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Transformer Calculations (NEC 450) — NEC 2023 Edition

Free NEC 2023 practice test on transformer installation and calculations. Covers primary/secondary overcurrent protection sizing (Table 450.3(B)), secondary conductor sizing, and dry-type transformer installation locations. Simple English, full explanations.

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The National Electrical Code (NFPA 70) is the benchmark standard for safe electrical installation in the US. It is updated every 3 years by the NFPA.

Study the edition adopted by your state. As of 2025, most states use the 2020 or 2023 NEC. These tests are written to the 2023 edition.

The most tested articles are 100 (Definitions), 200–250 (Wiring & Grounding), 300–314 (Wiring Methods), 400–411 (Conductors), 430 (Motors), and 700–701 (Emergency Systems).

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This Transformer Calculations (NEC 450) — NEC 2023 Edition has 30 questions with detailed answers and NEC references. Take the full test, review every wrong answer, and look up the NEC article it references before moving on.

Tip: Use the NEC index (back of the book) — find answers by keyword, not by article number.

Topics covered: NEC articles 100, 210, 220, 240, 250, 300, 310, 430, and special occupancies.
Passing score: Aim for 80%+ on NEC code tests before exam day.
Questions: 30 multiple-choice, one correct answer each

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Transformer Calculations (NEC 450) — NEC 2023 Edition — Question List

Question 1: Does the "next size up" overcurrent device rounding rule of NEC 240.4(B) apply when sizing protection for transformer secondary conductors under 240.21(C)?

True
False
Only for three-phase transformers
Only for autotransformers

NEC Reference: NEC 240.4(B) / 240.21(C)

Question 2: What is the primary overcurrent device rating for a 2-wire, 480V-to-120V transformer rated at 1.8 kVA?

6 A
7 A
4 A
10 A

NEC Reference: NEC Table 450.3(B)

Question 3: What is the primary overcurrent protection required for a 2-wire, 480V-to-120V transformer rated at 900 VA?

NEC Reference: NEC Table 450.3(B)

Question 4: What is the primary overcurrent protection required for a 2-wire, 480V-to-120V transformer rated at 7 kVA?

15 A fuse
20 A fuse
25 A fuse
10 A fuse

NEC Reference: NEC Table 450.3(B)

Question 5: What is the minimum size secondary conductor required for a 2-wire, 480V-to-120V transformer rated 2.2 kVA, with terminals rated 60 C?

12 AWG
14 AWG
8 AWG
10 AWG

NEC Reference: NEC Table 310.16

Question 6: Dry-type transformers rated 600 volts or less and NOT MORE than what kVA rating are permitted to be installed in hollow spaces of buildings, such as above a dropped ceiling?

50 kVA
100 kVA
85 kVA
75 kVA

NEC Reference: NEC 450.13(B)

Question 7: A dry-type transformer rated over 50 kVA is generally required to be installed where?

A readily accessible location
Anywhere, with no restriction
Only underground
Only outdoors

NEC Reference: NEC 450.13(A)

Question 8: Can a transformer be protected by overcurrent devices on the secondary side alone, without individual primary protection, under certain conditions?

Yes, under specific conditions
No, primary protection is always mandatory
Only for autotransformers
Only for three-phase units

NEC Reference: NEC 450.3(B)

Question 9: Per NEC Table 450.3(B), the 300% maximum primary protection tier applies when the transformer's rated primary current is below what value?

2 amperes
9 amperes
20 amperes
50 amperes

NEC Reference: NEC Table 450.3(B)

Question 10: Per NEC Table 450.3(B), the 167% maximum primary protection tier applies when the transformer's rated primary current is between what range?

2 amperes to less than 9 amperes
9 amperes to 20 amperes
Less than 1 ampere
Over 100 amperes

NEC Reference: NEC Table 450.3(B)

Question 11: Per NEC Table 450.3(B), the 125% maximum primary protection tier applies when the transformer's rated primary current is what?

9 amperes or more
Less than 2 amperes
Exactly 5 amperes
Under 1 ampere only

NEC Reference: NEC Table 450.3(B)

Question 12: A single-phase transformer delivers 240 volts at a full-load secondary current of 25 amperes. What is its kVA rating?

