Convert ohms and percent values into per‑unit results. Handle base changes for new voltage levels. Download tables, share PDFs, and document calculations confidently always.
| Base kV | Base MVA | R (ohm) | X (ohm) | Zbase (ohm) | Rpu | Xpu | |Zpu| | Angle (deg) |
|---|---|---|---|---|---|---|---|---|
| 11 | 10 | 0.15 | 0.75 | 12.1 | 0.012397 | 0.061983 | 0.063211 | 78.690068 |
Per‑unit work starts by choosing a consistent base for the project. A common practice is 1 MVA or 10 MVA for distribution studies, paired with the nominal line‑to‑line voltage at each bus. From that base, the calculator computes Zbase = kV²/MVA, letting every feeder and transformer be compared on the same scale. Consistency matters more than the specific base value. on site.
In commercial construction, transformer nameplate impedance often falls around 4–8% for many dry‑type and oil‑filled units, while larger utility‑style units can differ by design. Cable and busway segments add mostly reactance at higher voltages, and resistance becomes more visible on long low‑voltage runs. Tracking Rpu and Xpu separately helps estimate X/R ratio, which influences peak fault current and protective device duty.
Percent impedance is simply per‑unit magnitude: Zpu = %Z/100. If you also know R% and X%, the calculator converts them directly into Rpu and Xpu for more realistic studies. When only %Z is available, assuming Rpu ≈ 0 is a conservative simplification for many short‑circuit calculations, but it can understate heating losses. Adding an X/R ratio improves the split when vendor data includes it.
Per‑unit values streamline short‑circuit and voltage‑drop checks because the same arithmetic applies across voltage levels. After you obtain Zpu, you can combine series elements by summing Rpu and Xpu, then convert back to ohms on any chosen base if needed. Smaller |Zpu| usually means higher available fault current, while larger Rpu raises I²R losses and affects conductor temperature rise on long feeders.
Projects often mix 33 kV, 11 kV, and 400/230 V sections, so base conversion is essential. The calculator applies Zpu,new = Zpu,old·(Snew/Sold)·(Vold/Vnew)², preserving the physical impedance while changing the reference. This step is helpful when a transformer vendor provides data on one base, but your coordination study uses another. Always keep voltage bases line‑to‑line and three‑phase MVA.
Pick one MVA base for the study and use each bus’s nominal line‑to‑line voltage as its kV base. The per‑unit system stays consistent as long as every element is converted to the same chosen bases.
Yes. Select the polar option and provide magnitude and angle. The calculator converts to R and X internally, then reports both rectangular and magnitude‑angle results in per‑unit.
Six percent means |Zpu| = 0.06 on the transformer’s base. Lower values generally allow higher fault current, while higher values limit fault current but increase voltage drop under load.
X/R affects the DC offset and peak asymmetrical current during faults. Higher X/R can increase peak duty on breakers and fuses even when the symmetrical RMS value is unchanged.
Use it whenever a datasheet or model is referenced to a different MVA or kV base than your study. Converting bases prevents mixing incompatible per‑unit values in the same network.
No. Per‑unit supports electrical coordination, fault, and comparative studies. Conductor sizing, derating, and temperature rise still require code‑based thermal checks and manufacturer limits.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.