Stepped down conductor size

Discussion in 'Electrical Systems' started by Scott Carter, Aug 9, 2010.

  1. Scott Carter
    Joined: Oct 2006
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    Location: Annapolis

    Scott Carter Senior Member

    In installing a windlass I find that the #6 AWG leads coming out of the new Lewmar H3 windlass are about 4' too short to reach the power post of the old windlass installation. This is all about 25' forward of the battery, in the V-berth. The conductors coming from the battery/circuit breaker are #2 AWG, adequate for the max 12 VDC 110 amp load protection over that run.

    So my conundrum is this:

    Instead of having two power posts/junctions in the overall run like this:

    Battery/CB to post #1 = 25' (#2 AWG)
    Post #1 to post #2 = 5' (not yet run)
    Post#2 to windlass = 3' (windlass pigtails #6 AWG)

    I want to do this: completely replace the windlass' 3' factory leads with 8' ones of a larger gauge, but for space restrictions within the windlass housing smaller is better. #2 won't work. My calculator tells me that #4 is fine for the 8' run at the same load, which fits physically. But this, in effect, is reducing the gauge of the overall run from the battery to the load. To take this to an unrealistic extreme: as the run gets closer to the load the size could be repeatedly reduced (neglecting any loss at junctions, this is purely theoretical). A stepped, or tapered, conductor effectively? Thought I understood this concept hands down, but now that I'm overthinking it I think I don't really.
  2. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    #6 gauge or heavier wire is OK for the short term 85A current draw of the H3 so #4 will be fine. The important issue is the voltage drop in the wires. That is 1V for both cases, assuming you run #2 gauge between posts 1 & 2, so there is no difference. In very hot weather it might rise to 1.3V or so which should be no problem.
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    In a closed electrical circuit, the current is equal at any point, so tapering a conductor towards a single load doesn't make sense.

    AWG #6 copper wire has a resistance of approx. 0.0004 ohms/ft, at 100 Amps load the voltage drop is only 40 mV/ft. With 8 ft between winch an post #1, the total voltage drop is 16 * 40 = 0.64 V, acceptable by any standard.

    The AWG #2 part can be left out of the equation, the voltage drop there is negligible.
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