6 kVA
9.6 kVA
4 kVA
12 kVA

NEC Reference: NEC Article 450

Question 13: A 3-phase transformer is rated 75 kVA, 480V/208V. What is the approximate full-load secondary line current?

About 208 amperes
About 90 amperes
About 360 amperes
About 500 amperes

NEC Reference: NEC Article 450

Question 14: A separately derived system, such as the secondary of an isolation transformer, generally requires what at the source?

Its own grounding electrode connection and bonding jumper
Nothing extra at all
A connection to a telephone line
A padlock only

NEC Reference: NEC 250.30

Question 15: Can a transformer's overcurrent device serve to protect it against sustained thermal overload as well as short circuits?

Yes, generally both
No protection is ever provided
Only short circuits, never overload
Only overload, never short circuits

NEC Reference: NEC 450.3

Question 16: Dry-type transformer ventilating openings must generally not be blocked by what?

Walls or other obstructions
Nothing -- blocking is fine
Paint only
It does not matter

NEC Reference: NEC 450.9

Question 17: Transformers must generally be marked with a nameplate showing which of the following, among other data?

Rated kVA and voltage
Nothing at all
Only the paint color
Only the installer's name

NEC Reference: NEC 450.11

Question 18: Transformer vaults required for certain larger or liquid-filled transformers must generally be constructed with what type of materials?

Fire-resistant, structurally adequate materials
Cardboard
Untreated wood
No specific material is required

NEC Reference: NEC 450.42

Question 19: Oil-insulated transformers installed indoors generally require what, due to fire risk from the insulating oil?

A vault, with limited exceptions
No special measures at all
Only a warning sign
Installation is never permitted indoors

NEC Reference: NEC 450.26

Question 20: Secondary conductors from a transformer feeding a panelboard must generally be sized based on what?

Secondary current and applicable tap rules
A random guess
Only the transformer's weight
The primary current only

NEC Reference: NEC 240.21(C) / Table 310.16

Question 21: A 3-phase transformer is rated 30 kVA at 480 volts primary. What is the approximate primary current?

About 36 amperes
About 62.5 amperes
About 18 amperes
About 100 amperes

NEC Reference: NEC Article 450

Question 22: A transformer must have a disconnecting means within sight, unless what alternative condition is met?

Field marked location, lockable open
No alternative is ever allowed
A sign is enough with no lock
Any random condition

NEC Reference: NEC 450.14

Question 23: When two transformers are operated in parallel, what electrical characteristics generally must be matched for safe operation?

Voltage ratio, impedance, and phase relationship
Only the paint color
Nothing needs to match
Only physical size

NEC Reference: NEC Article 450 / general engineering practice

Question 24: For a transformer 600 volts or less, protected only on the secondary side, what is the maximum secondary protection generally allowed as a percent of secondary current?

125%
300%
50%
400%

NEC Reference: NEC 450.3(B)

Question 25: A transformer's core and copper losses generally cause its output power to be what, compared to its input power?

Slightly less than input power
Always exactly equal
Always greater than input power
Completely unrelated to input power

NEC Reference: General electrical theory

Question 26: Under the transformer secondary conductor tap rules, secondary conductors are generally limited to what maximum length when not separately protected, unless additional conditions are met?

25 feet
10 feet
100 feet
There is no length limit

NEC Reference: NEC 240.21(C)(6)

Question 27: A 3-phase, delta-delta transformer bank is rated 45 kVA at 480 volts primary. What is the approximate rated primary current?

About 54 amperes
About 94 amperes
About 27 amperes
About 150 amperes

NEC Reference: NEC Article 450

Question 28: A zig-zag grounding autotransformer is generally used in power systems to create what, when one is not otherwise available from the source?

A neutral point for grounding
More horsepower
A higher voltage only
Nothing useful

NEC Reference: NEC Article 450

Question 29: Transformers installed indoors, other than in a vault, must generally have exposed energized parts what?

Guarded against accidental contact
Left fully exposed
Painted a bright color only
Nothing is required

NEC Reference: NEC 450.8

Question 30: Potential (voltage) transformers used for instrumentation, rated for circuits over 600 volts, must generally be protected by what on the primary side?

