For a standard 5 kVA, 230V single-phase AC to AC converter operating at 85% efficiency and a 0.9 load power factor, the maximum continuous real output power is 3.825 kW, yielding a maximum output current of 16.63 Amps. The formula used to substitute these values is: I_out = (S_in × η × PF) / V_out, which becomes I_out = (5000 × 0.85 × 0.9) / 230 = 16.63A. This calculation assumes a unity voltage conversion ratio (230V in, 230V out) and a linear load.
The Assumptions That Fix Your Converter Math
To get a reliable output current from an AC to AC converter—whether a solid-state matrix converter, a cycloconverter, or a traditional autotransformer—four variables must be locked in: input apparent power (S_in), system voltage (V), power factor (PF), and conversion efficiency (η).
If any of these are floating, your conversion is meaningless. The most common bench mistake is assuming a power factor of 1.0 for motor loads. If the PF is unknown, or if the load is highly non-linear (Total Harmonic Distortion > 20%), converting apparent power (VA) to real power (W) or RMS current will result in dangerously undersized conductors. In high-THD environments, the distortion power factor drops the true PF well below the displacement PF. This means your converter will draw significantly more current than the basic formula predicts, leading to overheated neutrals and nuisance trips. Always measure true RMS current with a clamp meter capable of reading non-linear loads rather than relying on nameplate VA ratings alone.
How the Math Shifts: 120V vs 230V vs 3-Phase Systems
Presenting a single-voltage calculation as universal is a fast track to tripped breakers. The denominator in your current calculation shifts fundamentally based on the phase architecture.
- 120V Single-Phase: Common in North American residential branch circuits. The current doubles compared to 230V for the same power:
I = P / 120. - 230V Single-Phase: European standard or US split-phase line-to-line. The formula remains
I = P / 230. - 400V/480V 3-Phase: You must introduce the square root of 3 (≈ 1.732) to account for the phase angles:
I = P / (√3 × V_LL × PF). A 5 kW load on a 480V 3-phase system draws only about 7.5 Amps per phase, whereas that same load on a 120V single-phase system pulls over 41 Amps.
| Input Apparent Power | Real Output Power | Output Current (Amps) | Recommended Conductor (Copper, 60°C) |
|---|---|---|---|
| 4.0 kVA | 3.06 kW | 13.30 A | 14 AWG |
| 4.5 kVA | 3.44 kW | 14.96 A | 14 AWG |
| 5.0 kVA | 3.825 kW | 16.63 A | 12 AWG |
| 5.5 kVA | 4.20 kW | 18.26 A | 12 AWG |
| 6.0 kVA | 4.59 kW | 19.95 A | 10 AWG |
Topology Costs and Sizing: Transformers vs. Solid-State
The term "AC to AC converter" covers a massive price and performance gap. If you only need to change voltage amplitude (e.g., buck-boost 208V to 230V), a traditional copper-wound autotransformer is the correct tool. A 5 kVA buck-boost transformer typically costs between $150 and $250, operates at >98% efficiency, and introduces zero harmonic distortion. For foundational theory on transformer operation, refer to the All About Circuits AC textbook chapters.
However, if you need to change frequency (e.g., 60Hz to 50Hz for testing aerospace components or running imported machinery), you need a solid-state AC to AC frequency converter. These units rectify AC to DC, then invert it back to AC at the target frequency using IGBT switching. A 5 kVA solid-state frequency converter (such as the GoHz SFG-5000 series) will cost between $1,200 and $1,800. Because of the double-conversion process and internal switching losses, efficiency drops to 80-88%, and you must account for the heat dissipation in your enclosure sizing. According to power quality guidelines from Fluke, solid-state converters also inject harmonic currents back into the source, meaning you may need to oversize your upstream neutral conductors or install harmonic filters.
AC to AC Converter Sizing and Application FAQ
Can I use a standard AC to AC voltage converter to change frequency from 60Hz to 50Hz?
No. A standard transformer or autotransformer only changes voltage amplitude; it is completely transparent to frequency. If you feed 60Hz into a transformer, you get 60Hz out. To change frequency, you must use an active solid-state AC to AC converter (often called a frequency converter or variable frequency drive if driving a motor) that breaks the AC waveform down to a DC bus and synthesizes a new AC waveform at the desired Hertz using pulse-width modulation (PWM).
Why does my AC to AC converter trip the breaker when the motor starts?
Induction motors draw Locked Rotor Amperage (LRA) that is typically 5 to 7 times their Full Load Amperage (FLA) during startup, as outlined in the NEMA MG-1 standard for motors and generators. If your AC to AC converter is sized strictly for the motor's running wattage, the inrush current will instantly trigger the converter's internal overcurrent protection or trip the upstream branch breaker. You must either size the converter to handle the LRA (which is expensive and inefficient) or use a soft-start mechanism integrated into the converter's V/f (Volts per Hertz) control logic to ramp the voltage and frequency up over 2 to 5 seconds.
Do I need to derate an AC to AC converter for high ambient temperatures?
Yes. Solid-state AC to AC converters rely on internal heat sinks and cooling fans to dissipate IGBT switching losses. Most manufacturers rate their units for a 40°C (104°F) ambient maximum. For every 5°C above 40°C, you must derate the continuous output current by roughly 10%. If you are installing a 5 kVA converter inside a sealed NEMA 4X enclosure on a factory floor where ambient temps hit 45°C, your 16.63A capacity drops to roughly 15A. Always install a thermostat-controlled exhaust fan or an enclosure air conditioner for sealed deployments to prevent thermal shutdown.
