Shared neutral?

Estragon
Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
Likely a bad idea, but here goes...

My boat has a 2000w MSW inverter run off a pair of 6v GC batteries.  It's only significant AC load is a small microwave used only rarely away from the dock.  I'm redoing wiring for it, and was wondering if it would be possible to wire the AC input and outputs with separate live (hot) conductors, but a single neutral conductor and chassis ground?  The wire path from the inverter to nav station panel is really tight, and having one less (10ga) conductor would make the run much easier.
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Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
    Typically when a shared neutral is used it is with a split phase system or branch circuit such as a kitchen split receptacle, the neutral caries the unbalanced load. Being the inverter is single phase the additional live conductor would be of little benifit as a single 10 gauge circuit would exceed the inverters capacity and distance would not be a factor in the confines of a boat.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #3
    I'd need to see a drawing.   

    I am available for custom hardware/firmware development

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    To clarify, one line conductor would be ACin (shore power when available), the other would be ACout. In thinking more about it, the neutral can't be shared.

    Too many projects on the go, and I was thinking the in and out wouldn't be live at the same time. Of course this isn't true, as the inverter out is passing shore power (from the ACin) through to the ACout, so obviously the neutral can't be shared.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #5
    Sometimes it helps to just draw it. But you do not need to now :)
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Boat wiring for AC can get very complicated (between shore/genset/inverter power, ground fault protection, isolation, etc.).

    But--Be very careful here. Most MSW (modified square wave) AC inverters do not have an Isolated AC output. There is not "neutral" for MSW inverters--The AC outputs are both "hot" with respect to the DC battery bus.

    If you make an AC "Neutral" and tie it to boat/safety/chassis ground. And have your (typical) DC negative battery connection connected to the same green wire/safety/chassis ground, you will effectively short out your MSW inverter and fry it.

    Most Sine Wave (pure/true PSW/TSW) AC inverters use an isolation transformer and you can can make a grounded neutral AC output.

    Notice I say "most"... You need to check the manuals to see if your inverter is "most" inverters or not. Most MSW inverters will say nothing about grounding the AC output/creating a neutral.

    Many TSW/PSW inverters over ~2,000 or 3,000 Watt output actually have a true grounded bonded neutral internal to the inverter (similar for AC gensets too).

    This can create issues... Multiple Neutral Neutral Ground bonds (at shore power, AC genset, AC inverter) is not a good thing either (in most cases)....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #7
    Definitely grounding on boats can cause electrolysis!   But, this is a working system right Estragon? You are simply moving wires around?
    No history with zincs cooking off too fast? 
    "we go where power lines don't" Sierra Nevada mountain area
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    E-mail offgridsolar@sti.net

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #8
    > @Dave Angelini said:
    > Sometimes it helps to just draw it. But you do not need to now :)

    Yup, shoulda drawn it... I more or less did a mental one when I got a chance to go for a walk and clear the deck a bit, which is when it dawned on me it couldn't work.

    It is a working system, but the admiral wanted the inverter going pronto (to run the nuke on the hook to foam milk for coffee, which it turns out the NG stove does better anyway), so I used parts available at the time from the local orange box etc. That was c.2009 or 2010. I know it isn't done right though, so hoping to get at it this year.

    No problems with galvanic corrosion, but it's a freshwater boat. We probably get away with a lot more than we should.

    I'm also redoing some of the DC side, but will start a new thread on some questions with that.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #9
    Bill - It's been a while since I've had the panel apart, so I'm guessing a bit. IIRC, there's nowhere the DC negative is bonded to AC neutral. The inverter transfer switch makes an AC NG bond when inverting, and lifts it when ACin is live from shore power or whatever, but I think the AC ground is separated from DC negative. Will need to check that though.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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