gauge,12' run,need breaker?

Yep, everything you say makes sense. I just looked at the wires coming from the Yamaha outboard and they look to me to be the same size or poss a little smaller than the 6 gauge awg stranded that I have. There is no size printed on them just says Yamaha. Your point on the start up amp draw is well taken. I have been researching that and have not been able to find the answer as yet. Maybe someone who reads this will know what a 2002 Yamaha 25HP 4 stroke draws at start?? I looked at the top of the motor (cover off) and there was a plastic cover bolted over the flywheel, I did not see a rope start capability but that does not mean there is not one there.............many thanks again