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Voltage drop: what it means for the Texas exam

Bonding Jumper editorial teamSources checked 3 min read

Voltage drop is the voltage lost along the conductors between the source and the load, because current flowing through a conductor's resistance uses up some of the voltage. The load receives the source voltage minus the drop. Longer runs, smaller wire and more current all raise it, and too much starves motors and dims lights.

A source on the left at 120 volts, a long pair of conductors to a load on the right reading 114 volts, and a small resistor symbol drawn on each conductor labeled conductor resistance, with the lost 6 volts shared between them
The wire is a resistor too. What it uses, the load never gets.
Use this diagram

Why it happens

Every conductor has resistance, small per foot but adding up over distance. Ohm's law says the voltage across a resistance is the current times the resistance, so the conductor itself has a voltage across it whenever current flows. On a two-wire circuit the current goes out on one conductor and back on the other, so both lengths count.

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What it does to equipment

  • Motors draw more current to make the same power at lower voltage, run hotter and may struggle to start.
  • Lighting dims, and some drivers misbehave.
  • Electronics can reset or shut down.
  • Energy is lost as heat in the wire, which you pay for whether or not the equipment works well.

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Every answer comes with a full explanation.

A worked run with invented numbers

A 120-volt single-phase circuit feeds a 16-ampere load 100 feet away on 12 AWG copper. Chapter 9 Table 8 gives 12 AWG as 6,530 circular mils, and copper's K is about 12.9 ohm-circular mils per foot.

  • Drop = 2 × 12.9 × 16 × 100 ÷ 6,530 = about 6.3 volts.
  • Percent = 6.3 ÷ 120 = about 5.3%.

That is a large drop for a branch circuit, so the conductor would be upsized. Moving to 10 AWG, at 10,380 circular mils, gives about 4.0 volts, or 3.3%. The NEC voltage drop guide covers three-phase runs and the sections.

The percentages for ordinary branch circuits and feeders sit in informational notes, which recommend but do not require. Some specific articles do set mandatory limits for particular equipment; read the question's wording.

Common mix-ups

  • Using the round trip for L. The 2 in the single-phase formula already covers both conductors.
  • Confusing voltage drop with ampacity. A conductor can be legal on ampacity and still drop too much voltage on a long run.
  • Dividing by the load voltage. Percent drop uses the source voltage.

On the Texas journeyman exam

Voltage drop is tied to the branch circuits and conductors area, 14 scored items across both portions under the PSI bulletin. Study to compute the drop, the percent or the smallest conductor that meets a target.

Practice it

Check your runs with the voltage drop calculator, which shows every step for single-phase and three-phase. 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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