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Using 2x30A breakers on AC main (neutral and hot), will they trip at 30A or 60A?

Discussion in 'General Electronics Discussion' started by alesserfate, Feb 27, 2017.

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  1. alesserfate

    alesserfate

    29
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    Dec 5, 2014
    Hello again,

    I'm upgrading some wiring on my old boat, I noticed that my main AC shorepower breaker is actually two breakers with the toggles physically joined with a pin while at the back they are individual breakers: the hot has its own 30A breaker and the neutral has its own 30A breaker. My question is will this trip at 30A or 60A ? The rest of my boat is wired for a maximum of 30A so I want to make sure I'm in the safe.

    Thanks in advance.
     
  2. Bluejets

    Bluejets

    4,833
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    Oct 5, 2014
    It's a double pole breaker, meaning it will break all supply conductors under any fault condition @ 30 Amp (depending on whatever curve condition it comes under.)
    By that it means there are several types of 30 amp breaker with differing fault curves and characteristics.
     
    alesserfate likes this.
  3. alesserfate

    alesserfate

    29
    3
    Dec 5, 2014
    Thanks @Bluejets , I feel much better they are proper bluesea marine breakers. I will read the labels on them and should be able to figure out the rest. I appreciate it and will ask if I need any help further.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Let's assume they are two different, independent breakers.

    Let's also assume that the same current flows in the active and the neutral conductor (a short cut for saying that there are no earth currents, or other power sources that screw with the balance of currents in the 2 conductors)

    For a moment, assume that one breaker is faulty and will never break. What current flows when the other breaks?

    Note swap the two breakers over. What current flows when the wiring one breaks?

    Now imagine that you have the two breakers, and one of them is faulty (you don't know which). At what current will the working one break at?

    And then with 2 working breakers?

    So why are there 2 breakers? Safety, redundancy, and the fact that there are faults which can cause an unbalanced current.

    Practically speaking, a double pole breaker opens both conductors in a fault condition (something that two independent breakers won't). This as extra safety in the circumstance that (say) neutral has been connected to another phase, or where active and neutral have been reversed.
     
    alesserfate likes this.
  5. alesserfate

    alesserfate

    29
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    Dec 5, 2014
    Thanks @(*steve*) . Great explanation. The other thing I was always curious about, my boat also has an inverter, and when I'm not plugged in to shorepower and using the inverter to power my AC circuits at sea, they all essentially ground back to the inverter (the inverter has hot, neutral and ground connections where the AC starts during inverter operation). If there are currents escaping back through the green AC ground wire back to the inverter, what does the inverter do with them ? In a house they would go into earth using the waterpipe but what about when that's not an option ?
     
    Last edited: Feb 27, 2017
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    If boat is a steel hull then neutral would be bonded there along with any earth from the gen set and any other metal also. If timber, not much point Boats are usually all double pole switching as shore power may be supplied via flex lead with possibility of a and n being transposed.
     
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