In NFPA 70E, the short-circuit symmetrical ampacity from a bolted 3-phase fault at the transformer terminals is calculated with the following formula:

where

** Isc** = Amperes,

**= Volts,**

*V***= transformer MVA.**

*%Z*A typical value for the maximum power (in MW) in a 3-phase arc can be calculated using the following formula:

where:

MVA_{bf} = maximum bolted fault, MVA

The Flash Protection Boundary distance is calculated in accordance with the following

formulae:

where:

** Dc** = distance in feet of person from arc source for a just curable burn (i.e., skin temperature remains less than 80 degrees)

**= bolted fault MVA at point involved**

*MVA*_{bf}**= MVA rating of transformer.**

*MVA**For transformers with MVA ratings below 0.75 MVA, multiply the transformer MVA rating by 1.25.*

**= time of arc exposure in seconds**

*t*
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