Short answer: A transformer changes voltage up or down, and the current changes the opposite way, so the power in and out stays about the same. A motor turns electrical power into turning force, and it is started and stopped by a control circuit: the start and stop buttons switch a contactor coil, the contactor switches the motor power, and an overload relay opens the control circuit if the motor draws too much current for too long. The calculator below shows how voltage, turns ratio and current relate on a transformer.
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Transformer voltage ratio and current
Transformer voltage ratio and current calculator
Enter the nameplate kVA and the primary and secondary voltages. The example is a 25 kVA, 480 to 240 volt single-phase transformer. For three-phase, enter 3 for phases and use line-to-line voltages. This assumes an ideal transformer at full load. For single-phase transformers, the voltage ratio matches the winding turns ratio. For three-phase transformers, this calculator shows the line-to-line voltage ratio, which may differ from an individual winding’s turns ratio depending on the winding connections.
- Transformer rating, kVA
- Primary voltage, volts
- Secondary voltage, volts
- Phases (1 or 3)
How a transformer works
A transformer has two windings on an iron core. Alternating current in the primary winding sets up a changing magnetic field, which makes a voltage in the secondary winding. The voltage ratio follows the turns ratio: if the secondary has half the turns, it has half the voltage.
Power in is about equal to power out, so when voltage goes down, current goes up by the same ratio. For the same transferred power, the lower-voltage side carries more current. Its conductors must be sized for the applicable load and installation requirements, not just the voltage ratio. A transformer needs a changing current, such as alternating current; steady direct current does not keep passing power through it.
| Part | What it does | What you will hear on the job |
|---|---|---|
| Transformer | Changes AC voltage up or down | “Step-down from 480 to 208/120” |
| Motor | Turns electrical power into turning force | “Check the nameplate full-load amps” |
| Contactor | A heavy-duty switch worked by a coil | “The coil pulled in but the motor did not run” |
| Overload relay | Opens the control circuit on too much current for too long | “It tripped on overload” |
| Start/stop station | Push buttons that control the contactor coil | “Three-wire control with a holding contact” |
Motors and the start/stop circuit
A motor started directly across the line typically draws much more current during startup than during normal running. Soft starters and variable-frequency drives can limit starting current. Motor nameplates list ratings such as voltage, full-load amps and horsepower. OSHA defines a controller as a device that governs the electric power delivered to the equipment it is connected to; for a motor, the motor starter is a common example.
In a basic three-wire start/stop circuit, the control circuit and the motor power circuit are separate:
- Pressing start sends control power to the contactor coil.
- The coil closes the contactor, which connects the motor to power.
- A holding contact on the contactor keeps the coil energized after you let go of start.
- Pressing stop breaks the coil circuit, the contactor drops out and the motor stops.
- The overload relay senses motor current. OSHA describes an overload as running above the full-load rating long enough to cause damage or overheating. When that happens, the overload opens its contact in the control circuit and the contactor drops out.
The overload relay protects the motor from running hot for too long. Short circuits and ground faults are a different job, handled by the overcurrent protection ahead of the motor. Learn these circuits using drawings and equipment placed in an electrically safe work condition with your supervising electrician. Never assume that supervision alone makes energized troubleshooting safe or authorized. Follow your employer’s electrical safety procedures and work only within your training and qualifications.
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Ugly’s Electrical References, 2026 Edition
A pocket reference of electrical formulas and tables, handy for checking a turns-ratio or full-load current calculation.
Why does the current go up when a transformer steps the voltage down?
Because the power in and the power out are about the same. If the voltage is cut in half, the current has to about double to carry the same power. Losses in a real transformer are small, so the ideal math is a good starting estimate.
What is the difference between a contactor and an overload relay?
The contactor is the switch that connects the motor to power when its coil is energized. The overload relay watches the motor current and opens the control circuit if the motor draws too much for too long, which drops out the contactor.
Is the overload relay the same as the breaker for the motor?
No. The overload relay protects the motor from running overloaded. The breaker or fuses ahead of the motor protect against short circuits and ground faults. They are sized by different rules, so look up both in the 2026 NEC with your supervising electrician.
