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Joined: Jul 2001
Posts: 599
J
JBD Offline
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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.

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Joined: Apr 2002
Posts: 197
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Gwz Offline
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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.

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