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Old 19-06-2022, 20:21   #1
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Common negative

I am planning on replacing my catamaran's electrical system. I installed it 22 years ago with nice tinned wire but I would like to make it much simpler and easy to diagnose.

I thought one good idea would be to use a common negative for all circuits leading to one hull. A nice big wire with some nice terminal blocks allowing quick testing at each intersection.

On reading Nigel Calders book, he states that you should not have a common negative. Can anyone advise why this is the case? My catamaran is composite. I have no metal skin fittings, no large engine with grounding and no AC power. I can't see the problem with having all the circuits sharing a single negative feed. Surely I can just switch the positives and have a large breaker on the negative in case something goes awry. I think that using a single negative with terminals would be just like a long bus bar. Saving lots of room and making it easier to work out wiring and problems.

I was hoping that I could simplify the switch panel if I only had the positives in the board area and that at the moment, testing whe a circuit is malfunctioning is tricky with the lack of terminal blocks and long wires leading to the switch board.

cheers

Phil
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Old 19-06-2022, 21:22   #2
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Re: Common negative

Quote:
On reading Nigel Calders book, he states that you should not have a common negative.
I think you misread or misinterpreted what NC wrote. A common negative is required by ABYC Standards and good practice. If you want one in each hull, they need to be interconnected by heavy cable to ensure they are both at the same potential.
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Old 19-06-2022, 21:48   #3
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Re: Common negative

If I understand what you are suggesting do not see a problem as long as you do not share a large surge load with some more sensitive circuits. As an example would not share a starter return with say my autopilot. Also do not make the common negative/ return too long and be generous with conductor size.

Under most conditions with a ground/ bonded system there is no overcurrent protection in the negative side. One often overlooked advantage of a proper distribution system (other than avoiding an electrical fire) is device/ circuit selectivity. This means when the wiring/ equipment has a problem the overcurrent device (breaker or fuse) clears that problem and only that problem. Don't want to be holding a flashlight in your teeth because the compass light shorted out and took the interior lighting with it. The downside of too much sharing.

Depending on complexity of electrical system might have several bus type terminal strips all connected together on one end with the common return. Kind of helps out with the book keeping.

IMO grouping the common/ returns on a fixed backboard behind the movable front circuit breaker panel makes a lot of sense and is pretty typical. I have 7 sub panels on my 32' yacht and each one has at least one bus terminal strip with a return conductor back to the main ground bus/ shunt and battery negatives. Also a couple of fuse blocks for the pesky things that need 12V but no switch or have their own off switch (holding tank full light, FM/ stereo ect).

I have a pdf diagram of the system if you are interested in how an slightly anal old EE set up his system, but it is on another computer.


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Old 20-06-2022, 13:34   #4
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Re: Common negative

Thanks, great points.

Phil
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Old 20-06-2022, 13:55   #5
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Re: Common negative

Personally I like a common negative bus. The only real issue I have seen is that when the DC current flows in widely separated locations you'll get a magnetic field from the wiring that can disturb your compass or other sensitive gear. When + and - wires travel together as a pair they almost cancel each other out (almost because to be a perfect cancellation they would have to occupy the same space - > coax). When they are widely separated they will not cancel. You will end up with a static magnetic field around your wires, and its strength will depend on how much current they are carrying -> the distance required to keep your compass from being affected will depend on the current.
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