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Impedance: what it means for the Texas exam

Bonding Jumper editorial teamSources checked 3 min read

Impedance is the total opposition a circuit offers to alternating current, measured in ohms and written Z. It combines resistance, which turns energy into heat, with reactance, which comes from inductance and capacitance and stores energy and returns it each cycle. In a DC circuit or a purely resistive AC circuit, impedance and resistance are the same.

A right triangle with resistance R along the base, reactance X up the side and impedance Z along the slope, with example values 6, 8 and 10 ohms, next to a small coil labeled inductive reactance and a capacitor labeled capacitive reactance
Resistance and reactance add like the sides of a right triangle.
Use this diagram

Reactance

  • Inductive reactance comes from coils, such as motor windings and transformers. It rises with frequency and inductance: 2 times pi times frequency times inductance. It makes current lag voltage.
  • Capacitive reactance comes from capacitors. It falls as frequency or capacitance rises: 1 divided by (2 times pi times frequency times capacitance). It makes current lead voltage.

The two act in opposite directions, so the net reactance is the difference between them.

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The formula and a worked example

Impedance equals the square root of resistance squared plus net reactance squared. With invented numbers: a coil with 6 ohms of resistance and 8 ohms of inductive reactance has an impedance of the square root of 36 + 64, which is the square root of 100, or 10 ohms. On 120 volts it draws 120 ÷ 10 = 12 amperes, not the 20 amperes its resistance alone would suggest. Its power factor is resistance divided by impedance, 6 ÷ 10 = 0.6.

Ohm's law works for AC with impedance in place of resistance: volts equal amperes times ohms of impedance.

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Every answer comes with a full explanation.

Why electricians meet it

Fault current. The impedance of the transformer, the service conductors and the circuit limits how much current flows in a short circuit. A transformer's nameplate impedance, given as a percent, is a quick guide: the lower the percent, the higher the available fault current at its terminals. That number drives the interrupting rating of breakers downstream.

Voltage drop on long AC runs. The NEC's Chapter 9 tables give AC resistance and reactance for conductors in raceways, for more exact voltage drop work than the DC resistance method.

Ground-fault paths. The bonding path must have low impedance so a fault draws enough current to trip the breaker quickly.

Common mix-ups

  • Adding resistance and reactance directly. They add as sides of a right triangle.
  • Treating impedance as fixed. Reactance changes with frequency.
  • Mixing up lag and lead. Inductive lags, capacitive leads.

On the Texas journeyman exam

Impedance sits in definitions, theory and plans, 5 scored items across both portions under the PSI bulletin, and supports power factor, transformer and fault current topics. Study to find impedance from resistance and reactance, or current from voltage and impedance.

Practice it

The Ohm's law calculator handles the volts, amperes and ohms step; apply it with impedance for AC loads. Every term in this set is in the electrical terms glossary.

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Not affiliated with or endorsed by TDLR, PSI or NFPA. NEC and National Electrical Code are registered trademarks of NFPA.

Sources

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Facts checked 26 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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