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AC ground / DC negative connection on an outboard boat

AaronD

.
Aug 10, 2014
468
Catalina 22 9874 Newberg, OR / Olympia, WA
I've been doing some reading in preparation for a small shore power installation. It's clear that I should connect the AC ground (green) to the DC negative (consistent practice, from ABYC specs, Nigel Calder, @Maine Sail's posts at AC-DC ground, and other reliable sources).

And it's clear that I should make that connection on the boat side of the galvanic isolator - yes, I read Testing A Galvanic Isolator and found a fail-safe isolator (and no, I couldn't quite spring for an isolation transformer).

So far as I can tell, that ground -> DC negative connection would often be at the engine block. But it's unclear to me where to do that in a boat without an inboard. Should I use:

1) The boat side of the battery meter shunt (where all other DC negative connections come together)
2) A negative battery terminal (breaking the rule that all connections go through the shunt, but in theory this one shouldn't be carrying current, so perhaps a direct-to-the-battery connection is better?)
3) The negative bus nearest the shore-power inlet
Pro: within a couple feet of the inlet, galvanic isolator, and AC panel
Cons: An extra 10' or so from the battery bank (on AWG 2 cable); connects to the boat side of the shunt (so similar to #1, but with an extra 10' or so of (heavy-gauge) wire in between).
4) Some other location I'm not thinking of

How heavy should that AC ground -> DC negative connection be? 10 AWG (sufficient for the 30A shore-power current)? Or is there some reason it would need a heavier cable?
 
Feb 17, 2006
4,875
Lancer 27PS MCB Camp Pendleton KF6BL
There should only be one ground point. All grounds lead to that one point. You can have many branches of ground returns such as lights, radio, autopilot, tv, dishwasher, bathtub jets, massage machine, but all the grounds will lead to one ground point. The purpose is to prevent what is called a ground loop which if there is a difference of potential at two different ground points there there will be current flow. This is not good.

So, in a boat that does not have an internal motor it makes no difference. Even if it did have an inboard the single ground point is the battery negative terminal. AC ground and DC ground can share the same connection. That is how it is on my boat and I have had no issues.

JMHO
 

Jan11

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Apr 9, 2012
29
Ericson 35 Albany
AaronD - What Brian says in theory is correct, BUT in reality the main DC negative bus is the engine ground point on an inboard boat. The battery negative(s) is connected there, possibly through a shunt. On an outboard boat the ground point is in the same place, but without the engine - either at the battery terminal or if there is a shunt at the shunt terminal away from the battery.

Your guess #1 is correct.

He also says that all DC negatives must go to the main DC ground point. While this is ultimately true, but you can have as many negative DC sub buses as you want. Each of the buses is then connected to the ground point either directly or through another bus. What you can't have are multiple paths back to the ground point.

An example of what you don't want would be bus A tied to the ground point, bus B tied to the ground point and buses A & B tied together. This would be a ground loop and a source of noise on electronic systems.

The AC ground wire should be at least the same size as the line and neutral conductors.
 

AaronD

.
Aug 10, 2014
468
Catalina 22 9874 Newberg, OR / Olympia, WA
AaronD - What Brian says in theory is correct, BUT in reality the main DC negative bus is the engine ground point on an inboard boat. The battery negative(s) is connected there, possibly through a shunt. On an outboard boat the ground point is in the same place, but without the engine - either at the battery terminal or if there is a shunt at the shunt terminal away from the battery.

Your guess #1 is correct.

He also says that all DC negatives must go to the main DC ground point. While this is ultimately true, but you can have as many negative DC sub buses as you want. Each of the buses is then connected to the ground point either directly or through another bus. What you can't have are multiple paths back to the ground point.

An example of what you don't want would be bus A tied to the ground point, bus B tied to the ground point and buses A & B tied together. This would be a ground loop and a source of noise on electronic systems.

The AC ground wire should be at least the same size as the line and neutral conductors.
Thanks; that's exactly what I was looking for. DC shunt (away from the battery) it is. With 10 AWG to match the 30A shore power wiring.

FWIW, I do have DC negative buses in various places; I think I've wired them all correctly to avoid ground loops (at least so says my wiring diagram, although there's no guarantee that I followed said diagram perfectly :)).

Thanks again.