Transformer questions on a Texas paper, and the number that moved under everybody

How the exam actually asks it

Candidates revise this topic expecting ratios and then meet something else. Of the 14 questions in our 500 bank that involve a transformer, two are pure arithmetic in the theory area, six sit in the grounding area, four in the installation area and two in the code administration area. So roughly one question in seven on this topic is a calculation and the rest are about where the rules live and which source a fault returns to.

That distribution should change how you revise. The ratio work takes an afternoon and then stays learned. The rest asks you to hold four separate obligations apart from each other, keep a separately derived system distinct from a service, and know that one section number moved two places this cycle. Those are the parts worth rehearsing with the book open.

The number that moved this cycle

Transformer overcurrent protection is NEC 450.5 in the 2026 edition. It was 450.3 through the 2023 edition, and Texas examinations reference the 2026 code from 1 September 2026 under 16 TAC 73.100.

The move came from a template the 2026 edition ran through the whole book, putting listing requirements at .2 of an article and reconditioned equipment at .3, with everything that had been in those slots sliding down. So Article 450 now opens with 450.2 for listing, 450.3 for reconditioned equipment, and the overcurrent rules at 450.5.

The permitted percentages sit in two tables. One covers transformers of 1000 volts or less and is the one most journeyman questions use. The other covers transformers over 1000 volts and carries more conditions. Check which band your transformer is in before you open either, and then read off whether the installation has primary protection only or protection on both sides, because those two rows give different answers for the same machine.

Volt-amperes are what stays fixed

Power passes through, near enough, so the volt-amperes on the two sides match apart from losses that exam questions set aside. Voltage up means current down in the same proportion, and that single sentence generates every ratio on this topic without a formula full of subscripts.

Two bank questions work it in opposite directions. A transformer with a 480 volt primary and a 120 volt secondary has a 4 to 1 ratio, so 10 amperes in the primary becomes 40 amperes in the secondary. Check it by multiplying: 480 times 10 is 4,800 volt-amperes, and 120 times 40 is the same 4,800. The other question gives 600 turns on the primary and 150 on the secondary with 480 volts applied, which is the same 4 to 1 ratio, putting 120 volts on the secondary.

The useful check is directional. Current and voltage move opposite ways through a transformer, so any answer where both of them fall is wrong before you verify the arithmetic. An answer that squared the ratio has borrowed the relationship that applies to impedance. An answer that put more voltage on the winding with fewer turns has the ratio upside down.

On a three phase transformer the same division happens with the square root of three in the denominator alongside the voltage. That factor belongs in three phase work and the questions are written so that dropping it still produces a number in a plausible range, which is why it is the single most common place points go on the arithmetic side. The theory area drills the same denominator in several other settings.

Three obligations in three places

Almost every wrong answer on this topic comes from answering one obligation with the rules that govern another. There are three of them and they live apart.

• Protecting the transformer itself is Article 450, at 450.5 in the 2026 edition.
• Protecting the conductors on the primary side is ordinary conductor protection under Article 240.
• Protecting the conductors on the secondary side is 240.21(C), which is where the ten foot and twenty-five foot secondary allowances live.

A design can satisfy 450.5 in full and still leave the secondary conductors with no lawful basis for existing, because passing the transformer rules answers a different question. A candidate who looks for the twenty-five foot rule inside Article 450 loses time they will want later in the paper.

One carve-out is worth carrying. A transformer serving as a motor control circuit transformer steps outside the 450.5 table entirely and follows 430.72(C). Where a stem says control circuit, you are in Article 430, and the motor protection page covers where that section sits among the other devices on a motor circuit.

The secondary conductor trap

Transformer secondary conductors are generally treated as unprotected by the primary overcurrent device, under 240.4(F). Two configurations escape that and both are narrow: a single-phase transformer with a two-wire secondary, and a delta-delta connected transformer with a three-wire secondary. Even for those two the permission carries conditions, so read the whole sentence in your own book.

Where the secondary conductors have no overcurrent device at the secondary, what makes them lawful is 240.21(C), which sets out six cases with their own length limits and ampacity conditions. That is the section a twenty-five foot secondary run is testing.

The definitions article backs this up and generates a question of its own. Conductors running from the secondary terminals to the first overcurrent device downstream are feeder conductors, because Article 100 defines a feeder as running from the service equipment, the source of a separately derived system, or another power supply source, up to the final branch circuit overcurrent device. A transformer secondary is a source. How those conductors get protected is a separate question answered in Article 240, and the fact that they lack upstream protection has no bearing on what they are called.

The transformer as a separate source

Six of the 14 transformer questions in our bank sit in the grounding area, which makes this the busiest corner of the topic. A transformer secondary is a new source, and fault current on it returns to that source.

The path runs on the equipment grounding conductors to the transformer or to the first disconnecting means, crosses the system bonding jumper required by 250.30(A)(1), and returns on the derived grounded conductor to the winding. That is why the derived system needs a bonding jumper of its own and why the grounding conductor of the primary circuit is unable to stand in for it. Answers that send a secondary fault up the primary grounding conductor, or out into the earth through an electrode, are both on the page for candidates who have the wrong source in mind.

The location rule generates the other common question. 250.30(A)(1) permits the system bonding jumper at the source or at the first disconnecting means, and at one of those places only. Installing both and calling it redundancy closes a loop, and derived neutral current then comes back partly on the metal between the transformer and the disconnect. A permission to choose a location is a permission to pick one.

