Transmission & Grid calculator
Per-Unit Calculator: Impedance & Base Conversion
Convert ohms, per-unit impedance and percent impedance. Calculate base current and change between three-phase MVA and line-to-line kV bases.
GridBasis: Three-phase per-unit normalization
Results
- Enter values to calculate
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How to use this calculator
The per-unit system expresses each quantity as a fraction of a selected base. It keeps transformer and network calculations manageable by putting equipment data on consistent voltage and power references. A value of 0.10 p.u. corresponds to 10% on that same base.
Choose the MVA and kV base before comparing impedances. Across a transformer, carry the voltage bases through the turns ratio and keep the common power base. This calculator accepts a nonnegative impedance magnitude; it does not track transformer phase shifts or zero-sequence connection paths.
Formula
Zbase (Ω) = kVLL,base² / MVA3φ,base
Ibase (A) = 1000 MVA3φ,base / (√3 kVLL,base)
Zpu = Zactual/Zbase; Zactual = Zpu Zbase; %Z = 100 Zpu
Zpu,new = Zpu,old (Sbase,new/Sbase,old) (Vbase,old/Vbase,new)²
These are three-phase conventions: total three-phase base power and line-to-line base voltage. The same scalar base-change factor can be applied separately to real and imaginary impedance components.
Worked example
On a 100 MVA, 13.8 kV base, Zbase = 1.9044 Ω and Ibase = 4183.70 A. An impedance of 0.19044 Ω is 0.10 p.u. If 0.10 p.u. was originally on a 50 MVA base at the same kV, it becomes 0.20 p.u. on a 100 MVA base.
Assumptions and standards
- Balanced three-phase base convention; base kV is line-to-line.
- Convert all data to consistent bases before adding impedances.
- This tool handles an impedance magnitude. Retain complex R and X separately in network studies.
Calculation model checked: November 4, 2026
Frequently asked questions
Does 10% impedance mean 0.10 per unit?
Yes, on the same voltage and power base: percent impedance is 100 times the per-unit value.
Why does impedance increase when I increase MVA base?
At unchanged kV, base impedance decreases as MVA increases. The same actual ohms therefore occupy a larger per-unit value.
Can I use these results for final design?
Use them for preliminary design and checking. Final designs must be verified by a qualified engineer against the code edition and standards adopted where the work is installed.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you type is uploaded or stored on our servers.
References and further reading
Power system calculations: formulas and model guide