Checked against primary sources 2026-08-24

Transformer questions punish you for forgetting which side you are standing on

The arithmetic is a ratio. The mistakes are almost all bookkeeping about which winding the number belongs to, and about which article answers which question.

What a transformer actually conserves

Power, near enough. Volt-amperes in equals volt-amperes out, minus losses that exam questions almost always ignore.

So if voltage goes up, current goes down in proportion. That single sentence generates every ratio you need and it means you never have to remember a formula with subscripts in it.

Write down the kVA once. It is the same number on both sides. Everything else follows from dividing it by the voltage on whichever side you want.

The three-phase factor, and when it appears

On a single-phase transformer, current is the volt-amperes divided by the voltage.

On a three-phase transformer, the same division happens but with the square root of three in the denominator alongside the voltage.

That factor is the single most common place points are lost here. It belongs in three-phase calculations and nowhere else, and questions are written so that omitting it produces a number in a plausible range.

Sizing the conductors on each side

Primary and secondary conductors are sized from their own side currents. They are not the same size and they are not sized from the same number.

Work out the current on the side you are being asked about, then size the conductor for that current the way you would any other: base ampacity, corrections, termination limit (NEC 110.14(C)).

A question that hands you a kVA rating and both voltages is usually checking whether you compute the right side before you start looking up wire.

Three separate obligations, three separate places

Almost every wrong answer in this topic comes from answering one of these with the rules for another.

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

Passing the transformer rules does not answer the conductor question and passing the conductor rules does not answer the transformer question. A design can satisfy 450.5 and still leave secondary conductors with no lawful basis to exist.

Where Article 450 sits in the 2026 book

Transformer overcurrent protection is NEC 450.5 in the edition Texas examinations reference from September 1, 2026. It was 450.3 in the 2023 book.

The 2026 edition put Listing Requirements at 450.2 and Reconditioned Equipment at 450.3, and the rest of Part I slid down by two to make room. So a tab reading 450.3 now opens on Reconditioned Equipment, whose subsections are both reserved, and a candidate who trusts the tab loses a minute finding out.

The permitted percentages sit in two tables. Table 450.5(B) covers transformers of 1000 volts or less and is the one most journeyman questions use. Table 450.5(A) covers transformers over 1000 volts and carries more conditions. Check which band your transformer is in before you open either one.

Within Table 450.5(B), what decides your row is whether the installation has primary protection only or protection on both the primary and the secondary. Read that off the question before you read the table, because the two rows give different answers for the same transformer.

One carve-out is worth carrying: a transformer installed as a motor control circuit transformer is taken out of the 450.5(B) table entirely and follows 430.72(C) instead. If the question says control circuit, you are in Article 430, not Article 450.

Secondary conductors are a known trap

Transformer secondary conductors are generally not considered protected by the primary overcurrent device (NEC 240.4(F)).

The two configurations that escape that are narrow: a single-phase transformer with a two-wire secondary, and a delta-delta connected transformer with a three-wire secondary. Everything else is on its own. And even for those two, the permission is conditional rather than automatic, so read the sentence rather than the summary.

When 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 question about a twenty-five-foot secondary run is testing, and looking for it in Article 450 wastes the time you do not have.

So a candidate who assumes the primary breaker covers everything downstream has skipped a requirement, and the question was almost certainly built around that assumption.

The method

1. Write the kVA down. It is the same on both sides.
2. Identify which side the question is asking about.
3. Divide by that side voltage, including the square root of three if it is three-phase.
4. Size conductors for that side from the resulting current.
5. Size transformer protection from 450.5, after checking whether protection exists on one side or both.
6. Handle the secondary conductors under 240.21(C), not under Article 450.

Label your two currents primary and secondary before you do anything else. Almost every wrong answer here is the right arithmetic applied to the wrong side.

What this page cites

• NEC 450.5 Overcurrent protection for transformers. 450.5(A) transformers over 1000 volts with Table 450.5(A), and 450.5(B) transformers 1000 volts or less with Table 450.5(B). A third subsection 450.5(C) exists; the draft read did not print its heading and this page makes no claim about its subject. This was 450.3 through the 2023 edition. In the 2026 edition 450.2 is Listing Requirements and 450.3 is Reconditioned Equipment, both reserved. Read in NFPA second revisions 8259 and 8262, panel 9, A2025 cycle.
• NEC 240.21(C) Transformer secondary conductors. Permits a set of conductors feeding a single load, or each set feeding separate loads, to connect to a transformer secondary without overcurrent protection at the secondary, in the six cases at 240.21(C)(1) through (C)(6). This is where the ten-foot and twenty-five-foot secondary conductor rules live, not in Article 450.
• NEC 240.4(F) Single-phase other than two-wire, and multiphase other than delta-delta three-wire, transformer secondary conductors are not considered protected by the primary device. The section does permit the primary device to protect a two-wire single-voltage secondary and a delta-delta three-wire single-voltage secondary, but conditions that permission, so read the whole sentence.
• NEC 430.72 Where 450.5(B) sends a transformer that is serving as a motor control circuit transformer.
• NEC 110.14(C) The termination temperature limit applied when sizing either side.