Then 250.30(A)(8) bonds the metal water piping system and the exposed structural metal in the area served by the derived system, connected at the same point where the system bonding jumper is made. Bonding done back at the service ties that metal to the utility source and puts it nowhere on the fault path of this transformer secondary, which is why a floor-level transformer in an office building brings its own bonding obligation with it. The grounding area covers the rest of that sequence.

What the bookkeeping costs on the job

Two of these errors survive an inspection and go on costing money.

Two system bonding jumpers is the first. Everything works, the lights come on, and neutral current flows on building steel and on the raceway between the transformer and the disconnect for as long as the installation stands. It shows up later as stray voltage complaints, as a data problem nobody can place, or as a reading somebody takes on a clamp meter and cannot explain. Finding it means tracing a path through metal that was never meant to carry current.

The second is the pad mounted transformer at the edge of a parking lot. Article 110 requires outdoor electrical equipment to be protected from accidental contact and from vehicular traffic, and 300.6 carries the parallel requirement for the wiring. A pickup rolling at walking speed will shift an enclosure and open an energized part at chest height in a place the public walks. Bollards are the usual answer and siting the equipment out of the drive path is an equally good one. An enclosure type rating covers water and dust, and it does nothing about impact.

Worth knowing for scope: a utility owned pad mounted transformer feeding a building service sits outside the code under 90.2, which excludes installations under the exclusive control of an electric utility for generation, transformation, transmission, distribution and metering, wherever they stand. The premises wiring is covered in full.

What to do before test day

Five habits cover this topic, and four of them are written down before you start the arithmetic.

1. Write the kVA once and label it. It is the same figure on both sides and everything else comes from dividing it by the voltage on the side the question asks about.
2. Label your two currents primary and secondary before you calculate either. Most wrong answers here are correct arithmetic applied to the wrong winding.
3. Retab Article 450. A tab reading 450.3 now opens on reconditioned equipment, and overcurrent protection is at 450.5.
4. Tab 240.21(C) as well, because that is where a secondary conductor question is answered and hunting for it in Article 450 wastes the time you will want at the end of the paper.
5. Ask which source a fault is trying to reach before you answer any grounding question about a transformer. Six of the 14 questions in the area turn on that one decision.

For the ratios and the conductor sizing worked through with figures on the page, the question sets carries the transformer calculations page, which is where the worked arithmetic lives. Then take the thirty free questions here with the timer on, because the bookkeeping habits above only pay off under time pressure and that is the condition to practice them in.

Questions people ask

Where is transformer overcurrent protection in the 2026 code?

At NEC 450.5. It sat at 450.3 through the 2023 edition, and the 2026 edition moved it when a uniform section template put listing requirements at .2 of an article and reconditioned equipment at .3. Turning to 450.3 in a 2026 book now gives you a heading for reconditioned equipment with its subsections marked reserved, which reads as though you are in the right place. Texas examinations reference the 2026 edition from 1 September 2026 under 16 TAC 73.100, so check any tab you wrote against an older book.

Which side of a transformer do I size the conductors from?

Each side from its own current. Primary conductors come off the primary current and secondary conductors come off the secondary current, and the two are different sizes because the two currents differ by the turns ratio. Work out the current on the side the question asks about first, then size that conductor the way you would any other: base ampacity, corrections, and the termination temperature limit at 110.14(C). A question that hands you a kVA rating and both voltages is usually checking whether you computed the right side before you started looking at wire.

Does the primary overcurrent device protect the secondary conductors?

Generally no. NEC 240.4(F) treats transformer secondary conductors as unprotected by the primary device, with two narrow escapes: a single-phase transformer with a two-wire secondary and a delta-delta connected transformer with a three-wire secondary. Both of those permissions carry conditions, so read the whole sentence. Where there is no overcurrent device at the secondary, what makes the conductors lawful is 240.21(C), which sets out six cases with their own length limits and ampacity conditions, and that is where the ten foot and twenty-five foot runs are answered.

Where does the system bonding jumper go?

At the source of the separately derived system or at the first disconnecting means, and at one of those two places only, under NEC 250.30(A)(1). Installing one at each and calling it redundancy closes a loop, and derived neutral current then returns partly on the metal between the transformer and the disconnect. The permission names a choice of location, which is a permission to pick one. The same section governs where the metal water piping and exposed structural metal in the area served get bonded, which is at the same point the jumper is made.

Where does a ground fault on a transformer secondary return to?

To the transformer secondary. A separately derived system has its own source, so a fault on a branch circuit downstream returns there. The path runs along the equipment grounding conductors to the transformer or the first disconnect, crosses the system bonding jumper, and goes back on the derived grounded conductor to the winding. The grounding conductor of the primary circuit protects the primary circuit and is unable to stand in for that jumper, and the electrode is never the fault path on any system, so both of those answers are distractors.

What about a transformer supplying a motor control circuit?

It steps outside the ordinary transformer overcurrent table and follows NEC 430.72(C). Article 450 hands that case to Article 430, so a stem mentioning a control circuit, a start-stop station or a pilot device has told you which article to open. The wider point is worth carrying: motor control circuits have their own protection section at 430.72, working from a table indexed by conductor size and by whether the conductor leaves the enclosure, and it becomes a fourth plausible answer whenever a motor question says control circuit.