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BASIC NEC 2023

Three-Phase Transformer Connections Practice Test

Free online three-phase transformer connections practice test. 25 basic electrical questions on wye-wye, delta-delta, delta-wye connections, open-delta banks, and three-phase transformer sizing. Good for electrician exam prep.

TOPICS COVERED

Three-Phase Transformer Bank Basics Wye-Wye Connection Delta-Delta Connection Delta-Wye Connection Use Wye-Delta Connection Limitation Open-Delta Capacity

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A three-phase transformer bank can be one integrated three-phase unit, or three single-phase transformers wired together. Four basic connections matter: wye-wye, delta-delta, delta-wye, and wye-delta. US utilities favor delta-wye most, because the wye secondary gives a neutral point for both three-phase power and single-phase 120 V loads (like 208Y/120), while the delta primary helps contain harmonic currents. An open-delta bank uses just two transformers to supply three-phase power at about 58% of full delta capacity — handy when one transformer of a bank fails.

25

Questions

38 min

Time

75%

Pass

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Three-Phase Transformer Connections Practice Test — Question List

Question 1: A three-phase transformer bank can be built using:

Either a single three-phase transformer unit or three separate single-phase transformers connected together
Only a single-phase transformer, with no other option
A minimum of six separate transformers, with no exceptions
A capacitor bank, with no transformer involved at all

Question 2: In a WYE-WYE (Y-Y) transformer connection, both the primary and secondary windings are connected:

In a wye (star) configuration, each with a common neutral point
In a closed delta loop, with no neutral at all
One side wye, the other side always disconnected
In series with a single resistor

Question 3: In a DELTA-DELTA (Δ-Δ) transformer connection, both the primary and secondary windings are connected:

In a closed triangular loop, with no neutral point on either side
In a wye configuration, with a shared neutral on both sides
Completely isolated, with no windings connected to each other
In series with a capacitor bank

Question 4: A DELTA-WYE (Δ-Y) transformer connection has a delta-connected primary and a wye-connected secondary. This arrangement is very commonly used by utilities because it:

Provides both three-phase power and a neutral point for single-phase 120 V loads from the same transformer
Cannot supply any single-phase loads at all
Only works with DC power
Requires no grounding of any kind

Question 5: A WYE-DELTA (Y-Δ) transformer connection has a wye-connected primary and a delta-connected secondary. This arrangement does NOT provide:

A neutral point on the secondary side
Any three-phase power at all
Any voltage transformation whatsoever
A primary winding of any kind

Question 6: An open-delta (V-connection) transformer bank uses only two transformers instead of three to supply three-phase power. Compared to a full delta bank of the same individual transformer size, an open-delta bank can supply approximately:

58%
100%
67%
25%

Question 7: One practical use of an open-delta connection is:

Allowing three-phase service to continue, at reduced capacity, if one transformer of a delta bank fails or is removed
Permanently increasing total system capacity beyond the original design
Eliminating the need for any grounding system
Converting three-phase power into single-phase power only

Question 8: Regardless of how three transformers are connected together as a bank (wye or delta), the voltage transformation (turns) ratio of each INDIVIDUAL transformer winding:

Still follows the same basic primary-to-secondary turns ratio as a single-phase transformer
No longer applies at all in a three-phase bank
Automatically becomes zero
Only applies if the bank uses a delta connection

Question 9: In a wye-connected transformer secondary supplying 120 V phase voltage, the line-to-line voltage is approximately:

208 V
120 V
240 V
360 V

Question 10: Grounding the neutral point of a wye-connected transformer secondary is common practice mainly to:

Establish a stable voltage reference point for the electrical system
Increase the transformer's total kVA rating
Convert the wye connection into a delta connection
Eliminate the need for any secondary winding

Question 11: A "corner-grounded delta" system is a less common arrangement where:

One phase conductor of a delta system is intentionally grounded, rather than using a neutral point
All three phase conductors are grounded simultaneously
No part of the system is ever grounded under any condition
The system is converted permanently into a wye configuration

Question 12: A three-phase transformer's total kVA rating represents:

The total apparent power capacity across all three phases combined
The capacity of only one single phase, with the others ignored
The transformer's physical weight in pounds
The transformer's operating temperature limit

Question 13: A three-phase transformer bank is rated 300 kVA total. Approximately how much kVA does each individual phase (or single-phase unit, if built from three separate transformers) handle?

100 kVA
300 kVA
900 kVA
30 kVA

Question 14: A specialized transformer connection called a "zig-zag" is primarily used to:

Create a grounding reference point (neutral) on a system that would otherwise have none, such as a delta system
Double the output voltage of any transformer
Convert three-phase power into DC power
Eliminate the need for any overcurrent protection

Question 15: Compared to using three separate single-phase transformers, a single integrated three-phase transformer unit is generally:

Smaller, less costly, and more efficient for the same total capacity
Always less reliable than three separate units
Incapable of supplying any three-phase load
Only usable on DC systems

Question 16: Compared to a single integrated three-phase transformer, using three separate single-phase transformers offers the advantage of:

Better redundancy — the bank can often keep running at reduced capacity (open-delta) if one unit fails
Guaranteed lower total cost in every situation
A smaller physical footprint in every situation
No need for any grounding of any kind

Question 17: Before connecting two three-phase transformers to operate in PARALLEL, it is essential that they have matching:

Voltage ratios, impedance, and phase relationships (vector group)
Only the physical color of the transformer casing
Only the manufacturer's name on the nameplate
Nothing — any two transformers can always be safely paralleled

Question 18: A common US utility secondary voltage system supplied by a delta-primary, wye-secondary distribution transformer is:

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

Question 19: A three-phase transformer secondary delivers 208 V line voltage and 50 A line current to a load with a power factor of 0.85. What is the approximate total real power delivered? (P = √3 × V<sub>L</sub> × I<sub>L</sub> × PF)

About 15,314 W
About 8,840 W
About 10,400 W
About 18,016 W

Question 20: The term "vector group" for a three-phase transformer describes:

The phase angle relationship (shift) between the primary and secondary voltages, based on how the windings are connected
The physical weight of the transformer in pounds
The transformer's cooling method only
The color-coding of the nameplate

Question 21: Wye-wye (Y-Y) transformer connections are used less often than delta-wye connections partly because a wye-wye connection can have problems with:

Third-harmonic distortion, unless a special tertiary winding is added to help control it
Complete inability to transform any voltage at all
An inability to be grounded under any condition
Excess DC current on the secondary side

Question 22: One reason delta-wye transformer connections are widely favored for stepping down utility power to a building is that the delta primary side helps:

Contain and absorb harmonic currents, keeping them from passing through to the wye secondary side
Increase harmonic currents passed to the secondary side
Eliminate the need for any secondary winding at all
Convert the entire system to DC power

Question 23: In a delta-connected transformer secondary, the neutral point:

Does not exist, since delta windings form a closed loop with no common center point
Is always exactly the same as the wye neutral point
Always carries the full line voltage
Is required by definition in a delta connection

Question 24: Whether a three-phase transformer bank is connected wye or delta, the individual transformer windings still must be sized to handle their own individual:

Winding voltage and current ratings, based on the specific connection used
Nothing — individual winding ratings never matter in a three-phase bank
Only the total bank kVA, with no other consideration
Only the color of the winding insulation

Question 25: In summary, the two most common three-phase transformer connections used for typical US utility-to-building power distribution are:

Delta-wye and, less commonly, wye-wye
Only single-phase connections, with no three-phase option ever used
Only DC-to-DC conversion, with no transformer involved
A connection type that does not exist in practice

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score75%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 (NFPA 70)