DC Bonding

A/C & D/C Wiring, GPS, Radios, etc.
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jtalberts
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DC Bonding

Post by jtalberts »

With all the investigation into my current AC/120 issues I will be bonding my new electrical system to my DC ground. Looking over the engine grounding in the engine compartment, I have notice that each engine is grounded to a separate, factory installed, grounding point. It also appears there is a wire that bonds these two points together. From looking at other parts of the boat, I have noticed a few things that have been grounded to the hull in some areas. I have read that this isn't good and I should have them all bonded to a DC grounding bus and only the main connection at the engine block to the hull.

Do any of you have any experience with this? Should the engines be connected directly at the same spot or are the two spots being tied together correct? For the other items I have seen randomly grounded to the hull, should I rewire them to be connected directly to a grounding bus or are they fine? I want to get the electrical system on this boat cleaned up a bit and I hate doing work twice.
1973 Marinette Seacrest Executive
41x14
Twin Chrysler 340's with Velvet Drive 71C
"Figment"
Cabinfever
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Re: DC Bonding

Post by Cabinfever »

I have always been told to have your 12V grounds on the engine block. On my M, the grounds are on the floor support next to the engine, all in one place, except for the fuel sending unit ground. I would move your grounds off the hull and put them where the factory grounds were installed. I haven't got into my 120V wiring yet.
Joe
1977 28 Express
Single 318
(RESTORATION IS ALL THE ROUTINE MATIANANCE AT ONCE)
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GB49
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Location: Catawba Island, OH.

Re: DC Bonding

Post by GB49 »

To avoid a web of circuits throughout the hull all devices should be bonded to specific, common location(s). The designated location(s) are the battery negative connections. These do contact the hull but only in a couple places. The engines are also bonded to these locations.
The only 12DC hardware that appears to be factory connected to other areas of the hull are the fuel sending units (the sending units themselves are restive and don't use voltage to read fuel level), trim tabs and possibly the windshield wipers (not verified the wipers yet but I see a little ground wire coming out of case to mounting screw).

I have bus blocks around the boat but everything ultimately runs back to a battery switch for positive and back to negative lugs in engine room. Nothing else touches the hull, anywhere. So far so good.

As far as tying the 2 engines together, I see no reason since each engine should be grounded to its respective lug which is welded to the hull. Obviously the lugs are tied together via the hull and are generally a couple feet apart. Zero wires or 100 wires connecting the two engines does nothing but create a bunch of parallel connections for no reason, in my opinion.

-Karl
1986 Sedan 32'. Twin Chrysler 360in^3, 275hp.
Forum member since 1998.
jtalberts
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Re: DC Bonding

Post by jtalberts »

Thanks for the replies. I now need to buy some grounding busses. One of the POs felt the need to skip the grounding block that was installed at the fly bridge and take it directly to the hull. The 12volt system is in good shape except for these few things. I would like to run a main power feed to the bridge so I can eliminate a few runs through the conduit. It is getting tight.
1973 Marinette Seacrest Executive
41x14
Twin Chrysler 340's with Velvet Drive 71C
"Figment"
barkleydave
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Location: Kentucky

Re: DC Bonding

Post by barkleydave »

Just so everyone knows what the topic is addressing.

1. Our M's have two main grounding points on most it is on the forward bulkhead an ear where the negative ground cable goes from the battery directly to this point.
2. Other ground busses are located at each station and that ground wire also goes to the same point. The engines each have a separate ground attaching to the main stringer port and starboard. THESE ARE GROUNDING POINTS NOT BONDING POINTS.

Ok for clarification:

Bonding: Bonding is used on boats (NOT OUR M'S!) to provide some protection from lightning by providing a path to ground. (that being the water)
There are numerous articles discussing this "Cone of protection" . Each metal component on a fiberglass boat has a solid copper wire connected to a through hull etc. Sea Cocks, stations, rudder etc. On some boats they all connect to a central point (called a Dynaplate) This plate is hundreds of small metal pellets pressed together. This acts as a large metal plate for electrons to pass through to ground. It is NOT used as an electrical ground.

Due to the nature of our aluminum boats metal fitting must not be bonded. (thus the isolation of shafts, through hulls etc. )

There is more to it...but I hope it may help understand the differences.

Why do we avoid using the hull as a DC ground? By reducing the grounding points... to an isolated area you are reducing resistance. The more direct the bath to a clean grounding source (your battery) the less voltage drop. We are not actually grounding with DC current. A more appropriate term would be the negative return path to complete the circuit. It also has been referred to as a "ground loop".

One other note: When figuring the gauge of wire you are going to use you must calculate the length of the total round trip run. Not just one way.
In light circuits less that 10 amps this is not a big deal but in applications like a windlass, inverters etc. it becomes very important.

safe boating,
dave
1987 Marinette 29 FB Sedan
Retired Boat Accident UL and USCG trained investigator
Retired USCG Captain
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