12v Circuits - Rules of Thumb?

Discussion in 'Electrical Systems' started by SeaJay, Jul 6, 2010.

  1. SeaJay
    Joined: Jun 2007
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    SeaJay Senior Member

    I’m starting to conceptualize the wiring for the 46’ motorsailer I have under construction and am somewhat at a loss when dealing with 12v wiring strategies. I’ve done a fair amount of 120v residential wiring where wire size and voltage drop are pretty straight forward…you grab either the 12g or 14g and start pulling. However, I can’t see yanking a bunch of 8g boat cable to every light fixture on the boat, so maybe someone can point me in the right direction.

    IN GENERAL, is it more efficient (in the fullest sense of the word…i.e. Initial cost, energy use, labor, maintenance, weight, complexity, reliability, etc.) to pull a lot of small individual circuits from a centrally located panel, or is it more desirable to use a few perimeter circuits with larger cable and junction boxes/subpanels to supply the individual users?

    Are there some rules of thumbs like in residential wiring where you generally put the lights on one set of circuits and the duplex outlets on another?

    Regards,

    SeaJay
     
  2. CDK
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    CDK retired engineer

    There is one rule of thumb to be considered:

    Your 12v circuit uses currents that are 10 times higher, so the wire cross section must also be 10-fold that of residential wiring.
    Because normal fuses and circuit breakers have current ratings of 6, 10 or 16 amps the number of appliances that can be fed from one point is much lower than you are used to at home.
     
  3. SeaJay
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    SeaJay Senior Member

    Thanks CDK. I wasn't aware of the issue of smaller fuses/circuit breakers. That helps explain why I see such massive amounts of wiring entering panels on boats.
     
  4. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

    On smaller boats it's probably more economical to pull smaller runs to individual loads (one panel). On a larger boat, as you mention, running larger cables to supply separate nodes (buss's) is also common. You'll have to know the full load capacity of each node to correctly size the wire. I can see this is extremely advantageous on a cat where you're essentially wiring two separate boats. I have two buss's on my boat. One on the lower helm that supplies most of the boat's major systems and one on the overhead console that power most of the accessories. Both are fed from the main battery bank by individual circuits.

    Generally a good idea as well.
     
  5. SeaJay
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    SeaJay Senior Member

    Thanks Rick. I can see where on a cat the loads logically fall into a neat subdivision. For for some reason, I intutively want to use subpanels, although CDK's comments bear consideration.
     
  6. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

  7. missinginaction
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    missinginaction Senior Member

    CDK is as usual correct. It's going to be difficult in this format to get you all the information you need. You allready have a leg up on many people with your understanding of 120/240 volt residential wiring.

    I'd respectfully recommend that you go to a book store or your local library and find the following:

    http://www.amazon.com/Boatowners-Me...=sr_1_1?ie=UTF8&s=books&qid=1278442944&sr=8-1

    This well written and comprehensive text will provide you with all the information that you will need to properly configure your boats electrical system.

    With a boat of your size you might want to consider a 24 volt system.

    Regards,

    MIA
     
  8. SeaJay
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    SeaJay Senior Member

    MIA,

    I have, or I should say I had, lost, and have re-ordered Calder's book...a most valuable resource.

    Knotty,

    I've read a bit on the distributed systems and was interested in Calder's recent evaluation of the one he installed on his boat. They sound appealing with their promises of simplicity but I don't know if I'm quite sold on them. Nonetheless, they do seem to address the issues I'm concerned about. I don't have to make any decisions today so I'll keep an open mind.
     
  9. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

    That was just something I stumbled across awhile ago. I'm not sure either but there seems to be something there worth investigating. I think KISS is in order when doing that kind of wiring. 120 VAC and I don't get along.
     

  10. SeaJay
    Joined: Jun 2007
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    SeaJay Senior Member

    Knotty,

    Yes, KISS is the order of the day. Electrical problems can be such a headache to resolve which is why I want to really simplify and bullet-proof my wiring when I put it in.
     
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