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MartysHammer

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Posts: 29
Reply with quote  #16 
Makes sense. That’s a risk not worth taking. I’ll replace the Atwood switch with a 1-OFF-2 type. Thanks
jimh

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Posts: 6,384
Reply with quote  #17 
I don't know how your boat's primary 12-Volt DC power distribution is presently wired, so it is difficult to comment on what exactly would be the best approach to installing a selector switch for the source of HOUSE loads.

A conventional high-current OFF-1-BOTH-2 switch could be used, but, as I mentioned earlier, anytime that switch is in the BOTH position, the two batteries are wired in parallel.

If the switch is in the BOTH position and you cranked over an engine for starting, some portion of the current being supplied to the engine would come from its existing connection to one of the batteries, and some portion of the current will come from the second battery, flowing through the OFF-1-BOTH-2 switch. This means that the wiring to the OFF-1-BOTH-2 switch must be done with conductors that can handle the typical engine starting current, about 200-Amperes. 

Electricity works in a consistent manner, and if you wire the two batteries in parallel by setting the switch to BOTH, then engine starting current is going to flow through the switch. Electricity doesn't know that you wanted the switch to only handle 10-Amperes of HOUSE loads.

My earlier caution was to make you aware of this so you didn't plan on wiring the switch with small conductors between the switch and the two batteries.

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MartysHammer

Registered:
Posts: 29
Reply with quote  #18 
Yes sir, I did overlook the risk of starting with BOTH on the house selector. Thanks for pointing this out.

I went ahead and ordered the one you suggested from Mouser (no BOTH) to be on the safe side.
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