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 Post subject: 30 A Breaker Protection of #10???????
PostPosted: Tue Feb 25, 2014 12:17 pm 
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Joined: Mon Oct 10, 2011 4:07 pm
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Location: Phoenix
I am curious to see others opinions on this subject and hoping that maybe some of the breaker "folks" are on here and can explain. I recently looked at a TCC with a # 10 conductor which is protected by a CH Type HFD 30A breaker. NEC compliant. In plotting the damage curve for the cable with the 30A breaker the conductor damage curve crosses into the "tolerance boundary" of the breaker suggesting that the cable is not adequately protected. The cable has no de-rating factors and is rated 75 degrees C. I am curious if anyone else has run across this situation and how you handled it. Thanks in advance.


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PostPosted: Tue Feb 25, 2014 1:33 pm 
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Cable damage-curves generally aren't applicable to low voltage protection. Don't worry about it.


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PostPosted: Tue Feb 25, 2014 3:03 pm 
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Joined: Mon Jan 18, 2010 11:35 am
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The UL test for that breaker includes 4' of rated conductor. This way the breaker is tested for actual protection of the conductor, during overloads and short circuits.


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PostPosted: Fri Feb 28, 2014 8:04 am 
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Strange. Usually the conductor damage curve is so far to the right of the protective devices time current curve that this is only a problem if it is medium voltage and the protection approaches the NEC upper limit of MV protection of 600% of the ampacity. The other case is if the maximum short circuit current is so high that the TCC extends way to the right of the graph and intersects the damage curve.

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PostPosted: Mon Mar 03, 2014 8:01 am 
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Joined: Sat Nov 21, 2009 10:23 pm
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Location: Ohio
This has always been the case, there have been multiple IEEE papers written on the topic. The circuit breaker cannot, in many cases protect the wire from a short circuit. Compare the same curve with a Mersen A6K30R or Buss KTSR30, you now have total protection. My guess even a common TRS30R or FRSR30 will also protect the wire.

If you want a further shock, plot the withstand rating of an IEC contactor/starter with the let-through curve of a common molded case circuit breaker. The UL test on a contactor or starter was watered down at UL in the early 90's and they now allow failure of the components as long as the damage to the enclosure is not dramatic.


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