Wiring Gauge Question
 

Apologies if this has been covered already, had a search around but couldn't find anything similar.
I am adding a second battery and updating the wiring in the boat but keep getting conflicting information on what gauge wire I need between the two batteries???

Existing start battery is right next to the engine and wired with what looks like 4 or 2 gauge.
Future setup will have a voltage sensitive relay next to the start battery and a 100AH deep cycle house battery under the seats in the bow so the cable run will be 15-18 feet each way, see pic.

Some guys have told me 4 gauge will be fine, others have said 0 gauge or even 00 but I dont think i could even connect 00 to the VSR! There is a massive price difference and so I don't want to waste money but it needs to be safe? What do I need?

Notes
Engine is a black scorpion, alternator is rated 70 amps @ 2000 rpm.
The VSR is rated at 140 amps max.
Main items connected to the house battery will be headunit, amp (1500W), perfect pass and heatercraft heater.
We don't hang out on the lake for hours just listening to music, we are almost always towing someone.
If the house battery gets really low I'm happy to disconnect it and charge when we get home, boat always lives on the trailer.

Thanks

i don't have the expertise to advise you on the gauge you need. i've done a fair amount of wiring and i always have to ask for advice. the last "long" run (15-20ft) i did "0" was recommended. the one piece of advice i have is to look here for wire:

http://www.genuinedealz.com

i have zero affiliation with them, over the years i've made several purchases from them and a) their prices are usually better than anyone else and b) they've always gotten my order right. for a few bucks extra they will go ahead and terminate the wire and apply the head shrink.

There's an ABYC table on this page that can help you: http://www.marinewiring.com/abyc-mar...byc-standards/

ABYC is the standard-bearer for vessel construction so I'd try to meet their specs. The length of the run matters as well as the current.