Transformer Calculations Practice Questions

A. 31.3 amperes

Single-phase current is the volt-amperes divided by the voltage, so 15,000 divided by 480 is 31.3 amperes. The 62.5 figure is the secondary current if the secondary is 240 volts, and it belongs to the other side of the problem. Writing primary and secondary on the page before dividing anything is what keeps the two apart.

NEC 2026 450.5

B. 90.2 amperes

Divide 75,000 by 480 and by the square root of three: 480 times 1.732 is 831, and 75,000 over 831 is 90.2 amperes. Leaving the square root out gives 156.3, which lands in a plausible range and is the single most productive error on this subject. The factor belongs in three-phase work and nowhere else.

NEC 2026 450.5

C. The kilovolt-ampere rating

Power in equals power out apart from losses the paper ignores, so the kVA is one number written once. Voltage and current move in opposite directions in proportion, which is where every ratio in the topic comes from. Conductor size follows from the current on each side and comes out different on each side for that reason.

NEC 2026 450.5

D. 208.2 amperes

Same kVA, different divisor: 208 times 1.732 is 360, and 75,000 over 360 is 208.2 amperes. The 360.6 option is what you get by dividing by the voltage alone and then treating the square root of three as a multiplier, which reverses the arithmetic. Divide by both the voltage and the factor, every time.

NEC 2026 450.5

A. 450.5

Listing requirements went in at 450.2 and reconditioned equipment at 450.3 for this cycle, and the rest of Part I slid down by two to make room. A tab set for the older book opens on reconditioned equipment, whose two subsections are both reserved, and the minute spent working out why is a minute you did not have.

NEC 2026 450.5

B. Table 450.5(B)

The section splits by voltage band before anything else. The over 1000 volt table carries impedance columns and a supervised location row, and using it on a 480 volt unit produces percentages that do not belong to the problem. Check the band, then open one table and stay in it.

NEC 2026 450.5(B), Table 450.5(B)

C. Whether the installation has primary protection only or protection on both the primary and the secondary

Two rows, two different answers for the same transformer, and the question tells you which one applies before it asks for a number. Impedance decides rows in the table for units over 1000 volts and in the footnote about coordinated thermal overload protection, but it does not pick your row here. Read the protection arrangement out of the stem first.

NEC 2026 Table 450.5(B)

D. A current transformer

The scope excludes a list of eight kinds of transformer, and current transformers head it. Also out: dry-type units that are a component part of other apparatus, transformers integral to X-ray and similar apparatus, Class 2 and Class 3 transformers, sign and outline lighting, electric-discharge lighting, power-limited fire alarm, and research and testing units under safeguards. Everything else is in.

NEC 2026 450.1

A. 125 percent

That is the ordinary case and the one most journeyman items use. It also carries the footnote that lets you take the next higher standard rating when 125 percent does not land on one. The 250 percent figure belongs to the primary side of an installation that also has secondary protection, and mixing the two rows is the error the table is designed to expose.

NEC 2026 Table 450.5(B)

B. 300 percent

The primary only row has three current bands and the percentage rises as the current falls, because a device sized at 125 percent of a very small current is smaller than the transformer inrush. Nine amperes and two amperes are the breakpoints. A 500 volt-ampere control transformer on a 480 volt primary draws about 1 amperes, which lands in the bottom band.

NEC 2026 Table 450.5(B)

C. 50 amperes

Primary current is 30,000 over 480 times 1.732, which is 36.1 amperes, and 125 percent of that is 45.1. That is not a standard rating, so the footnote lets you go up to the next standard rating, which is 50. A candidate who rounds 45.1 down to 45 has read the footnote backwards; it permits a higher rating, not a lower one.

NEC 2026 Table 450.5(B), Table 240.6(A)

D. 175 amperes

Primary current is 112,500 over 831, which is 135.3 amperes, and 125 percent of that is 169.2. Standard ratings run 150, 175, 200 through that stretch, so the next one above 169.2 is 175. Stopping at 150 protects the transformer but answers a question the table did not ask, which was for the maximum.

NEC 2026 Table 450.5(B), Table 240.6(A)

A. 250 percent

Adding a device on the secondary lets the primary device be much larger, because the secondary device is now the one holding the load. That single figure covers all three primary current bands in this row, so unlike the primary only row you do not have to check whether the current is above or below 9 amperes.

NEC 2026 Table 450.5(B)

B. 125 percent

The secondary side of this row reads like the primary only row: 125 percent above 9 amperes and 167 percent below it, with the next higher standard rating permitted on the 125 percent figure. Where the secondary protection is required as many as six breakers or six sets of fuses may share the job, grouped together in one place, with their ratings added up and measured against the single device figure.

