Connecting 120V grounding to DC negative

[mikeporterinmd]

I've been installing a shore power system on Shelly IV. Nigel Caulder's
book discusses at length why you should connect the 120V grounding
connection (green) to the DC negative system in the boat. While this can
cause galvanic corrosion, safety concerns should prevail.

Briefly: the concern is what happens if a 120V/DC device, such as
a battery charger, shorts 120V to the DC negative? If that happens,
then 120V flows through the DC negative and generally into the water.
Also, 120V is then live on the DC negative, which could be dangerous on
the boat as well. So, by connecting the DC negative to the 120 V
grounding, we give the electricity a good path to ground, hopefully
resulting in a breaker tripping. The downside is galvanic corrosion.

However, when installing the Blue Sea panel, there were no directions
to make such a connection.

How are newer C-Dory's wired? Do they have the 120V grounding/DC
negative connection? How about galvanic corrosion? Are your boats
equiped with galvanic isolators? Or, given the small amount of
metal in the water, is galvanic corrosion on an outboard boat not
much of an issue?

Or did I misread the book (It's outside right now!)?

Thanks,

Mike

[thataway]

Hi Mike,
The TC 255 has a galvanic isolator. My asumption is that is because of the aluminum Armstrong bracket--rather than the one thru hull for the raw water intake.

The only place I see a potential for green wire--neutral to the negitive part of the 12 volt system is thru the battery charger itself. The outlets have GFI on them. The water heater is not specifically protected--I just wired the Air conditioner to the green neutral--same with second battery charger and microwave ciruits.

ABYC code does call for the green wire to be bonded at one point to the engine block or a single point. .
http://www.marinco.com/docs/guides/Boater'sGuidetoACElectrical.pdf
this is a Marinco PDF brochure on wiring--but it does give essences of ABYC code--and also shows the various plugs and adaptors--probably not a bad link to have for wiring projects.

Nigel Calder points out that Negitive 12 volt from the batteries, the engine block, the green wire, central bonding strip and ground connections for radio ground should be to the same point. Also Nigel's book is geared to the boat kept in the water--and basicially an inboard--as trailerable boats there can be slightly different issues.

I am not sure I agree--and there is certainly some disagreement about the lightning and radio grounds. many prefer to leave these "floating"--to avoid negitive ground loops.

I will check the TC 255 in the AM to see if there is a direct connection between the green wire, and ground--but to my recollection the only place might be at the isolator and the battery charger negitive.

I would not do this ground connection at the switch terminal. I would do it at a battery or battery charger if I was to do it. One issue is that european wiring is different than US. In the US the neutral is tied to the ground at some point--usually near the house meter. On our boat docks I always put in a second ground point right at the boat. What I have found with long runs of wire, even code certified for wet environment and run in conduit, is that there can be a small amount of voltage leakage and this can raise the voltage potential in the neutral--so I want it back down at ground right before the cord goes to the boat. (this probably only applies to peole who have their own docks or lifts).

[mikeporterinmd]

We'll be keeping the boat in the water most of the summer, hence interest in
galvanic corrosion. Of course, I have the option of a quick haulout to check
the condition of the outboard's zincs. What I haven't checked yet is if the
underwater components of the outboard are grounded to DC negative or
if they are isolated.

At this time, I have the green connected to the grounding bus in the panel.
From the grounding bus in panel, a short piece of 10 gauge to an external DC neutral
bar, and then about 16 feet of six gauge back to the main neutral post where the
batteries are connected, along with everything else.

Oddly, there only appears to be the batteries and two fairly lightweight
wires on the post. The batteries only have a single cable to each post.
I'm not at the point where I'm changing existing wiring, but
the lack of DC neutral cabling would explain why whenever I trim the
outboard, the low voltage indicator on the GPS goes off (with the engine
running of course.) The low voltage setting is 12V.

I like the way the project is working out. The only downside is that I needed
6 gauge between the charger and the batteries because the charger can put
out 30 amps to either bank. I wanted to stay near 3% voltage drop, and the
round trip is about 30'.

Technically, 6 gauge won't do it, but the West Marine catalog has a neat graph
showing voltage drops. The graph shows 30', 30 amp, 6 gauge just slightly
into 4 gauge. I could have those numbers slightly off since I'm writing this
from memory. Coulda run a bunch of 10 gauge, but the 6 gauge cost less than
3x10 gauge. Plus, power strips would be needed, which should be covered, etc.

Anyhow. It's been fun so far. Made a nice little teak trimmed frame for
the cutout for the panel, and a sliding 'window' to keep spray out. Panel
is port side, over top of the water tank, along with the charger and an outlet.
I'll post photos sometime when things look a little neater.

So, to pop for the galvanic isolator or not, that is the question.

Thanks,

Mike

[Sea Wolf]

Mike-

Whether to spring for the ~$400 galvanic isolator is a good question: Some thoughts:

1. What type of water will you be mooored in? Salt, brackish, fresh?

2. Can you insert a sensitive volt meter in your boat in series between the green (ground) 120v lead and the 12v ground (formerly black, now yellow) and see what voltage potential exists in your slip between the ground and the water, then base the isolator decision on the voltage push? I 'd think a sensitive voltmeter would be a good tool, but I also understaand that Guest makes a special meter for this: LINK .

