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Old 30-05-2006, 16:50   #1
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Ship's Common Ground

I am installing a chatplotter at the helm and was trying to decide if I should run the ground wire all the way back to the electric panel or take the shorter route to the battery ground, which goes to the engine block. I then read an article on that said that all grounding wires, including the battery cables, should go to one common grounding plate (a heavy bus bar). It said not to use the engine block as the ground and that using the engine block as the common ground will cause corrosion problems.

Should I run my ground to the battery, to the ground back at my electrical panel, or should I install a bus bar and send everything to it? How big would this bus bar need to be on a 30 foot boat?


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Old 31-05-2006, 00:56   #2
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I don't believe there is any real harm in using the engine block as the common negative battery terminal on your boat. However, that is probably not the best solution. If you have more than one battery and a switch to select one or the other or both, then you probably have a common negative battery post for the batteries installed, most likely near the batteries or the switch. Trace the negative battery cable from your engine back to the batteries to find it. If that is convenient to your wiring for the chart plotter use it. Otherwise run duplex wire back to your electical panel.
If you want to replace whatever you have as a common negative terminal with a heavy duty bus bar (makes adding additional high load circuits easier), some of the more knowledgable electricians on this forum problem can provide specific guidance. I would just determine the maximum current required for the starter motor, double that value, then buy a bus bar rated for that current load.


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Old 31-05-2006, 03:11   #3
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Your Chartplotter (and other DC devices) has a DC Positive & a DC Negative (which is “grounded”. Get in the habit of thinking Pos. & Neg. power supply, leaving the terms Ground, Grounding, and Grounded for their other, more appropriate, applications.

As John implies, you probably have more than one DC Negative (ground) bus (ie: at the DC Distribution Panel, Accessory Panel, and Battery Bank, etc.). All of these DC Negatives may be grounded at the Engine Block, or an immersed Ground Plate.

Provided that all the DG Negative cables (connecting the various Negative Blocks) are adequately sized & installed, it does not really matter where you connect the Chartplotter Negative - so chose the nearest most convenient Negative block (probably proximate to your DC Positive source).

Take a look at the (2) diagrams.

Basic DC Grounding Diagram:

Basic DC Wiring Diagram:

Don’t hesitate to ask further questions, or reiterate your query, if we’ve been (will be) unclear.

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 31-05-2006, 09:06   #4
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Zippy, it is electrically okay to run your negative lead to any ground bus, however, when it comes time to truoubleshoot an electrical problem you are going to have real problems. Unless: your cables are all very well labled and your wiring diagrams are kept up to date. Even so you are making life more difficult for yourself.

Running duplex cable, as John mentioned, makes troubleshooting so much easier. That is, I believe, why switch and breaker panels are equipped with both positive and negative terminals. Having equipment postive and negative wires coming back to the same panels allows you to go directly to the wire at either end of it, which is a great help. That is so long as your switch/breaker/fuse panel is labled properly.

Troublshooting electrical problems is something you will be doing as you go boating. Adding or removing electrical devices is never ending in todays world. Being able to find wireing connections easily will make the process so much easier.

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Old 31-05-2006, 09:12   #5
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Zippy, further to my last post: Do not connect negative leads together at the equipment and run one wire back to a negative bus. This is a dangerous situation and can lead to fire. Your negative wires are not fused and by connecting them together at the equipment you run the risk of overloading and so overheating the wire.

You must make sure that when adding equipment that all of the wire is protected by the breaker/fuse.

Another good reason (perhaps the best of reasons) to run duplex wire back to your panel.


PS: I know of at least one fire that was caused when a yard installed a larger alternator to an engine. The old alternator had an internal negative path (through the case) but the new alternator had a separate, isolated negative. The yard didn't install a new negative cable and so the only negative path back to the batteries was through the regulator. When the batteries reached a discharge level where the alternator started putting out more than the negative lead could handle there was a fire which resulted in the total loss of the alternator. Luckily the boat and people were okay, however the alternator had been installed in a rush to start an offshore trip and so resulted in a very uncomfortable trip to the Azores and to having no engine on arrival. Besides, fire extinguishers can cause a lot of corrosive damage.

PPS: did I mention that they had counted on a watermaker which of course they couldn't use?
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