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  1. #1
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    Aug 2007
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    Default Bowthruster battery cable sizes

    I have just installed a Vetus 75kgf bowthruster and need to connect up the battery or batteries which are located adjacent to the thruster.

    Reading the Vetus instructions I require cables that are 150mm2 cross section area. To me this seems very excessive. In a previous boat I has a bowthruster that had much smaller cables (something like 35mm2). I can get 35mm2 cables made up very easily, but not for the recommended size. I can't even visualise the bigger size.

    Are Vetus being too particular? Are they over specifying the cable size? I plan to use either a single 120Ah AGM battery or 2x 90Ah AGM batteries. The cable run will be under 1 metre.

    What cable size does the forum recommend using?

  2. #2
    Join Date
    Nov 2006
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    I would recommend the size Vetus recommend, because they recommend it....

    You've just spent (presumably..) some k on it, why scrimp on 50 of cable?

    They've done the maths, worked out volt drops and safety factors and arrived at 150mm2. Use that

  3. #3
    Join Date
    Aug 2005
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    The cable run seems very short, do you have your battery bank well forward? The run includes out and back, so at least 2 M on your measurements. If it is this short then the drop won't be too bad, for the cables the biggger the better as the power these units use in short bursts is big.

  4. #4
    davidej is offline Registered User
    Location : West Mersea. north Essex
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    Is 150 mm2 really required if a separate battery is fitted 1m away from the thruster - sounds more like the requirement if you are using the main batteries and a 6-10m (each way) run.

    I would have thought the OP could use something much more modest.
    davidej

  5. #5
    Join Date
    Aug 2007
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    Each battery will be mounted on a platform directly on top of the thruster tunnel, almost touching the motor so the cable runs are very very short. In fact if I use two batteries I will be putting one each side of the motor so both (total) runs would be 1 metre maximum.

    So what size cable do you actually have? I have seen plenty of boats with bow thrusters fitted but I've never seen any 'monster' cables. Does anyone ever fit the recommended size, or do manufacturers fit what they think they can get away with?

  6. #6
    pvb's Avatar
    pvb is offline Registered User
    Location : UK East Coast
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    Quote Originally Posted by Bloater View Post
    Reading the Vetus instructions I require cables that are 150mm2 cross section area. To me this seems very excessive. In a previous boat I has a bowthruster that had much smaller cables (something like 35mm2). I can get 35mm2 cables made up very easily, but not for the recommended size. I can't even visualise the bigger size.

    Are Vetus being too particular? Are they over specifying the cable size? I plan to use either a single 120Ah AGM battery or 2x 90Ah AGM batteries. The cable run will be under 1 metre.
    The Vetus instructions specify 150mm2 cable for up to 18m of cable run, and this is to limit voltage drop to a max of about 10%. They could have usefully quoted different cable sizes for shorter runs. Interestingly, in the Vetus catalogue, they quote 70mm2 cable for up to 8m cable run. Obviously, if your cable run is only 2m total, you can use smaller cables. I'd suggest 35mm2 would be fine, and should result in a voltage drop below 0.7v, which is acceptable.

    However, the same instructions you refer to specify a minimum 143Ah battery (although the current Vetus catalogue suggests a minimum 120Ah battery). Regardless, I wouldn't run a 550A thruster on a single battery. Your suggestion of having 2 x 90Ah batteries is much better.

  7. #7
    Talulah's Avatar
    Talulah is offline Registered User
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    Quote Originally Posted by pvb View Post
    The Vetus instructions specify 150mm2
    Where could you actually source that size of cable from?

  8. #8
    bammylovechops's Avatar
    bammylovechops is offline Registered User
    Location : Live Manchester, Boat Conwy
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    You could try this site, it gives a cable calculation table.

    http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

    Worth checking twice and buying once
    It is better to have memories than dreams.

  9. #9
    pvb's Avatar
    pvb is offline Registered User
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    Quote Originally Posted by Talulah View Post
    Where could you actually source that size of cable from?
    I'm sure it's available somewhere, but most people would just use 2 or more smaller cables in parallel.

  10. #10
    markdj is offline Registered User
    Location : Strangford Lough, Northern Ireland
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    If you visit the Merlin Power Store website, they have a free program for calculating cable size and volts drop.

    I'd go for less than 10% volts drop and if you only have a short run ,then 5% is better.

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