Battery cables - how big?

I have a 30 amp hour AGM powering the vhf and autohelm on my small cat. The max load is maybe two amps with everything running. I want to add a second 30 amp hour AGM in parallel. Do I have to use the big thick battery cables that are used in cars and boats with big amp draws, or can I just use a thinner cable sized for say twice the expected draw (4 amps)?
Thanks

 

You should use a reasonable sized cable that can accept heavy duty terminals and support the physical loads that might occur. There is no reason to go smaller than 6 gauge. 3 or 4 gauge would be better.

 

The most important thing to consider, is to have a cable of a gauge that is rated to a larger current than the fuse or breaker in the circuit

 

You underestimate the currents involved.
The VHF typically draws more than 4 Amps when transmitting, the autohelm may also draw a lot, depending on wind an wave load. And how about charge current? If that comes from an outboard it may be 8 to 10 Amps.

To make sure both batteries maintain the same charge level you really need 3 or 4 gauge cables, certainly when the batteries are far apart.

 

Thanks folks - much appreciated.

 

I normally work with maximum 5 Ampere per mm2 copper wire. Although I know that copper is expensive, but not more than 6 Ampere per mm2.
Bert

 

What is the wire length between batteries and the electrical panel or junction box?
And what is the maximum admissible voltage drop?

 

I believe that at least the VHF could be considered of "critical application" which means no more than 3% drop.

 

The largest current will probably be when charging. Size the cables for less than 3% loss at maximum charging current.

 

The dimensioning chart on this website: https://www.bluesea.com/resources/1437 is made according to the ABYC E-11 standard for DC systems on board, and will give you the minimum required wire gauge for a given current and a given wire length. It is based on a 3% voltage drop.
As CDK said, it is important to correctly estimate the amps which will be drawn from the installed electric devices, plus a margin for possible future additions.

Direct link to the chart: https://www.bluesea.com/files/resources/newsletter/images/DC_wire_selection_chartlg.jpg

Cheers

 

This link is a good resource too.

http://www.bulkwire.com/wireresistance.asp

Figue out all the max amp draws, add them together, then allow room for some extra equipment you may want to add in the future. Enter that in my link along with the cable run length. Select the size that gives you the desired voltage drop.

Eg I entered 10A for 20' and it recommends 14awg for a 3.7% voltage drop. If you want under 3% drop 12awg gives 2.3% voltage drop.

I would use a the biggest sensible size wire you can just to join the 2 batteries in parallel, so the batteries charge and discharge equally. That won't be expensive for short link lengths. Then use the smaller size cable you worked out above for the longer run.

 

If you don't already have a copy, I can't recommend highly enough, to pick up a copy of Nigel Calder's "Boatowner's Mechanical and Electrical Manual". Ditto on pretty much everything else which he's published. It/they, will help you with a lot of projects like this. Plus, if you go to Amazon, & do a search using his name, in the books section, a lot of good reference & how-to books are listed. Most of which are good to have in one's reference library. Whether one's a true Newbie boat owner, or an experienced hand & tool guy.
Call it a Christmas present to yourself. And also, good luck with the project.