Arc flash 3phase 208/120panel

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cornbread

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Please correct me if Im wrong, but I thought if a XFMR feeding a 208/120 panel was under 75KVA the panel could be rated a Cat 0? In doing some calcs with E-tap it appears not to be the case. Any help in this matter would be greatly appreciated.
 

jim dungar

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Please correct me if Im wrong, but I thought if a XFMR feeding a 208/120 panel was under 75KVA the panel could be rated a Cat 0? In doing some calcs with E-tap it appears not to be the case. Any help in this matter would be greatly appreciated.

Calculations, on transformer secondaries, will almost always result in a value >1.2 cal/cm^2.
IEEE 1584 makes a comment that faults on certain circuits (e.g. maximum125kVA transformer) are unlikely to occur, so performing calculation is not necessary.

Although, IEEE 1584 makes no reference to possible incident energy values, through a series of 'short cuts and misinterpretations' it has become a common industry practice to equate 'no calculations needed' to 'no hazard'.
 

cornbread

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I see NPPA 130.5 Info note 5, references IEEE1584 but I don't think that is binding and if I look at the Exceptions where it says Table 130(C)(15) can use in lue of calcs. When I look at the tables it has condistions associated with it Max 25KVA witha max clearing time of 0.03 sec (2 cycle). Still confused , seem calcs are needed unless your system can clear in 0.03 sec, how do you know the clearing time unless you do the calcs?
 

jim dungar

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...how do you know the clearing time unless you do the calcs?
You don't, kind of.
You need to make short circuit and time-clearing calcs (about 90% of the work), you just do not need to make incident energy ones.
 
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