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BASIC NEC 2023

Single-Phase vs Three-Phase Power Basics Practice Test

Free online single-phase vs three-phase power practice test. 25 basic electrical questions on wye and delta connections, line vs phase voltage, and US residential vs commercial power systems. Good for electrician exam prep.

TOPICS COVERED

Single-Phase Definition Three-Phase Definition Three-Phase Offset Why Three-Phase Preferred Three-Phase Power Smoothness Wye Connection

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US homes run on single-phase power (technically "split-phase" — 120/240 V from a center-tapped transformer, not true two-phase). Commercial and industrial buildings typically run on three-phase power, which delivers smoother, more efficient power and lets motors start on their own. Know your two three-phase connections: WYE (star, has a neutral, line voltage = √3 × phase voltage) and DELTA (triangle, no neutral, line voltage = phase voltage). Common US commercial systems include 208Y/120 V and 480Y/277 V.

25

Questions

38 min

Time

75%

Pass

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Single-Phase vs Three-Phase Power Basics Practice Test — Question List

Question 1: Single-phase AC power delivers electricity using:

One alternating voltage waveform
Three alternating voltage waveforms
A constant, non-alternating DC voltage
No voltage waveform at all

Question 2: Three-phase AC power delivers electricity using:

Three alternating voltage waveforms, evenly offset from each other
One alternating voltage waveform
A constant, non-alternating DC voltage
Two identical, unoffset waveforms

Question 3: In a standard three-phase system, the three voltage waveforms are offset from each other by:

120 degrees
90 degrees
180 degrees
45 degrees

Question 4: Compared to single-phase power, three-phase power is generally preferred for industrial and commercial loads mainly because it:

Delivers power more efficiently and smoothly, allowing smaller conductors for the same amount of power
Is always cheaper to install in a small residential home
Never requires any special equipment
Cannot be used with motors of any kind

Question 5: A major practical advantage of three-phase power over single-phase power is that three-phase power delivery is:

Nearly constant and smooth, rather than pulsating to zero twice per cycle like single-phase power
Just as pulsating as single-phase power, with no real difference
Completely constant with no waveform at all, like DC
More erratic and unpredictable than single-phase power

Question 6: In a WYE (star) three-phase connection, the three windings are connected:

Together at a common point, forming a neutral
End to end, forming a closed triangular loop
Completely isolated with no connection to each other
In series with a single resistor

Question 7: In a DELTA three-phase connection, the three windings are connected:

End to end, forming a closed triangular loop with no neutral point
Together at a common center point
Completely isolated with no connection to each other
In a straight series chain to ground

Question 8: In a WYE-connected three-phase system, if the phase voltage is 120 V, what is the approximate line-to-line voltage?

About 208 V
120 V
240 V
360 V

Question 9: In a DELTA-connected three-phase system, the line-to-line voltage compared to the phase voltage is:

The same — line voltage equals phase voltage in a delta connection
Line voltage is always √3 times the phase voltage
Line voltage is always half the phase voltage
There is no relationship between the two

Question 10: In a WYE-connected three-phase system, the line current compared to the phase current is:

The same — line current equals phase current in a wye connection
Line current is always √3 times the phase current
Line current is always zero
There is no relationship between the two

Question 11: In a DELTA-connected three-phase system, the line current compared to the phase current is:

Higher — line current equals the phase current multiplied by the square root of 3
The same — line current always equals phase current
Lower — line current equals the phase current divided by 3
Always exactly zero

Question 12: Which of the following is a common three-phase voltage system used in US commercial buildings?

208Y/120 volts
9 volts DC
1.5 volts DC
12 volts DC only

Question 13: Single-phase power is most commonly used to supply:

Residential homes and small individual loads
Large industrial factories exclusively
Utility power transmission lines exclusively
Nothing — single-phase power is never used

Question 14: Three-phase power is most commonly used to supply:

Commercial and industrial facilities, especially large motors
Only single-family residential homes
Only handheld battery-powered devices
Nothing — three-phase power is never used

Question 15: One key advantage of three-phase induction motors over single-phase induction motors is that three-phase motors:

Are self-starting and don't need a separate starting mechanism like a capacitor
Always run on DC power instead of AC
Cannot be reversed once installed
Require a starting capacitor, unlike single-phase motors

Question 16: What is the total real power of a balanced three-phase load with a line voltage of 208 V, a line current of 20 A, and a power factor of 0.9? (P = √3 × V<sub>L</sub> × I<sub>L</sub> × PF)

About 6,485 W
About 3,744 W
About 3,600 W
About 7,488 W

Question 17: The formula for total real power in a SINGLE-phase circuit, P = V × I × PF, differs from the three-phase formula mainly by:

The addition of the square root of 3 factor to account for the three phases
The complete removal of voltage from the formula
The complete removal of current from the formula
There is no difference between the two formulas

Question 18: Standard US residential 120/240 V single-phase service is created by:

A center-tapped transformer winding, giving two 120 V "hot" legs that combine for 240 V
Three completely separate transformer windings
A single 240 V battery bank
A rectifier converting AC directly to DC

Question 19: Despite having two "hot" legs, standard US residential 120/240 V service is technically NOT considered two-phase power because:

Both legs come from the same single transformer winding, just tapped at the center
It actually is two-phase power, and the common name is correct
It uses three completely separate transformers
It is actually DC power, not AC at all

Question 20: Compared to single-phase motors, three-phase motors generally produce:

Smoother, more constant torque
Much rougher, more pulsating torque
No torque output at all
Torque that only exists at startup

Question 21: If one phase of a running three-phase motor is lost (a condition called "single-phasing"), the most likely result is:

The motor continues trying to run on the remaining phases, overheating and risking damage
The motor immediately and safely shuts itself off with no risk
Nothing changes — the motor runs exactly as if all three phases were present
The motor instantly reverses direction with no other effect

Question 22: Before connecting a three-phase motor to a new supply, checking phase sequence (rotation) is important because an incorrect sequence will cause the motor to:

Run in the reverse direction
Run at exactly double its rated speed
Refuse to start under any circumstance
Automatically convert to single-phase operation

Question 23: A common three-phase FOUR-wire system used in US commercial buildings includes:

Three hot conductors plus a neutral conductor
Four completely separate neutral conductors and no hot conductors
One hot conductor and three neutral conductors
Four hot conductors with no neutral at all

Question 24: A three-phase load has a real power of 10,000 W, a line voltage of 208 V, and a power factor of 0.8. What is the approximate line current? (Rearrange P = √3 × V<sub>L</sub> × I<sub>L</sub> × PF)

About 34.7 A
About 27.8 A
About 48.1 A
About 60.1 A

Question 25: Compared to a single-phase system (which uses one voltage waveform), a three-phase system uses how many separate voltage waveforms?

Three
One
Two
Six

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score75%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)