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 Post subject: NFPA70E 2012 Table D.7.7
PostPosted: Tue Oct 22, 2013 5:55 pm 
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Good evening all. I'm new here and this is my first post. This is a sweet site! I have read a lot of great information here so far but was hoping for a little help. I am considering bidding on and performing a short circuit/arc-flash study on a small school. The specifications they provide state that for 480V equipment and lower (which is the highest they have on site) to use NFPA 70E Table D.7.7 for the IE and AFB instead of the methods found in D.7.2, D.7.3, D.7.4 and D.7.5.

I am fairly fluent in D.7.2-D.7.5 but I am a bit confused regarding table D.7.7. It seems simple enough on the surface but I feel that I am missing something.

Looking at Table D.7.7, it offers various equations for different breaker types and ratings. Part of the equation uses Ibf. My confusion if that I am not sure if I am to use the Ibf at my equipment based on the short circuit study done previously or if I have to calculate the Ibf based on NFPA 70E D.7.7(a), D.7.7(c) and D.7.7(d) on page 70E-71 (opposite of Table D.7.7) Any help here would be appreciated.

Also, when using Table D.7.7, if I have a MCCB or ICCB that has a 1600 amp rated frame yet plugged with a 1400 amp plug, am I going to use the 4th breaker down listed in Table D.7.7 (1600-6000 MCCB or ICCB) or am I not able to use Table D.7.7 in this case because it is set up with a 1400 amp plug?

I welcome and appreciate any thoughts on this.

M


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PostPosted: Wed Oct 23, 2013 1:44 pm 
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I would use Ibf from the short circuit study done previously as long as the parameters and inputs from that study are still valid. For instance did the study use the minimum available fault current from the utility or was it an infinite bus? An infinite bus will not provide conservative results for arc flash.

The question on MCCB with a 1400A plug is interesting. It seems there is a gap in the table as you could have a 1400A plug in a 1600A frame MCCB or ICCB and have the LTPU set for 0.9 (1260A), then what? Seems to be a gap between 1200A to 1600A, unless the Rating refers only to breaker frame? Now that I write this I think it does refer to frame size. Look at the first rating, it is 100 - 400, what would you do with a 70A breaker? That would surely be a 100A frame, minimum.

Sorry for the free flow thought process above but I think it is frame rating.

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PostPosted: Mon Nov 04, 2013 9:09 am 
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MichaelE - Consider using FlashTrack. The data collection part of the project is very important and time consuming. FlashTrack help you ensure that all the data required is captured and its done in a uniformed manner. Watch a [url='http://facilityresults.com/W1/flashtrack-tutorials/']video[/url] of FlashTrack in action at http://facilityresults.com/W1/flashtrack-tutorials/

[url='http://facilityresults.com/W1/flashtrack-tutorials/'][ATTACH=full]331[/ATTACH][/url]


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[url='http://facilityresults.com/W1/']Facility Results[/url]


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PostPosted: Tue Nov 05, 2013 3:33 pm 
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Bryan,
What you posted is not relevant to the OP's question but is more of an advertisement for a product your company sells. I am sure that your posting could be in violation of the forum policy on commercial postings.

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PostPosted: Wed Nov 13, 2013 5:51 am 
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Michael-

Whenever there are gray areas, inconsistencies, or ambiguities, document your work and determine the most conservative possible outcome.


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