3. Another "trick" I've read about is to hang a zinc near your boat tied directly into the marina supply outlet's ground wire. This zinc effectively takes on most of the current potential between the ground wire and any stray currents in the marina water that would lead to galvanic corrosion in your boat. I could even see this leading to a wire loop going down both sides of your slip with multiple zincs on it to surround the boat with protection as much as possible.

Joe.

[thataway]

I checked and do not see a direct connection on the TC 255 between the green wire (neutral) and a common negitive. The engine block and bracket (including the zinc on the bracket) will be attatched to the engine ground--or negitive.

I ran two more #8 pair from the batteries to the dash pannel-and run the electronics off the house when starting. I have an ARC which keeps the house isolated from the start battery. Although we don't usually get into electronics protection on smaller outboard boats, I think that this is definately worth while for the C Dory 22, with a good set of electronics. My impression is that most of the older 22's are under wired--in fact the current boats are underwired for what most of us put on them.

The reason for the high capacity battery charaging is that you run a genset and charge the batteries rapidly. But a rule of thumb is not to charge batteries more than 25 % of capacity. If you have a 100 amp hour battery, generally charge at no more than 25 amps. (You may get by with a bit more charge with an AGM), but need a temperature sensor from the battery charger. Rather than use an older 30 amp charger I had, I sprung for a new solid state with temp compensenation and various battery setable perameters.

The yellow ground (called "safety wire"--so that it is not confused with the black negitive 12 volt and hot 110 volts)--is optional--and my C Dory has black and red scheme for the 12 volt circuit--except on some runs when I have put in safety wire (also helps to identify which circuit). I also like to put a plastic label around each wire at the ends--so you can trace them back when trouble shooting in the future.

Good wiring!

[mikeporterinmd]

[mikeporterinmd]

Bob,

I was wiring in the battery charger tonight. It called for a sep. connection
to the DC bonding circuit in addition to the connection for battery neg.

Perhaps that's where you have a connection between AC grounding and
DC bonding. I did not verify conductivity between the grounding and
bonding points on the charger. I'll take a look at that some time.

Mike

[thataway]

Mike,
Yes, most battery chargers have a green wire and in many it goes to the case & chassis (at least in the past). The Guest which the TC came with does have a green wire connection. Good point to check continuity to the negitive.

If you do potentials in the boat, one usually uses a silver chloride electrode to dangle in the water. Not real cheap but I do check my boats regularly. Most good digital volt meters are adequate for this measurement.

[Sea Wolf]

My Sea Ray* has a "Mercathode" system on both engines and the combined boat metal structure which has a system which senses the galvanic corrosion potential and effectively blocks it by applying a reverse potential (voltage).

There's a very good discussion of galvanic corrosion from Mercury Marine HERE.
It's about stern drives, but has a lot of information that applies to outboards, too!
Joe.

*Sorry, Don, the Sea Ray Intruder strikes again!

[flapbreaker]

Hmmm. All this shore power stuff seems complicated. I have not used it yet but I thought I had a system all figured out. Someone tell me if I'm gonna blow something up with this.

Basically I have a 12 amp on board battery charger that has a wire going to the positive and one to the ground on each battery. The battery charger is then plugged into a GFI recepticle box that also has a breaker fuse. From there I was going to plug the 3 prong cord from the GFI into a 15amp to 20 amp converter so I could plug into the marine power. Seems pretty straight forward but I'm no electrician. Anyone see potential problems?


To

To

To

To the dock cord.

[Sea Wolf]

flapbreaker-

If I understand your system correctly, it looks workable to me!

With a positive and negative wire going to each battery, I'm assuming that the charger has two separate charging circuits, right?

Im also assuming there are individual switches to shut off or isolate each battery? E.G., a 1/both/2/ off switch? It's best to isolate them during charging, or they'll be in parallel, and a short in one can result in the other one discharging through the shorted one and the electrolyte can boil and become steam!

Are both batteries of the same type? (flooded wet cell, AGM, or gel cell?) Most chargers are not designed to mix types, and some are designed for wet cells only.

Does your instructions with the charger say to connect it separately with a ground wire to the 12 volt negative ground wires in the boat?

Good Luck!

Joe.

[mikeporterinmd]

You should put inline fuses or breakers on the positive wires from the
batteries to the charger. The fuses should be near the batteries.

Boats, at least old ones, were often wired without these if the wires
were protected and the batteries not too far from the charger.
So, if the charger is relatively close...

Also: what does plug into marine power mean? Remember that
most marinas are wired for 30 amp service. So, whatever wiring
you use between the marina and your boat needs to have an
ampacity of 30 amps (meaning it can carry 30 amps without
catching on fire). Once you get to the breakers on your boat,
wiring past that point can be sized to match the breaker on your
boat.

So, putting some adapters on to a 16 gauge extension cord is a
bad idea.

Some marinas have 15 or 20 amp service available, but it usually
installed for those working on the docks and not really for boater use.
So, the outlets tend to be rather spread out.

Mike

[oldgrowth]

[flapbreaker]

[flapbreaker]