In NFPA 70E, the short-circuit symmetrical ampacity from a bolted 3-phase fault at the transformer terminals
is calculated with the following formula:
Isc = {[MVA Base × 106] ÷ [1.732 × V]} × {100 ÷ %Z}
where
Isc = Amperes,
V = Volts,
%Z = transformer MVA.
A typical value for the maximum power (in MW) in a 3-phase arc can be calculated using the following formula:
P = [maximum bolted fault in MVAbf] × 0.7072 MW
The Flash Protection Boundary distance is calculated in accordance with the following
formulae:
P = 1.732 × V × Isc × 10–6 × 0.7072
Dc = [2.65 × MVAbf × t]½
Dc = [53 × MVA × t]½
where:
Dc = distance in feet of person from arc source for a just curable burn (i.e., skin temperature remains less than 80 degrees)
MVAbf = bolted fault MVA at point involved
MVA = MVA rating of transformer. For transformers with MVA ratings below 0.75 MVA, multiply the transformer MVA rating by 1.25.
t = time of arc exposure in seconds
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