Single-phase transformers

It does: change AC voltage level, isolate circuits (isolation type), and match impedances.

It does not: create power from nothing. Ignoring losses, Pp ≈ Ps, so Vp · Ip ≈ Vs · Is.

Transformers are among the most efficient machines in the trade—often 90% to 99% at full load—but efficiency never excuses guessing load amps from “it looks small.”

All values scale with the turns ratio. Ideal relationships:

Vp / Vs = Np / Ns = Is / Ip

If the secondary has fewer turns, it is a step-down: lower voltage, higher available current. If it has more turns, it is a step-up.

You do not need the exact turn count on every job. You need the ratio and the nameplate voltages. Example: 480 V primary, 120 V secondary → ratio 4:1. At 12 A secondary, primary current is about 3 A (ideal).

Primary and secondary are electrically separated and magnetically coupled through the core. That isolation reduces voltage spikes coupled from the line and lets you reference the secondary as the design requires (including a center tap for 120/240 V).

A 1:1 isolation transformer keeps the same voltage but still gives galvanic separation—useful for sensitive equipment and for breaking ground loops when the application allows it.

With the secondary open, the primary draws only a small excitation current to magnetize the core. When you connect a load, secondary current rises and primary current rises with it.

At energizing, inrush can briefly be many times full-load current depending on where the AC wave is when the switch closes. Clamp meters with peak-hold show that spread. Protection must allow for inrush without nuisance trips—or you chase ghosts.

Copper losses (I²R) and core losses (hysteresis and eddy currents) become heat. Hot at no load points toward core / overvoltage issues. Hot only under load points toward overload or poor ventilation.

Habit: convert the job to kVA and rated current before you argue about “weak” voltage.

Many isolation transformers provide more than one secondary, or a center-tapped secondary for 120/240 V. Treat each secondary’s VA budget as real. You can overload one secondary while the other looks light—the primary still has to supply the sum.

When a center tap is used as a neutral, bond and ground it exactly as the design and code require. A floating “neutral” on a center tap is not a feature; it is a hazard and a source of wild line-to-neutral voltages under unbalanced load.

Technicians sometimes remember only “480 to 120 is 4:1.” The deeper habit is volts-per-turn: each turn on the same core sees the same induced volts (ideal). That is why tap changes move voltage in predictable steps and why a shorted turn is catastrophic—it becomes a local shorted loop of huge current and heat.

Before you speak, write down:

1. What topology or machine you have in front of you.
2. Voltages and currents with the measurement point.
3. Frequency or rpm if they apply.
4. What the nameplate or diagram says.
5. What changes if you isolate one part of the circuit.

A diagnosis without those data is conversation, not the trade.

1. Explain the central block out loud to an imaginary helper.
2. Rewrite the field case with numbers from a real piece of equipment.
3. Complete the checklist without looking.
4. Mark which rows in the mistakes table have already happened to you.

If you cannot say the core idea in one minute, return to the first third.

Energized measurement needs PPE, a meter of the right category, and a plan if the reading does not make sense. Capacitors, inductive fields, rotating shafts, and power neutrals do not forgive haste. If the procedure says de-energize and verify absence of voltage, that rules.

Situation. A 480–208/120 V, 15 kVA transformer. The helper hangs an 80 A load at 208 V and wonders why the primary breaker is at its limit.

How to think. Secondary full-load current ≈ 15 000 / 208 ≈ 72 A. At 80 A the unit is already overloaded. Primary at 480 V: ≈ 31 A full load. The numbers match power conservation.

Conclusion: always work in kVA and rated amps—not by eye.