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BASIC NEC 2023

Three-Phase Circuit Calculations Practice Test

Free online three-phase circuit calculations practice test. 25 basic electrical questions on wye and delta line/phase relationships, three-phase power, and line current calculations. Good for electrician exam prep.

TOPICS COVERED

Wye Configuration Delta Configuration Three-Phase Power Three-Phase Current Calculations Three-Phase Fundamentals Balanced Loads

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Three-phase power is standard for commercial and industrial buildings, and the math behind it revolves around one key number: 1.732 (the square root of 3). In a wye (star) connection, line voltage is 1.732 times the phase voltage, while line and phase current are equal. In a delta connection, it is the opposite: line and phase voltage are equal, while line current is 1.732 times the phase current. Getting comfortable with these relationships is essential for sizing conductors and calculating loads on three-phase systems.

25

Questions

38 min

Time

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Three-Phase Circuit Calculations Practice Test — Question List

Question 1: In a wye (star) three-phase configuration, how does line voltage relate to phase voltage?

Line voltage = phase voltage x 1.732
Line voltage = phase voltage exactly
Line voltage = phase voltage / 1.732
Line voltage = phase voltage x 3

Question 2: A wye-connected system has a phase voltage of 120 V. What is the line voltage?

120 V
208 V
240 V
360 V

Question 3: A wye-connected system has a line voltage of 480 V. What is the approximate phase voltage?

120 V
208 V
277 V
480 V

Question 4: In a wye (star) three-phase configuration, how does line current relate to phase current?

They are equal (line current = phase current)
Line current = phase current x 1.732
Line current = phase current / 1.732
Line current = phase current x 3

Question 5: In a delta three-phase configuration, how does line voltage relate to phase voltage?

They are equal (line voltage = phase voltage)
Line voltage = phase voltage x 1.732
Line voltage = phase voltage / 1.732
Line voltage = phase voltage x 3

Question 6: In a delta three-phase configuration, how does line current relate to phase current?

Line current = phase current x 1.732
They are always equal
Line current = phase current / 1.732
Line current = phase current x 3

Question 7: A delta-connected system has a phase current of 20 A. What is the line current?

11.5 A
20 A
34.6 A
60 A

Question 8: The formula for calculating three-phase real power is:

P = 1.732 x VL x IL x PF
P = VL x IL only, with no 1.732 factor
P = VL / IL
P = VL + IL

Question 9: A three-phase load operates at 208 V line voltage, draws 30 A line current, at a power factor of 1.0 (unity). Approximately how much real power does it use?

6,240 W
10,800 W
18,000 W
36,000 W

Question 10: The formula for finding three-phase line current from a known kVA and line voltage is:

I = (kVA x 1000) / (1.732 x V)
I = kVA x V
I = kVA / V only, with no 1.732 factor
I = V / kVA

Question 11: A three-phase transformer is rated 75 kVA at 208 V. Approximately what is its full-load line current?

75 A
120 A
208 A
360 A

Question 12: Why does the number 1.732 (the square root of 3) appear so often in three-phase electrical formulas?

It comes from the geometric/vector relationship between three AC waveforms that are each 120 degrees apart in phase
It is an arbitrary number with no mathematical basis
It only applies to single-phase circuits
It represents the number of wires in every three-phase circuit

Question 13: A "balanced" three-phase load is one where:

All three phases carry approximately equal current
Only one phase carries any current at all
The load is physically balanced on a scale
The neutral carries the most current of any conductor

Question 14: A significantly UNBALANCED three-phase load (one phase carrying much more current than the others) can cause:

Uneven heating of conductors and equipment, and current flowing on the neutral conductor in a wye system
No effect on the system whatsoever
Automatically balances itself with no consequences
Increased efficiency compared to a balanced load

Question 15: A "208Y/120" three-phase system designation means:

It is a wye-connected system with 208 V line-to-line voltage and 120 V line-to-neutral (phase) voltage
It is a delta-connected system with no neutral at all
It only works with single-phase loads
The numbers represent the frequency in Hz

Question 16: A "480Y/277" three-phase system designation means:

It is a wye-connected system with 480 V line-to-line voltage and 277 V line-to-neutral (phase) voltage
It is a single-phase-only system
It has no relationship to the wye or delta configuration concept
The 277 represents the frequency in Hz

Question 17: A "240 V delta" three-phase system, in contrast to a wye system, typically:

Has no true center-point neutral in the same way a wye system does (some delta systems use a mid-point "high-leg" tap instead)
Always has an identical neutral setup to a wye system
Cannot supply any three-phase loads
Is only used in single-phase residential wiring

Question 18: One practical advantage of a wye-connected system (like 208Y/120) is that it can supply:

Two useful voltages from the same system -- the higher line voltage for three-phase loads, and the lower phase voltage for single-phase loads
Only one single voltage under any condition
DC power exclusively
No usable voltage at all

Question 19: A delta-connected system has a line current of 52 A. What is the approximate phase current?

15 A
30 A
52 A
90 A

Question 20: A wye-connected system has a phase voltage of 277 V. What is the approximate line voltage?

277 V
416 V
480 V
554 V

Question 21: A basic three-phase, three-wire system (no neutral) has how many ungrounded (hot) conductors?

3
1
2
4

Question 22: In a standard three-phase system, each of the three phase voltages is separated from the others by how many degrees?

90 degrees
120 degrees
180 degrees
360 degrees

Question 23: You are given a wye system's LINE voltage and need to find the PHASE voltage. What operation should you perform?

Divide the line voltage by 1.732
Multiply the line voltage by 1.732
Multiply the line voltage by 3
No calculation is needed, they are always equal in wye

Question 24: You measure a three-phase system and find that line voltage and phase voltage are exactly equal. This system is most likely configured as:

Delta
Wye
Single-phase
This measurement is impossible in any configuration

Question 25: A three-phase transformer is rated in kVA rather than kW mainly because:

kVA represents apparent power, which does not depend on the power factor of whatever load happens to be connected
kVA and kW are exactly the same thing with different names
kW cannot be measured in three-phase systems
Transformers never carry any current at all

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)