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Thread: a trip curve for a plain-vanilla resi

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    Homeowner Thatguy's Avatar
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    Default a trip curve for a plain-vanilla resi

    15A breaker to be used with #14 AWG copper; anyone have a link?

    Thanks.

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Thatguy View Post
    15A breaker to be used with #14 AWG copper; anyone have a link?

    Thanks.

    Just what are you looking for?

    A good rule of thumb concerning the trip curve of a breaker is six times the rated current.

    Your 15 amp breaker will carry 90 amps for 12 cycles without tripping.

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by jwelectric View Post
    Just what are you looking for?

    Reverse engineering the NEC.

    A good rule of thumb concerning the trip curve of a breaker is six times the rated current.

    Your 15 amp breaker will carry 90 amps for 12 cycles without tripping.
    Good. That's exactly what I'm looking for.

    So the I-squared-T rating for this breaker is 8100 x 12/60 = 1620 amps-squared-seconds.

    For one-half cycle at 60 Hz this should trip at 440A.
    With a dead short at the end of 1/4 ohm worth of Romex the rms current for this half cycle would be 120/(1/4) = 480A.
    Close enough.

    If #14 AWG copper fuses at ~170A (in one second?) then its rating is about 30,000 (A^2)-sec. So the breaker interrupts the current before the wire gets anywhere near the melting point, maybe when the wire reaches 90C or so. . .?

    Thanks.
    Last edited by Thatguy; 11-19-2008 at 02:34 PM.

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