NEC 2026 Table 450.5(B)

C. 225 amperes

Primary current is 90.2 amperes and 250 percent of that is 225.5, so the largest standard rating that does not exceed it is 225. The next higher standard rating footnote rides on the 125 percent cells only, so there is no rounding up out of this cell. That difference between the two rows is worth more marks than the arithmetic is.

NEC 2026 Table 450.5(B), Table 240.6(A)

D. The motor control circuit provisions in Article 430

The transformer section hands this case straight to the motor control circuit rules and takes it out of the table entirely. So a stem that says control circuit has told you which article you are in, and working the percentages anyway produces a defensible number that answers the wrong question. Read the description of the transformer before you read its rating.

NEC 2026 450.5(B), 430.72(C)

A. 175 amperes

Secondary current is 45,000 divided by 208 times 1.732, which is 360, giving 124.9 amperes. At 125 percent that is 156.1, which is not a standard rating, so the footnote takes you up to 175. Answering 150 uses the next rating below and reads as the cautious choice, but the item asked for the maximum the table allows.

NEC 2026 Table 450.5(B), Table 240.6(A)

B. A single-phase transformer with a two-wire secondary, or a delta-delta connected transformer with a three-wire secondary

Everything else is not considered protected by the primary device, which is the sentence a candidate who assumes the primary breaker covers the whole system has never read. Even for those two arrangements the permission is conditional: the primary protection has to comply with the transformer section, and the value may not exceed the secondary conductor ampacity times the secondary to primary voltage ratio.

NEC 2026 240.4

C. 23.1 amperes

Take one tenth of the primary device rating, which is 10, and multiply by the primary to secondary voltage ratio of 480 over 208, which is 2.31. That gives 23.1 amperes. Dividing by the ratio instead of multiplying gives 43.3 and is the same arithmetic upside down. This floor sits on top of the ordinary sizing to the load and to the device the conductors terminate in, so it does not replace them.

NEC 2026 240.21(C)

D. 133 amperes

The rule takes one third of the primary device rating and multiplies it by the primary to secondary voltage ratio. One third of 200 is 66.7, the ratio is 480 over 240, or 2, and the product is 133.3 amperes. Stopping at 66.7 is the whole calculation with the ratio left out, and it is the answer of somebody applying a feeder tap rule instead.

NEC 2026 240.21(C)

A. The impedance, for transformers 25 kVA and larger

The nameplate list runs to eight items: manufacturer, rated kilovolt-amperes, frequency, primary and secondary voltage, impedance at 25 kVA and larger, required clearances where the unit has ventilating openings, the amount and kind of insulating liquid where one is used, and for dry-type units the temperature class of the insulation system. Impedance is the one candidates forget, and it is the input the protection tables need for the coordinated overload footnote.

NEC 2026 450.13

B. It is within sight of the transformer, or in a remote location that is lockable open and field marked on the transformer

The section gives two ways to satisfy it and puts conditions on the second. A remote disconnect has to be lockable open and its location has to be marked on the transformer itself, so somebody arriving at the unit can find the way to kill it. An option that permits a remote disconnect with no marking has dropped the half of the rule that matters.

NEC 2026 450.16

C. In a transformer room of fire-resistant construction with a minimum 1 hour fire rating

The 112.5 kVA figure is the line. At or below it the rule is a 12 inch separation from combustible material unless a fire-resistant heat-insulating barrier is used, and above it the unit goes in a rated room. The two exceptions turn on a Class 155 or higher insulation system, with either a barrier and stated clearances or a completely enclosed construction. A vault is what dry-type units rated over 35,000 volts get.

NEC 2026 450.21

D. A unit rated 1000 volts or less and not exceeding 50 kVA that meets the ventilation and separation rules

Three conditions ride together here: the voltage band, a 50 kVA ceiling, and compliance with the ventilation section and the 12 inch separation from combustible material. A unit that meets all of them is not required to be readily accessible, which is the point of the permission. Dropping the kVA limit turns a narrow allowance into a general one.

NEC 2026 450.15

B. A transformer or a polyphase bank of two or more single-phase transformers operating as a unit

The section defines the word for its own purposes in its opening lines, and the bank case is what the definition is there for. Three single-phase units wired as a bank get protected as one transformer rather than three, so you work the percentages against the bank rating. Autotransformers have their own section and are not covered by this definition.

NEC 2026 450.5