Resistance: what it means for the Texas exam

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://bondingjumper.com/charts/bondingjumper/resistance.svg" alt="A long thin wire and a short thick wire of the same metal side by side, the long thin one labeled higher resistance and the short thick one lower, with the formula R equals K times L divided by area beneath them" width="100%"><p><a href="https://bondingjumper.com/guides/resistance?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=resistance.svg">Double the length, double the resistance. Double the area, half the resistance. by Bonding Jumper</a></p>

The four factors

• Material. Copper conducts better than aluminum, so an aluminum conductor needs a larger size to match copper. The constant K, about 12.9 ohm-circular mils per foot for copper, with a higher constant for aluminum, captures this.
• Length. Resistance rises in direct proportion to length.
• Area. Resistance falls in proportion to the cross-sectional area, measured in circular mils for building wire.
• Temperature. For copper and aluminum, resistance rises as the conductor warms. A hot conductor drops more voltage and makes more heat than the same conductor cool.

Together: resistance equals K times length divided by circular mils. The NEC's Chapter 9 Table 8 lists the direct-current resistance of each conductor size at a stated temperature, which saves the arithmetic.

A worked example with invented numbers

A 100-foot, 120-volt circuit uses 12 AWG copper. The current travels 200 feet in total, out and back. Using K, 12.9 × 200 ÷ 6,530 circular mils gives about 0.395 ohm for the pair. At 16 amperes, Ohm's law gives a drop of 16 × 0.395, about 6.3 volts, the same answer the voltage drop formula gives for this run, because it is the same calculation.

Ohm's law and power

Resistance links voltage and current: volts equal amperes times ohms. Power lost as heat in a resistance is the current squared times the resistance, which is why doubling the current through a conductor quadruples its heating. That heat is what limits ampacity.

Resistance in circuits

Resistances in series add; in parallel they combine to less than the smallest. See series vs parallel circuits. In AC circuits with coils or capacitors, the total opposition is impedance, which includes resistance and reactance.

Common mix-ups

• Using one-way length for the resistance of a two-wire circuit. The current goes out and back.
• Thinking a larger number means a larger wire. In AWG, a larger number is a smaller conductor with more resistance.
• Ignoring temperature. Table values are at a stated temperature; real conductors may be hotter.

On the Texas journeyman exam

Resistance and Ohm's law sit in definitions, theory and plans, 5 scored items across both portions under the PSI bulletin, and appear inside voltage drop and conductor problems on the Calculations portion.

Practice it

The Ohm's law calculator solves for volts, amperes, ohms and watts from any two. Every term in this set is in the electrical terms glossary.

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Sources

Show the list

• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.