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Old 13-10-2020, 11:36   #1
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Common ground

Question.

Fiberglass boat is grounded to the battery, which goes to the engine transmission, are thereís any drawbacks as using this as a ground point for all 12v systems?

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Old 13-10-2020, 19:20   #2
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Re: Common ground

how did you ground a fiberglass boat?...

you should have a neg bus bar near the batteries. that join all the battery banks, engine, house panel loads, AC ground wire, and grounding bus wire at a single point.

your engine block is not a bus bar. (though often used as one...)
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Old 13-10-2020, 19:28   #3
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Re: Common ground

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how did you ground a fiberglass boat?...

you should have a neg bus bar near the batteries. that join all the battery banks, engine, house panel loads, AC ground wire, and grounding bus wire at a single point.

your engine block is not a bus bar. (though often used as one...)

Thatís what I have, but my question was is that a proper termination point for everything?
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Old 13-10-2020, 19:53   #4
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Re: Common ground

That is a pretty typical installation. It makes it easy to charge a starting battery + a house bank from the engine alternator. You need a common negative for both batteries and the alternator is usually grounded through the engine block.

There are some installations that disconnect both positive and negative at the engine when the engine switch is off. Overall these are not as common, and all of the negatives are still connected (and to the block) when the engine switch is on.
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Old 13-10-2020, 19:54   #5
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Re: Common ground

Second the suggestion to locate the 12V common bus bar near main battery bank. I share a common return for both the engine and alternator but others prefer independent conductors. You can make your own with a short section of copper bar (1" X 1/4" X ?) or buy one from Blue Sea. Mount it on dry plywood or starboard to keep it simple.


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Old 13-10-2020, 20:01   #6
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Re: Common ground

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Originally Posted by SalingSue View Post
Thatís what I have, but my question was is that a proper termination point for everything?
Yes that is the proper set up per ABYC
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Old 13-10-2020, 20:08   #7
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Re: Common ground

Why use a bus bar for negative instead of the engine block?

What's to gain?

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Old 13-10-2020, 20:13   #8
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Re: Common ground

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Why use a bus bar for negative instead of the engine block?

What's to gain?

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Old 14-10-2020, 22:31   #9
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Re: Common ground

And what if you dont have an engine block? (ie electric motor)
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Old 14-10-2020, 22:45   #10
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Re: Common ground

sounds like a stupid idea to use an engine block for more reasons than the simple fact that engines are stupid.


it is a different metal and will corrode like crazy at the connections and also thermal changes will loosen the connections and then corrode more. You need special screw connectors to connect aluminum to copper wire to avoid issues over time and thermal changes, so it's best to keep all wiring brass and copper which are the best conductors if possible.
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Old 15-10-2020, 06:56   #11
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Re: Common ground

Anyone who thinks that brass is a good conductor of electricity hasnít actually looked at the numbers. Most brass is only 20-25% as conductive as copper. Tin/lead solder is worse: about 11%. And stainless steel only about 5%. Cast iron is 5-10%. Thatís a good reason NOT to use anything but copper as a conductor.
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Old 15-10-2020, 07:17   #12
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Re: Common ground

The fact is that for the vast majority of engines sold the alternator case is the negative half of the electrical equation. The alternator case is electrically connected to the engine block through its mounting hardware. The same is true for the starter motor.

In order to start your engine from the start battery and then to charge the start battery from the alternator you have to have a negative connection from the battery to the engine block. With some very specific exceptions that's just the way it is, take it up with the engine designers.

If you want to charge the house battery from the same alternator then the house battery too needs an electrical connection to the block (unless you add an echo charger or similar). This is how the vast majority of boats are wired. And a large number of problems with engine starting, charging, and engine instruments can be chased to the negative connection at the block.

It''s pretty easy, though, to make that connection sound. And for those with a bit more skill you add a negative connection to the alternator case.
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Old 15-10-2020, 07:30   #13
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Re: Common ground

While it’s true that engine designers often use the alternator/starter case as the negative conductor, that’s for ease of manufacture and installation. For the starter motor, where it’s typically used for a few seconds, or for the OEM alternator with an output of a few tens of amps, using a block ground isn’t a big deal. But install a high-output alternator on the engine and it becomes a much bigger problem, which is one reason that bigger alternators are often sold with an isolated ground.

Will a block ground "work?" Probably. Is it a good place to tie all your negative wires? No.
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Old 15-10-2020, 07:47   #14
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Re: Common ground

Quote:
Originally Posted by Bycrick View Post
Anyone who thinks that brass is a good conductor of electricity hasnít actually looked at the numbers. Most brass is only 20-25% as conductive as copper. Tin/lead solder is worse: about 11%. And stainless steel only about 5%. Cast iron is 5-10%. Thatís a good reason NOT to use anything but copper as a conductor.
While this is literally true, it's not actually particularly important in this application. Usually the conductive path of the piece of brass or cast iron or even stainless steel is generally very short and large in size, for example an engine block or a brass terminal strip, so there is little measurable voltage drop at these points.
Low conductivity becomes important in longer conductive paths, for example in wiring. You can check this yourself, just measure the voltage drop across the engine block from one end to a connection at the other end.
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Old 15-10-2020, 08:11   #15
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Re: Common ground

Quote:
Originally Posted by Bycrick View Post
Anyone who thinks that brass is a good conductor of electricity hasnít actually looked at the numbers. Most brass is only 20-25% as conductive as copper. Tin/lead solder is worse: about 11%. And stainless steel only about 5%. Cast iron is 5-10%. Thatís a good reason NOT to use anything but copper as a conductor.

This is miss leading at best, not true at worst. Stainless and Iron are both *very* good conductors. Compared only to copper, copper is better. But both Copper and Stainless are both so much better than an insulator, that the difference isn't that relevant. It's worth noting that lead is pretty close to stainless, and lead is used for battery terminals. Stainless is better than mercury, which is used in mercury switches.

https://www.thoughtco.com/table-of-e...ctivity-608499
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