Per-Unit Calculator

A reactance of 0.1 pu on a 50 MVA base becomes 0.2 pu on a 100 MVA base at the same voltage, since per-unit impedance scales with MVA.

Impedance in per unit (old base):

Old base MVA:

Old base kV:

New base MVA:

New base kV:

Impedance on the new base =

0.2 pu

Precision

Base impedance kV²/MVA: 1.21 Ω
Actual impedance: 0.242 Ω
Base current MVA/(√3 kV): 5248.638811 A
Three-phase fault current behind this impedance: 26243.19405 A
Fault level MVA_base/Z_pu: 500 MVA

Step-by-step solution

Z_new = Z_old × (kV_old/kV_new)² × (MVA_new/MVA_old)
Z_new = 0.1 × (11/11)² × (100/50) = 0.2 pu
Z_base = 11² ÷ 100 = 1.21 Ω, so Z = 0.242 Ω

Per-Unit Formula

Z_new = Z_old (kV_old/kV_new)² (MVA_new/MVA_old)

Z_base = kV²/MVA; I_base = MVA/(√3 kV); fault MVA = MVA_base/Z_pu

Worked Example

A 0.1 pu reactance on 50 MVA, 11 kV, moved to a 100 MVA, 11 kV base:

Z_new = Z_old × (kV_old/kV_new)² × (MVA_new/MVA_old)
Z_new = 0.1 × (11/11)² × (100/50) = 0.2 pu
Z_base = 11² ÷ 100 = 1.21 Ω, so Z = 0.242 Ω

Actual impedance: 0.242 Ω · Base current MVA/(√3 kV): 5248.638811 A · Three-phase fault current behind this impedance: 26243.19405 A · Fault level MVA_base/Z_pu: 500 MVA

GATE tip

Choose one MVA base for the whole system and let the kV base change with each transformer ratio. Then every transformer’s per-unit impedance needs only the MVA correction.

More Power Systems Calculators

Symmetrical Components Calculator

V₁ = (V_a + aV_b + a²V_c)/3

V₀ = (V_a + V_b + V_c)/3, V₂ = (V_a + a²V_b + aV_c)/3, a = 1∠120°

Example: Unbalanced phases V_a = 100∠0°, V_b = 80∠−120°, V_c = 90∠120°: Positive sequence V₁ = 90 ∠ 0°.

Open the symmetrical components calculator →

Frequently asked questions

Why use per unit?

Per-unit values look the same on both sides of a transformer, fall in familiar ranges (0.05–0.2 for transformers) and make network calculations scale-free.

What is the fault level?

For a three-phase fault behind Z_pu, the fault MVA is base MVA ÷ Z_pu and the fault current is base current ÷ Z_pu.

Sources

The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.

Cite this page

APA

Scientific Converters. (2026, October 15). Per-Unit Calculator. Retrieved October 25, 2026, from https://scientificconverters.com/calculators/per-unit-calculator/

IEEE

Scientific Converters, "Per-Unit Calculator," scientificconverters.com. Accessed: Oct. 25, 2026. [Online]. Available: https://scientificconverters.com/calculators/per-unit-calculator/