Earth bonding (oh no not again)

[starman]

I’m plodding through my new electrical installation and have arrived at earth bonding: plenty has been written here about the AC side and I think I follow all that it’s the DC side that’s puzzling me. 
Im fitting a Victron shunt and following the instruction that all negatives return to the shunt and then a single connection to the battery. That includes the hull earth grounding lead (and also in Victron’s guide, the Multiplus case earth)

The question: what size cable for my 12v ground? Or how do I work it out?

 

Im also a bit bit bemused because it’s a trad engine solidly mounted to steel and therefore already earthed via the starter negative.  

[Tracy D'arth]

If your shunt is not designed to take the starter current, just take all the negs from all cabin supplies to the shunt then 50mm to the batteries.

[Jen-in-Wellies]

Assuming you have a separate engine starter battery from your house batteries, the engine battery negative terminal will be at hull and engine battery voltage. The house battery negative at a slightly different (mV levels) voltage, depending on what current is flowing through the shunt. You would use the same size wire from the engine battery to the engine as you've used for the starter motor positive. Personally, I'd use the same size wire to connect engine to hull. Also take a similar sized wire from the house negative on the house side of the shunt to the hull. The wire lengths will be short and then there is no question about their current carrying capacity. Currents flowing through them should be minimal, excepting the starter return wire from the engine.

The engine battery negative and house battery negative are not connected directly together, otherwise the shunt won't be measuring house batt current in and out.

[starman]

15 minutes ago, Jen-in-Wellies said:

Assuming you have a separate engine starter battery from your house batteries, the engine battery negative terminal will be at hull and engine battery voltage. The house battery negative at a slightly different (mV levels) voltage, depending on what current is flowing through the shunt. You would use the same size wire from the engine battery to the engine as you've used for the starter motor positive. Personally, I'd use the same size wire to connect engine to hull. Also take a similar sized wire from the house negative on the house side of the shunt to the hull. The wire lengths will be short and then there is no question about their current carrying capacity. Currents flowing through them should be minimal, excepting the starter return wire from the engine.

The engine battery negative and house battery negative are not connected directly together, otherwise the shunt won't be measuring house batt current in and out.

Now my old grey matter is seriously struggling. 
I already have a 50mm2 +ve to the starter solenoid, a 50mm2 -ve to the starter and a 50mm2 -ve from the start battery to the house side of the shunt. 
All correct so far?

You’re saying add a further 50mm2 cable from the house side to my hull bonding point (which will need to be close to my AC hull bond). 
 

I know one thing for sure - I wish I’d bought a Smartbank system like I had in the last boat. None of this complicated shunt stuff, just a relay a couple of wires and a box of tricks. 

Edited October 27, 2021 by starman

[David Mack]

 

20 minutes ago, Jen-in-Wellies said:

Also take a similar sized wire from the house negative on the house side of the shunt to the hull.

Doesn't that mean that the return current path from the negative of the domestic battery would run from the battery terminal, through the shunt and connecting wire to the hull, then through the hull and engine block back to the alternator body, which would be a BSS fail? Better surely to connect the house side of the shunt to the engine battery negative, and then share the return wire from there to the starter motor as the hull connection.

[Jen-in-Wellies]

9 minutes ago, David Mack said:

Better surely to connect the house side of the shunt to the engine battery negative, and then share the return wire from there to the starter motor as the hull connection.

Yes.

[starman]

17 minutes ago, David Mack said:

 

Better surely to connect the house side of the shunt to the engine battery negative, and then share the return wire from there to the starter motor as the hull connection.

Next question - how is that ‘close to’ any possible AC bonding point? Must be 12ins minimum on mine.