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Thread: Is resistance transferable ?

  1. #21
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    Quote Originally Posted by gar View Post
    190520-0739 EDT

    Sahib:

    Your statement is correct, but did you bother to read my assumptions which were there to define or frame the background for the scope of my discussion?

    .
    Sorry, gar. Your assumption is flawed: a short circuit assumes negligible resistance and 3 ohms is not negligible.

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    190520-0948 EDT

    Sahib:

    The 3 ohms was the resistance of the 1000 ft of #12 copper branch circuit wire. A dead short was placed at the end of the bramch circuit producing a 40 A load thru the circuit.

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    Now getting back to OP's application and questioning of a short length of 12 AWG at each end for termination, it is presumed the increased conductor size over the bulk of the run is low enough impedance it isn't much impact on magnetic trip functioning, and the short length of 12 AWG just to help facilitate connections, is pretty much a negligible resistance in this circuit.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    Now getting back to OP's application and questioning of a short length of 12 AWG at each end for termination, it is presumed the increased conductor size over the bulk of the run is low enough impedance it isn't much impact on magnetic trip functioning, and the short length of 12 AWG just to help facilitate connections, is pretty much a negligible resistance in this circuit.
    For long circuits such as 1000ft as stated by gar only thermal trip ie overload element of CB would operate and not the magnetic element of the CB for a short at the end of the circuit due to considerable resitance/impedance in the circuit.

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    Quote Originally Posted by gar View Post
    190520-0948 EDT

    Sahib:

    The 3 ohms was the resistance of the 1000 ft of #12 copper branch circuit wire. A dead short was placed at the end of the bramch circuit producing a 40 A load thru the circuit.

    .
    That 40A current is not short circuit current (even though it results from a short) but overload current due to considerable resistance in the circuit and so your conclusion that #12 copper wire is not suitable may not be correct since the thermal element of the CB would still operate and disconnect the circuit without any damage.

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    Quote Originally Posted by Sahib View Post
    For long circuits such as 1000ft as stated by gar only thermal trip ie overload element of CB would operate and not the magnetic element of the CB for a short at the end of the circuit due to considerable resitance/impedance in the circuit.
    Yes even with larger conductors to reduce voltage drop, there still can be enough resistance in entire run to limit fault current to a level below magnetic trip level of the breaker. This sort of can lead us into AIC rating discussions and incident energy discussions for arc flash issues. Conductors do have resistance which is current limiting.
    I live for today, I'm just a day behind.

  7. #27
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    The previous posts illustrate why the suggestion to use a higher voltage pops up so often.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    190520-2114 EDT

    Sahib:

    In my post #17 I made specific assumptions, and I stated them. This was to try to avoid all sorts of if, and, and but stuff.

    I will further clarify that a real world voltage source consists of an ideal voltage source with a series internal impedance. Further assume linear components.

    I consider a source to be where you connect a load. If I talk about a dead short somewhere, then that is some impedance that is very small compared to its source, and it is the load. For the problem I was illustrating the wiring to the load was part of the source, and not part of the load. Roughly speaking a dead short will have near zero voltage drop across it with current flow.

    You can define a load at any two terminal point you want in a circuit. By virtue of my use of the term "dead short circuit" I defined where those two terminals were.

    The purpose of my post was to try to define a simple circuit that would illustrate the problem of not using an EGC of adequate size. I used the trip time of 1/60 as a way of defining a specific trip characteristic.

    .


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    Quote Originally Posted by gar View Post
    190520-2114 EDT

    Sahib:

    In my post #17 I made specific assumptions, and I stated them. This was to try to avoid all sorts of if, and, and but stuff.

    I will further clarify that a real world voltage source consists of an ideal voltage source with a series internal impedance. Further assume linear components.

    I consider a source to be where you connect a load. If I talk about a dead short somewhere, then that is some impedance that is very small compared to its source, and it is the load. For the problem I was illustrating the wiring to the load was part of the source, and not part of the load. Roughly speaking a dead short will have near zero voltage drop across it with current flow.

    You can define a load at any two terminal point you want in a circuit. By virtue of my use of the term "dead short circuit" I defined where those two terminals were.

    The purpose of my post was to try to define a simple circuit that would illustrate the problem of not using an EGC of adequate size. I used the trip time of 1/60 as a way of defining a specific trip characteristic.

    .

    Thanks for your response, gar. I still have the impression that you considered only magnetic trip only 20 A CB, leaving out thermal-magnetic 20A CB. Supposing a 20 A thermal-magnetic CB is present, how will it change your reasoning in post #17 with regard to the problem of not using an EGC of adequate size ?

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