Transformer Calculations (NEC 450) — NEC 2023 Edition

Question 1: May the "next size up" rule of 240.4(B) be used when protecting transformer secondary conductors under 240.21(C)?

• Yes, always
• Yes, but only below 800 A
• Only for single-phase transformers
• No, 240.21(C) states that 240.4(B) does not apply

NEC Reference: NEC 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?

• 5 A
• 6 A
• 4 A
• 3 A

NEC Reference: NEC Table 450.3(B)

Question 4: A 2-wire, 480-to-120 V transformer is rated 7 kVA and has primary-only protection. What is the maximum standard primary fuse size?

• 30 A
• 25 A
• 20 A
• 15 A

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 1000 volts or less and not more than what kVA may 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: A transformer rated 1000 V or less has secondary protection at not more than 125% of its secondary current. What is the maximum primary (feeder) overcurrent device rating permitted?

• 125% of primary current
• 167% of primary current
• 250% of primary current
• 300% of primary current

NEC Reference: NEC Table 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

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: The secondary of a step-down transformer is a separately derived system. What does 250.30(A) require at the source or first disconnect?

• A second main bonding jumper at the service
• A ground rod at every panelboard it feeds
• A GFCI device on the secondary
• A system bonding jumper and a grounding electrode conductor

NEC Reference: NEC 250.30(A)

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: What does 450.9 require for transformers with ventilating openings?

• Openings located so walls or obstructions do not block them, with marked clearances kept
• Forced-air cooling in every case
• Openings covered with screens only
• A minimum of 10 ft to any wall

NEC Reference: NEC 450.9

Question 17: Which information must appear on a transformer nameplate?

• The installation date and installer
• Manufacturer, rated kVA, frequency, voltages and (25 kVA and larger) impedance
• The building circuit number
• Only the manufacturer's name

NEC Reference: NEC 450.11

Question 18: What minimum fire resistance is required for the walls and roof of a transformer vault?

• 4 hours
• 3 hours
• 2 hours
• 1 hour

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: Separate equipment grounding conductors and supply-side bonding jumpers are installed in a dry-type transformer enclosure. What must be provided?

• Connection to the neutral bar only
• A separate ground rod at the transformer
• A splice in the bottom of the enclosure
• A terminal bar for those connections, not over the vented part of the enclosure

NEC Reference: NEC 450.10(A)

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

Question 22: A transformer's disconnecting means is not in sight of the transformer. What does 450.14 require?

• Its location field marked on the transformer, and it must be lockable in the open position
• A sign at the service only
• Nothing more
• A second disconnect at the transformer

NEC Reference: NEC 450.14

Question 23: Two transformers will be operated in parallel and switched as a unit. What does 450.7 require?

• One overcurrent device sized at 100% of the combined rating
• Both transformers must be in a vault
• The overcurrent protection for each transformer must meet 450.3(A) or (B)
• No overcurrent protection on the secondary

NEC Reference: NEC 450.7

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 45 kVA, 480-208Y/120 V three-phase transformer is fully loaded. What is the approximate secondary line current?

• 54 A
• 94 A
• 125 A
• 217 A

Question 28: What is a grounding (zig-zag) autotransformer used for on a 3-phase, 3-wire ungrounded system?

• To raise the system voltage
• To filter harmonics only
• To correct power factor
• To establish a neutral point for grounding or for supplying line-to-neutral loads

NEC Reference: NEC 450.5

Question 29: What does 450.8 require for exposed energized parts of transformer installations?

• Guarding against accidental contact
• A kVA label only
• Insulating tape over each terminal
• No special requirement indoors

NEC Reference: NEC 450.8

Question 30: How must voltage (potential) transformers installed indoors or enclosed be protected?

• With a secondary breaker only
• With primary fuses
• With a ground-fault relay only
• No protection is required

NEC Reference: NEC 450.3(C)