Percent impedance on a clear electrical base
Convert a transformer nameplate percent impedance into per-unit and equivalent ohms on the voltage base you enter. The same normalized impedance also shows the ideal transformer-terminal short-circuit multiplier, while keeping downstream network and protection studies outside the result.
How to use this calculator
Enter the transformer nameplate kVA.
Enter the rated voltage of the winding to which you want the ohmic impedance referred; use line-to-line voltage for three-phase.
Choose single- or three-phase.
Enter the transformer nameplate/test percent impedance.
Read per-unit impedance, base impedance, referred ohms, rated current and the transformer-only terminal fault screen.
Where people use it
- •Converting transformer %Z into ohms on a selected winding base.
- •Checking per-unit/base-impedance calculations before a network study.
- •Understanding why the same %Z corresponds to different ohms when referred to different voltage windings.
- •Screening the relationship between transformer impedance and terminal symmetrical fault current.
Example: 1000 kVA, 415 V, 3φ, 5% Z
Zpu is 0.05. Zbase is about 0.172225 Ω, so transformer impedance referred to the 415 V side is about 0.008611 Ω. Rated current is about 1391.21 A and the ideal transformer-only terminal screen is about 27.824 kA.
What this conversion does not assume
- •Use the actual nameplate or factory-test impedance for engineering decisions; do not replace it with a generic typical value.
- •Ohmic impedance is winding/base dependent. Enter the voltage of the side to which the impedance is being referred and keep voltage and kVA bases consistent.
- •The terminal fault-current output assumes an infinite upstream bus and transformer impedance only. Real utility/source, cable, bus and motor impedances change available current.
- •Percent impedance is an impedance magnitude. This calculator does not invent separate R and X components or X/R ratio when they are not provided.
- •Do not use the terminal screen as automatic breaker, fuse, SCCR or arc-flash approval. Those require the complete system and applicable study method.
- •Tap position, temperature and test conditions can affect engineering data; use project/manufacturer test information when precision matters.
Frequently asked questions
How do I convert transformer percent impedance to ohms?+
Convert Z% to per-unit by dividing by 100, calculate base impedance V²/S on the chosen winding base, then multiply Zpu by Zbase.
Why does transformer impedance in ohms change between primary and secondary?+
Ohmic impedance depends on the voltage base and is referred through the turns ratio. Percent/per-unit impedance remains convenient because it is normalized to the transformer rating.
What does 5% transformer impedance mean?+
Approximately 5% of rated voltage applied under the specified short-circuit test condition produces rated current. On an ideal infinite source this corresponds to a 20× rated-current terminal short-circuit screen.
Should I use primary or secondary voltage?+
Use the rated voltage of the winding to which you want the ohmic impedance referred. For balanced three-phase calculations use that winding line-to-line voltage.
Is Z% the same as resistance?+
No. Z% is the magnitude of equivalent impedance, combining resistance and leakage reactance. Separate R and X need additional test/nameplate data.
Can this calculate breaker interrupting rating?+
No. The included fault current is transformer-only context. Equipment duty requires complete available-fault-current data and the applicable engineering study.
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