Cable lenght vs resistance in overall 12v circuit using 2 batteries in parallel

DIY
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Hi all, lots of contradictory answers here and there on the web so here it is:

I’m using 4/0 wire to build a system and was wondering if I take into account the lenghts between the posts on my class T fuse and from my main switch on the positive side and the same with shunt on the negative side. I’m attaching my diagram to help understanding. What I mean is that my fuse holde measures 3.5 inches between posts, my main switch is 2.5 inches, my Victron shunt around 3 inches so that’d make 6 inches to add on the positive side and 3 inches on the negative side of the system…

Also, do I have to calculate the overall system cables lenghts i.e. between the Multi Plus and Lynx PLUS between the Lynx and battery bank or JUST from Lynx to battery bank ?

Thanks in advance for clearing it up.

battVan
battVan513×1706 114 KB

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Calculate overall total cable length between power source and load, irrelevant of whats its connecting together.

Use one of many online dc cable size calculators to see cable size required for a given load over a given distance, if in doubt use a few different online calculators to get an average.

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And if that is a 12v system it is always best to use biggest size and max that will fit anyway there are huge amps there. Heat and all the other things that go with it.

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Thanks. Indeed, I used these tables and for every component, I go a notch ‘‘overkill’’ to cover for eventual upgrades. :smiley:

Lots of comments I saw said to measure resistance, which makes sense and measuring between my 24’’ and 21.5’’ 4/0 cables resistance (between MultiPlus and Lynx units) gave essentially no difference so my guess is that the actual battery charging process will be affected in some minor way, which would be compensated by the system ? (shunt + BMV-712) . Otherwise, no issue considering the fact that the system won’t really be pushed to its limits ?

Thanks for clearing up the different components not being taken into account in the calculations !

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Yes, 12v. And yes, bigger size for wires everywhere. More than needed on some branch circuits and occasionnally using one step up for wire and still keeping the fuse amperage for the actual wire gauge that’s under that size.