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Thread: Breakers of the same size in series

  1. #21
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    Quote Originally Posted by kwired View Post
    I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.
    That will happen if the short circuit current exceeds the magnetic trip rating of both the main and branch breaker. Typical molded case breaker trip at about 10x the handle rating instantaneously; so a 30 amp breaker starts at 300amps and a 200 amp breaker about 2,000. Some main breakers even start below 10x. So an 8000 amp utility source can easily trip both if the fault and cable impedance is low enough as 8,000 or even 2,500 is still higher than 2,000.
    What is esoteric knowledge today will be common knowledge tomorrow.

  2. #22
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    Quote Originally Posted by mbrooke View Post
    That will happen if the short circuit current exceeds the magnetic trip rating of both the main and branch breaker. Typical molded case breaker trip at about 10x the handle rating instantaneously; so a 30 amp breaker starts at 300amps and a 200 amp breaker about 2,000. Some main breakers even start below 10x. So an 8000 amp utility source can easily trip both if the fault and cable impedance is low enough as 8,000 or even 2,500 is still higher than 2,000.


    Absolutely. Overload conditions, the 200 amp main would not have tripped. Short circuits and ground faults, anything in the current path will open if it's instantaneous trip level is exceeded. Had the fault in my example happened on the load end of the circuit in question - the resistance of the branch circuit conductors may have reduced the fault current enough that the main doesn't trip. But in this case the fault was right at the fitting where the circuit leaves the panel - very short distance of smaller conductor and not nearly as much resistance results in higher fault current.

  3. #23
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    Quote Originally Posted by kwired View Post


    Absolutely. Overload conditions, the 200 amp main would not have tripped. Short circuits and ground faults, anything in the current path will open if it's instantaneous trip level is exceeded. Had the fault in my example happened on the load end of the circuit in question - the resistance of the branch circuit conductors may have reduced the fault current enough that the main doesn't trip. But in this case the fault was right at the fitting where the circuit leaves the panel - very short distance of smaller conductor and not nearly as much resistance results in higher fault current.
    Yup- thats how it works. Even POCOs have to worry about it. Often fuses in series on 12.47kv taps close to the substation will not coordinate due to the high fault current. 100K, 65K and 30K fuses will all blow at the same time for squirrel or failed transformer. On the other hand on long feeders a fault at the far end (say 20 miles away) may not get picked up the feeder breaker requiring more sensitive over current devices out on the line.
    What is esoteric knowledge today will be common knowledge tomorrow.

  4. #24
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    I love being accused of "overthinking" and "underthinking" the same issue in the same thread by different people.

    In Goldilocks terms, it sounds like I'm thinking "just right".


  5. #25
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    Quote Originally Posted by jaylectricity View Post
    I love being accused of "overthinking" and "underthinking" the same issue in the same thread by different people.

    In Goldilocks terms, it sounds like I'm thinking "just right".

    I'd say your just learning
    What is esoteric knowledge today will be common knowledge tomorrow.

  6. #26
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    Quote Originally Posted by mbrooke View Post
    I'd say your just learning
    you're
    The more they overthink the plumbing, the easier it is to stop up the drain

  7. #27
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    Quote Originally Posted by dkidd View Post
    you're
    I never passed spellin'!
    What is esoteric knowledge today will be common knowledge tomorrow.

  8. #28
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    Quote Originally Posted by mbrooke View Post
    I never passed spellin'!
    If you were from down south it should have been "yer".

  9. #29
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    Quote Originally Posted by kwired View Post
    If you were from down south it should have been "yer".
    Yesir indeed!
    What is esoteric knowledge today will be common knowledge tomorrow.

  10. #30
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    Quote Originally Posted by kwired View Post
    I once had a fault in a 30 amp circuit, my helper at the time thought tightening the clamp on the romex connector meant turn the screws until they don't turn anymore I guess. Not only did my helper need clean underpants, it tripped the 30 amp breaker as well as the 200 amp main breaker in the panel.
    Ha. Best I have is watching an industrial electrician create a phase to ground fault in a 480V cabinet for a 1/2HP shaft drive motor... that breaker never tripped, the incinerator building's 2000A main went first. I saw the arc flash, building goes dark, e-lights come on, hear the ID fan winding down... fun stuff.

    Last time I blew up something was cutting out some NM for a kitchen demo. Just got there, was told the circuit was dead by the boss. I didnt doublecheck... put my Kleins into the cable and squeeze...loud pop, bright flash, blown Kleins. That wasnt the first or last pair I blew up working with him, tho I havent blown up anything else since I moved on working for him. cuz, yanno, verifying power is off and securing a breaker takes a whole two minutes or so... two minutes is a small price to pay vs $20 paperweight pliers or looking like a j/a in front of the HO.

    Dual breakers for one circuit, not my favorite, tho no NEC issue. Jraef's points are valid. I run across dual GFCI protection much more often, even once saw numerous GFCI receps for a kitchen fed line/load off one another, coming from a 20A GFCI breaker (HO wired). If ground fault protection (x1) works, GFCI protection x4 must be 4x as good, right?
    Electricians do it until it Hertz!

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