AC to AC Converter Current Calculator: 240V to 120V Step-Down Sizing

If you are running a 2000W appliance through a 240V to 120V AC to AC converter, the 240V input side draws 8.33A while the 120V output side delivers 16.67A (assuming 100% efficiency and a 1.0 power factor). The core formula used to find the output current is I_out = P / (V_out × η × PF). Substituting our exact values: I_out = 2000W / (120V × 1.0 × 1.0) = 16.67A. When wiring this setup, always size your output conductors and overcurrent protection for the higher output current, not the lower input current.

The Assumptions That Fix Your AC to AC Converter Math

An AC to AC converter—whether a traditional toroidal step-down transformer or a solid-state voltage regulator—does not create power; it conserves it minus system losses. To get a reliable amperage calculation, four assumptions must be locked in:

• Voltage Ratio: The exact primary and secondary voltages under load. A 240V primary might droop to 235V under heavy load, slightly shifting the ratio.
• Efficiency (η): No converter is 100% efficient. According to the U.S. Department of Energy, modern distribution transformers typically operate between 97% and 99% efficiency at peak load, while smaller benchtop solid-state AC-AC converters often sit around 85% to 92%. If you assume 100% efficiency, you will undersize your input wiring.
• Power Factor (PF): The ratio of real power (Watts) to apparent power (Volt-Amps). A purely resistive load (like a space heater) has a PF of 1.0. Inductive loads (like motors) drop this to 0.7 or 0.8, meaning the converter must supply more current to deliver the same wattage.
• Phase Configuration: Single-phase math is straightforward. Three-phase systems require the square root of 3 (1.732) as a multiplier.

Output Current Table: 240V to 120V Step-Down (±20% Range)

Below is a reference table for a standard 240V-to-120V single-phase AC to AC converter, scaled ±20% around our 2000W baseline. This assumes a 95% converter efficiency and a 1.0 power factor (resistive loads). Wire sizes are based on 75°C THHN copper ampacity columns.

Voltage and Phase Shifts: 230V, 120V, and 3-Phase

Treating 240V as a universal baseline will cause sizing errors when dealing with international equipment or industrial power. Here is how the math shifts across different grid standards:

230V vs. 240V Nominal: In the EU and UK, the harmonized nominal voltage is 230V, whereas North American split-phase is nominally 240V (though often measured at 236V at the panel). If you plug a 2000W load into a 230V-to-120V converter, the input current rises slightly to 9.15A (at 95% efficiency). Always calculate using the measured voltage at the receptacle, not the nominal panel voltage, to account for voltage drop across long feeder runs.

Three-Phase AC to AC Converters: Industrial step-down converters (often matrix converters or heavy 3-phase transformers) distribute the load across three legs. The formula shifts to include the square root of 3: I = P / (√3 × V × PF × η). For a 6000W load on a 400V 3-phase input, the current per leg is only 9.12A, allowing for much smaller input conductors compared to a single-phase equivalent.

When Watt-to-Amp Conversion Becomes Meaningless

The conversion from Watts to Amps on an AC to AC converter becomes entirely meaningless when the Power Factor (PF) is unknown for an inductive load.

If you are stepping down voltage to run an AC motor, an air compressor, or a refrigerator, the nameplate wattage only tells you the real power doing useful work. It ignores the reactive power required to maintain the magnetic fields in the motor windings. As detailed in All About Circuits, apparent power (VA) is what the converter actually has to supply. If a motor draws 1000W but has a poor PF of 0.6, the converter must be sized for 1666 VA. If you size your AC to AC converter and output wiring based purely on the 1000W real power figure, the converter will overheat, and the output voltage will sag severely under load. For inductive loads, always calculate using the nameplate FLA (Full Load Amps) directly, bypassing wattage conversions entirely.

AC to AC Converter Sizing FAQs

Can I use a 2000W AC to AC converter for a 15A 120V power tool?

Yes, but you must account for startup surges. A 15A 120V power tool draws 1800W of running power, which fits within a 2000W converter's continuous rating. However, universal motors in tools like circular saws or routers can draw 2x to 3x their rated current for the first half-second of startup (Locked Rotor Amps). Ensure your converter has a stated surge capacity of at least 4000W for 1-2 seconds, or the internal thermal fuse will trip instantly upon pulling the tool's trigger.

Does an AC to AC converter change the frequency from 50Hz to 60Hz?

A standard transformer-based AC to AC converter does not change frequency; it only changes voltage. If you feed it 50Hz, the output will be 50Hz. Running a 60Hz North American appliance on 50Hz will cause motors to run 17% slower, overheat due to reduced cooling fan speed, and draw higher magnetizing current. To change both voltage and frequency, you need a solid-state AC-AC frequency converter (often called a variable frequency drive or VFD for motors), which rectifies the AC to DC and then inverts it back to AC at the target frequency.

Why does my AC to AC converter trip the breaker on startup?

This is caused by transformer magnetizing inrush current. When you first energize the primary winding of a heavy iron-core step-down converter, the core can temporarily saturate if the AC voltage cycle happens to switch on near the zero-crossing point. This results in a massive, momentary spike in current (up to 10x-15x nominal) that lasts for 3 to 5 AC cycles. A standard thermal-magnetic breaker might interpret this as a short circuit and trip. The fix is to use a breaker with a high magnetic trip threshold (like a Type C or Type D curve breaker in IEC regions, or a HACR-rated breaker in the US) that tolerates brief inrush spikes without tripping.