Battery Cable Size?

Discussion in 'Electrical Systems' started by Katoh, Oct 20, 2011.

  1. baeckmo
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    baeckmo Hydrodynamics

    You will do fine with 50 mm2 cables here. Use good quality, rubber shielded welding cable as proposed, with pressed cable fittings (NO soldering!!!!). The problem is seldom the cable resistance per se, but bad contact between fitting and copper strands causing heat with further loss aso.
     
  2. Katoh
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    Katoh Senior Member

    Gentlemen
    Again lots of information. You're all correct in the shock of the price of cable, excuse the pun! at nearly $30.00 meter for 4/0 is quite expensive and then add shipping, so nearly double that.
    The specs on the Alternator are only 60amp, I think this should not be part of the equation as we are designing for start-up load not charge load which is a lot less than that. With charging also as long as your cable can handle the current you are fine, I cant see how using bigger cable can overcharge batteries? Or am I reading this wrong?
    On my last vessel I had both batteries again forward undercover, a smaller run of 4.5m in cable and each battery went to its own outboard, (johnson 90's 2 strokes) and for the life of me I cant remember the cable size, but I know it was quite significant.
    I have been offered some cable that is double shielded water resistant and chemical resistant and heat resistant to 105DegC, but the problem with this cable its 4 core and each core is 7.55mm, I have been told by joining all the cores to one you will effectively have the capacity of a 28mm cable, could this be an option?
    Thanks
     
  3. MechaNik
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    MechaNik Senior Member

    I don't think anyone mentioned Voltage drop. It is the most important factor why you should have big cables for starting. Nothing worse than getting 6v at the starter when you turn the key.
    At lower voltages the amps increase in the starter motor giving it a hard life and poor starting ability as it will turn slower. Same deal when you spec battery V/ah.
    When pricing cable you can also consider using multiple lengths to make up the cross sectional area. Sometime big cable comes at an inflated premium due to the small quantities sold.
     
  4. aranda1984
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    aranda1984 aranda1984

    Battery cable

    First of all, you can run many wires in parallel, the final result is the same.

    Secondly: a very long cable run, with undersized cables will result in voltage drop. Charts are available for this in the business, but they are sized for 100% duty cycle!

    Quote: "The problem is seldom the cable resistance per se, but bad contact between fitting and copper strand"

    This so called bad contact is actually what shows up as a resistance (R) in the equation.
    The resistance will generate heat, resulting in voltage drop and accelerated corrosion.

    You need the right crimping tool to have the perfect connection between the lug and the cable.
     
  5. viking north
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    viking north VINLAND

    Better yet track down an old military scrap yard - I use aircraft wiring-- the ultimate tinned and super insulated -- with the added bonus each and every wire is numbered. Your tax money at work and finally of good use---
     
  6. Katoh
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    Katoh Senior Member

    I found a really good little program , free download what gives you everything you need, to those that are interested.
    http://www.midcoast.com/~aft/program/wiresz20.zip
    Not many old aircraft dismantler's round our part of woods, the biggest thing they sting you on the freight to get it over here from the states. Works out at double the price.
    thanks to all.
     
  7. cyclops2
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    cyclops2 Senior Member

    When in doubt.

    Look at a automobile starter cable for a engine of about the same size. Gas to gas & diesel to diesel. That will work. Not sure. Go 1 size larger. NO LARGER THAN THAT.

    The wire size does not cause the problem. It is the quality of the lugs, their compression around the wire. PURE Copper wire & lug? Clean & correctly tightened hardware.

    So much electrical stuff is way below minimum standards today. METALLIC TYPE WIRE Tin coated. I have a Copper magnet. YEPPER. It is attracted to modern Copper. Crap wire. :eek:
     
  8. Katoh
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    Katoh Senior Member

    Gentlemen
    When determining the length of cable to be used in the calculations is it the length from battery to starter? Or from battery to starter then back to battery, complete circuit length?
    Thanks
     
  9. cyclops2
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    cyclops2 Senior Member

    It is always the TOTAL length of wire between the + & - posts. Including any trips thru a AUX. relay or converter or anything else the HEAVY starting motor current MUST GO THRU.

    Some boats have series / paralell selectors. Some even allow you to patch in the "" house batteries "". A real NO NO.
    Dorky boat builders allow smaller cables for house batteries. EVEN if they allow them to be switched to starting duty in a emergency.
    " I had to change my house batteries abut a week after that emergency use of them. Of course you had to. The current flow they provided overheated them & twisted the plates. Did you notice the sudden corrosion on the terminals & other metal parts ? Any wet acid on top of the battery surface ?

    USCG regulations are very clear about that use.
     
  10. Katoh
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    Katoh Senior Member

    Without getting to technical, why is the length a round trip? In mains power or even larger DC power we design from power unit or outlet to motor or whatever is using the power, we don't add going back to the source.
    Seems strange, even in my case a run of 5.5m or 18' would suddenly double and you would be designing for cable double the distance 36'. If that was the case 4/0 cable would not be large enough to cope with the 260A at startup.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    Katoh,

    Because that's how far the electrons have to travel in a direct current (DC) circuit. It's a pretty basic rule of DC.

    -Tom
     
  12. cyclops2
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    cyclops2 Senior Member

    Same rule for a 120vac circuit also.
    If the electricty goes to the motor, it needs a return path for all the electricty that went to the motor. The goers need a seperate return path to get back to the battery as more keep leaving the battery each second.
     
  13. Submarine Tom

    Submarine Tom Previous Member

    They don't leave each second, they leave continuously until the circuit is opened or the battery goes dead.

    -Tom
     
  14. viking north
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    viking north VINLAND

    Exactly--Battery wise ---what goes out on the negative (conductor)must return on the positive (conductor) (electron flow) or visa versa (field flow)
     

  15. rxcomposite
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    rxcomposite Senior Member

    I just posted the 12 v cable sizing here Anchor windlass circuit breaker sizing.

    Consult Volvo penta manual for the cranking ampere of the starter motor. Also check the maximum cranking ampere (MCA) because you have to size your battery too. Batteries are also rated by their MCA.

    The ampere rating of the cable is the same going in or out. It is the voltage that is divided between the positive, the load, and the return (or ground).

    Viking,

    Aircraft wires are not tinned, it is aluminum alloy and cannot be soldered. If it is copper tinned, the wiring will be too heavy and the aircraft will not fly. I use them too and they are great. Lightweight too.
     
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