Impedance
Updated 2026-10-27
Data records
- Impedance is the complex ratio of the voltage across a circuit or circuit element to the alternating current through it; its magnitude is the ratio of the voltage amplitude to the current amplitude, and its phase angle is the angle by which the voltage leads the current.
- Because reactance depends on frequency, the impedance of a circuit that contains inductance or capacitance changes with the frequency of the applied voltage while its resistance, to a first approximation, does not; an impedance reading is therefore stated together with the test frequency at which it was taken.
- Impedance is read by loop impedance testers on the fault loop of a low-voltage installation, by battery impedance testers on cells and their inter-cell connections, and by LCR meters on components, each at a test frequency chosen for the application.
- In the NIST Guide to the SI, the ohm, symbol Ω, is the SI coherent derived unit of electric resistance, expressed in other SI units as the volt per ampere; impedance, being likewise a ratio of voltage to current, is stated in the same unit. Value: Ω.
- Resistance is the in-phase, energy-dissipating part of impedance and reactance is the quadrature part that arises from inductance and capacitance; impedance combines the two as a vector sum, so its magnitude is never smaller than the resistance alone.
Sources
- Electrical Testing Reference editorial taxonomy, Electrical Testing Reference editorial taxonomy of test methods, 2026-10 edition, Electrical Testing Reference. Held as: Own observation.
- NIST SP 811, NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes, web page updated 2025-08-18 edition, NIST. Cited: clause Table 3. Held as: Government work. Publisher page
Changelog (11 entries)
| Date | Revision | Change |
|---|---|---|
| 2026-10-27 | created (qty.impedance.definition) | |
| 2026-10-27 | revised (qty.impedance.definition) | |
| 2026-10-27 | review_status unreviewed -> software_reviewed (qty.impedance.definition) | |
| 2026-10-27 | created (qty.impedance.frequency-dependence) | |
| 2026-10-27 | review_status unreviewed -> software_reviewed (qty.impedance.frequency-dependence) | |
| 2026-10-27 | created (qty.impedance.measured-by) | |
| 2026-10-27 | review_status unreviewed -> software_reviewed (qty.impedance.measured-by) | |
| 2026-10-27 | created (qty.impedance.unit) | |
| 2026-10-27 | review_status unreviewed -> software_reviewed (qty.impedance.unit) | |
| 2026-10-27 | created (qty.impedance.versus-resistance) | |
| 2026-10-27 | review_status unreviewed -> software_reviewed (qty.impedance.versus-resistance) |
Cite this page
Cite the governing standard for the fact itself; cite this page as the finding aid.
"Impedance." Test Instrument Reference. 2026-10-27. https://www.testinstrumentreference.com/quantities/impedance/.
Test Instrument Reference, Impedance (Oct. 27, 2026), https://www.testinstrumentreference.com/quantities/impedance/.
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title = {Impedance},
howpublished = {\url{https://www.testinstrumentreference.com/quantities/impedance/}},
organization = {Test Instrument Reference},
publisher = {Test Instrument Reference},
year = {2026},
note = {last substantive change 2026-10-27}
}{{cite web |title=Impedance |website=Test Instrument Reference |publisher=Test Instrument Reference |url=https://www.testinstrumentreference.com/quantities/impedance/ |date=2026-10-27 |access-date=}}