Has anyone done a Dual Battery switch setup – 1979 140 hp Mercury

[Luigi Frassetto]

I bought a Blue Sea Systems m Series model 6011 dual battery switch, rated for 300 amps continuous operation, 450 amps intermittent, and 675 amps cranking.  The instructions state that I have to use 4/0 cable to meet the current ratings capability of the switch.  Not having done any electrical wiring in a boat before, I soon learned just how big such a cable is and how very expensive it is.    I estimate that I have to use 6 foot runs at the most of the new cable and if I use 1/0 cable instead, the max voltage drop just from 12 feet of line length in one circuit using 1/0 cable is about 0.5 VDC, if I assume 300 amps.   If I stuck with 4/0 cable, the voltage drop would be half of that.   I would appreciate any advice and experience from anyone that has done this kind of project already to advise on whether I really need to use 4/0 cable (3/4 inch diameter with the insulation).  By the way, I believe the cranking amps for the starter motor is max 190 amps according to Mercury (1979, 140 HP).  I don’t know how much extra current the motor uses while starting.  My only accessories on the boat are a fish finder and a 2-speaker stereo system that is almost never on.  I never boat at night so I don’t count the lights.  I would think 10 amps would cover these accessories easily.

[bobw]

I’ve never seen any outboard installation that needed 4/0 battery cable, even with a dual battery setup.   I have the same battery switch on my Evinrude 150 (although I only run 1 battery) and I have #2 battery cables.   Don’t know if this will help you but here’s a battery cable sizing chart published by Mercury.

Battery-Cable-Wire-Gage-Table

Bob

1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20

“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."

[Luigi Frassetto]

Thanks for sharing your experience and info, Bob.  I’ll take a look.

[billw]

Mercury’s own cable is only about #4. Most of the time, you see a half volt to a volt drop in those cables when everything is working right. The 4/0 is just recommended because that’s the most the switch can take, I would think. I bet if you used #2 over 12 feet, that would be plenty.If you want to see it in Black and White, Maybe ABYC has online technical specs that you could check.

Long live American manufacturing!

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Luigi Frassetto

[bobw]

You can google “marine wire sizing” and find a number of sizing charts and calculators.

Luigi, here’s a link to a supplier of pre-fab marine starting cables I’ve used before.   Just pick the cable size you want, specify the total length of each cable and what type of cable terminations you want and they will make them up for you.   High quality tinned copper wire with heat shrink lug connections, these are very good quality cables and at a reasonable price.

https://www.batterycablesusa.com/battery-cables

Bob

1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20

“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."

[Luigi Frassetto]

Thanks, Bill and Bob for your help.  I’ve started using the calculators already, Bill, good suggestion.  Marine cables at many online stores are so expensive so I’ll look at your link too, Bob.

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