This article will illustrate the basic theory of calculating a utility source impedance from fault data provided by power providers. Utility fault levels are usually expressed in MVA or Amperes (A).

## I. Calculating source impedance from fault level expressed in Amperes (A)

For sources other than power stations, Z_{2} can be taken as equal to Z_{1}.

where:

Z_{1} = positive sequence impedance of source (ohm)

Z_{2} = negative sequence impedance of source (ohm)

Z_{0} = zero sequence impedance of source (ohm)

V_{pp} = phase-to-phase voltage, V

I_{3f} = 3 phase fault current, A

I_{1f} = phase-to-earth fault current, A

## II. Calculating source impedance from fault level expressed in MVA

where:

kV_{pp} = phase-to-phase voltage, kV

MVA_{1} = fault level on a 3-phase base, MVA

MVA_{0} = single phase-to-earth fault on a 3-phase base, MVA

When the impedance is given in % rather than in ohms