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Single-phase vs three-phase: what it means for the Texas exam

Bonding Jumper editorial teamSources checked 3 min read

Single-phase power delivers one alternating voltage, which rises and falls through zero twice each cycle. Three-phase power delivers three alternating voltages of the same size, each a third of a cycle, or 120 degrees, behind the one before. Because one phase is always near its peak, three-phase delivers smooth, steady power and runs motors without help to start turning.

Two graphs stacked: one sine wave labeled single-phase with gaps at each zero crossing, and three overlapping sine waves labeled A, B and C spaced 120 degrees apart, labeled three-phase, with the sum shown as nearly flat
One wave dips to zero. Three waves, a third of a cycle apart, never all dip at once.
Use this diagram

Where each one is used

Homes and small shops usually get single-phase 120/240-volt service: two hot conductors and a neutral from a center-tapped transformer. Commercial and industrial buildings usually get three-phase, often 208Y/120 or 480Y/277 volts, to run motors, large HVAC and big lighting loads efficiently. A three-phase system can still serve single-phase loads by connecting between one phase and the neutral, or between two phases.

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Why three-phase is efficient

At the same voltage, three-phase delivers more power for each pound of conductor than single-phase does. Three-phase motors are simpler and smaller for their output, and they start without the capacitors or windings a single-phase motor needs.

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The formulas and the 1.732

  • Single-phase power: watts = volts × amperes × power factor.
  • Three-phase power: watts = 1.732 × line volts × line amperes × power factor.

The 1.732 is the square root of 3. It appears because the voltage between two phases is 1.732 times the voltage from either phase to the neutral in a wye system: 120 × 1.732 is about 208. A worked example with invented numbers: a three-phase 208-volt load draws 30 amperes per line at a power factor of 1.0. Power is 1.732 × 208 × 30, about 10,800 watts.

How the three windings are connected, in a wye or a delta, decides which voltages are available. See wye vs delta.

Common mix-ups

  • Multiplying by 3 instead of 1.732. Three-phase power uses the square root of 3 with line values.
  • Using 1.732 on a single-phase circuit fed from a three-phase panel. Two phases of a 208Y/120 system feed a single-phase load at 208 volts, with no 1.732 in its power formula.
  • Assuming the neutral carries the sum of the phase currents. In a balanced three-phase wye, the neutral conductor carries little or none.

On the Texas journeyman exam

Phase relationships sit in definitions, theory and plans, 5 scored items across both portions under the PSI bulletin, and every three-phase load, feeder or motor calculation depends on them. Study to choose the right formula and to read 208, 240, 277 and 480 volts correctly.

Practice it

The Ohm's law calculator includes three-phase power. 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.

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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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