Primary-side fuses
Nothing at all
A padlock only
A warning sign only, no fuse

NEC Reference: NEC 450.3

Key NEC References for This Test

Article	What It Covers
Ch. 1–4	General rules — apply to all installations
Ch. 5–7	Special rules — override Ch. 1–4 where they conflict
Ch. 8	Communications — completely independent of Ch. 1–7
Ch. 9	Tables — referenced by articles, not standalone enforceable rules
Art. 210.12	AFCI — major expansion in NEC 2026
Art. 230.67	Surge protection at service equipment — new NEC 2026

Study Tip: Learn the NEC index — use keywords, not article numbers. Practice finding 5 different topics in under 2 minutes each.

Transformer Calculations (NEC 450) — NEC 2023 Edition

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Key NEC References for This Test

Transformer Calculations (NEC 450) — NEC 2023 Edition

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Transformer Calculations (NEC 450) — NEC 2023 Edition — Question List

Question 1: Does the "next size up" overcurrent device rounding rule of NEC 240.4(B) apply when sizing protection for transformer secondary conductors under 240.21(C)?

Question 2: What is the primary overcurrent device rating for a 2-wire, 480V-to-120V transformer rated at 1.8 kVA?

Question 3: What is the primary overcurrent protection required for a 2-wire, 480V-to-120V transformer rated at 900 VA?

Question 4: What is the primary overcurrent protection required for a 2-wire, 480V-to-120V transformer rated at 7 kVA?

Question 5: What is the minimum size secondary conductor required for a 2-wire, 480V-to-120V transformer rated 2.2 kVA, with terminals rated 60 C?

Question 6: Dry-type transformers rated 600 volts or less and NOT MORE than what kVA rating are permitted to be installed in hollow spaces of buildings, such as above a dropped ceiling?

Question 7: A dry-type transformer rated over 50 kVA is generally required to be installed where?

Question 8: Can a transformer be protected by overcurrent devices on the secondary side alone, without individual primary protection, under certain conditions?

Question 9: Per NEC Table 450.3(B), the 300% maximum primary protection tier applies when the transformer's rated primary current is below what value?

Question 10: Per NEC Table 450.3(B), the 167% maximum primary protection tier applies when the transformer's rated primary current is between what range?

Question 11: Per NEC Table 450.3(B), the 125% maximum primary protection tier applies when the transformer's rated primary current is what?

Question 12: A single-phase transformer delivers 240 volts at a full-load secondary current of 25 amperes. What is its kVA rating?

Question 13: A 3-phase transformer is rated 75 kVA, 480V/208V. What is the approximate full-load secondary line current?

Question 14: A separately derived system, such as the secondary of an isolation transformer, generally requires what at the source?

Question 15: Can a transformer's overcurrent device serve to protect it against sustained thermal overload as well as short circuits?

Question 16: Dry-type transformer ventilating openings must generally not be blocked by what?

Question 17: Transformers must generally be marked with a nameplate showing which of the following, among other data?

Question 18: Transformer vaults required for certain larger or liquid-filled transformers must generally be constructed with what type of materials?

Question 19: Oil-insulated transformers installed indoors generally require what, due to fire risk from the insulating oil?

Question 20: Secondary conductors from a transformer feeding a panelboard must generally be sized based on what?

Question 21: A 3-phase transformer is rated 30 kVA at 480 volts primary. What is the approximate primary current?

Question 22: A transformer must have a disconnecting means within sight, unless what alternative condition is met?

Question 23: When two transformers are operated in parallel, what electrical characteristics generally must be matched for safe operation?

Question 24: For a transformer 600 volts or less, protected only on the secondary side, what is the maximum secondary protection generally allowed as a percent of secondary current?

Question 25: A transformer's core and copper losses generally cause its output power to be what, compared to its input power?

Question 26: Under the transformer secondary conductor tap rules, secondary conductors are generally limited to what maximum length when not separately protected, unless additional conditions are met?

Question 27: A 3-phase, delta-delta transformer bank is rated 45 kVA at 480 volts primary. What is the approximate rated primary current?

Question 28: A zig-zag grounding autotransformer is generally used in power systems to create what, when one is not otherwise available from the source?

Question 29: Transformers installed indoors, other than in a vault, must generally have exposed energized parts what?

Question 30: Potential (voltage) transformers used for instrumentation, rated for circuits over 600 volts, must generally be protected by what on the primary side?

Key NEC References for This Test

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