ECN Forum Forums Technical Discussion Electrical Theory and Applications Short Circuit 1000kva 3 ph 1.6% Impedance 277/480

The .9 factor is used to adjust for the tolerances of the transformer impedance.

All impedances are really specified with a +/- tolerance, but we usually here of them as a fixed number. By only using 90% of the quoted %Z Scott is taking into account the worst case available current.

These 50KVA 120/240V 1Ø 3W transformer have an Impedance of 0.9 Percent.

The local PoCo chart shows 23640 fault current @ 240 volts for these 0.9 % nameplate Impedance.

The .9 in the above fault current calculation(s), is as Don noted, for worst case scenario of transformers 25 KVA and larger do to the +/- 10% variation of the nameplate impedance.