To "convert" 20 amps to voltage for a standard 4,800-watt resistive load (like an electric water heater), the direct answer is 240 volts. You cannot directly convert amps to volts without a third variable—either Power (Watts) or Resistance (Ohms)—because current and voltage are related by the load's characteristics, not by a fixed exchange rate. Using Watt's Law (V = P / I), we substitute the values: 4800W / 20A = 240V. If you are instead looking for a physical electronic circuit to convert a 4-20mA current signal to a 1-5V voltage signal for a PLC, you need a transimpedance amplifier or a precision 250Ω shunt resistor.
| Current (Amps) | Voltage (Volts) | Deviation from Baseline |
|---|---|---|
| 16A | 300.0V | -20% |
| 18A | 266.7V | -10% |
| 20A | 240.0V | Baseline |
| 22A | 218.2V | +10% |
| 24A | 200.0V | +20% |
The Math: Assumptions That Fix the Answer
The fundamental assumption that fixes any amps-to-voltage conversion is that Power (Watts) or Resistance (Ohms) must be known and constant. In DC circuits or purely resistive AC circuits (like baseboard heaters), the math is absolute. However, in real-world AC environments, the answer shifts dramatically based on system phase and nominal voltage.
If you are trying to figure out what voltage is required to push a specific amperage for a fixed load, or how many amps a specific voltage will draw, you must account for the system architecture. According to Fluke's guidelines on AC power measurement, single-phase and three-phase systems distribute power differently, meaning the conversion formula changes.
- Single-Phase (120V/230V): V = P / (I × PF)
- Three-Phase (208V/400V): V = P / (√3 × I × PF)
Below is a data-dense matrix showing how the required current shifts for a fixed 7,200W (7.2kW) EV charger load across different global voltage standards, assuming a Power Factor (PF) of 1.0.
| System Type | Nominal Voltage | Formula Used | Resulting Current (Amps) | Typical Breaker Size (125% Rule) |
|---|---|---|---|---|
| 1-Phase (North America) | 120V | 7200 / (120 × 1) | 60.0A | 70A or 80A |
| 1-Phase (North America) | 240V | 7200 / (240 × 1) | 30.0A | 40A |
| 1-Phase (Europe/UK) | 230V | 7200 / (230 × 1) | 31.3A | 40A |
| 3-Phase (North America) | 208V | 7200 / (1.732 × 208 × 1) | 20.0A | 25A or 30A |
| 3-Phase (Europe) | 400V | 7200 / (1.732 × 400 × 1) | 10.4A | 16A |
When the Conversion is Meaningless (and Hardware Alternatives)
An amps-to-voltage calculation becomes entirely meaningless in AC circuits if the Power Factor (PF) is unknown. If you are measuring 15 amps on a large induction motor with a clamp meter, you cannot simply multiply by 240V to find the real power (Watts), nor can you work backward to find the effective voltage drop without knowing the phase angle between voltage and current. A motor with a 0.75 PF will draw significantly more apparent current for the same real work than a resistive heater.
The Physical "Amps to Voltage Converter" Circuit
In industrial automation and embedded electronics, the phrase "amps to voltage converter" rarely refers to math. It refers to a physical circuit used to translate a 4-20mA current loop signal (standard for pressure and flow sensors) into a 1-5V or 0-10V voltage signal that an Arduino, ESP32, or PLC ADC (Analog-to-Digital Converter) can read.
Because microcontrollers measure voltage, not current, you must convert the signal. As detailed in All About Circuits' breakdown of transimpedance amplifiers, you have two primary hardware choices:
| Feature | Passive Shunt Resistor | Active Transimpedance Amplifier (Op-Amp) |
|---|---|---|
| Core Component | 250Ω Precision Resistor (0.1% tolerance) | Op-Amp (e.g., OPA340) + Feedback Resistor |
| 4-20mA Output | 1V to 5V | Configurable (e.g., 0V to 10V) |
| Input Impedance | Adds 250Ω burden to the loop | Virtually zero burden (ideal for long runs) |
| Cost & Complexity | ~$0.50 / Trivial to wire | ~$3.00+ / Requires dual-rail power supply |
| Best Use Case | PLC analog inputs, short wire runs | 3.3V MCUs (ESP32), long cable runs, 0-10V scaling |
Bench Tip: If you are reading a 4-20mA sensor with an ESP32 (which has a 0-3.3V ADC limit), a standard 250Ω resistor will yield 5V at 20mA, frying your GPIO pin. Instead, use a 165Ω shunt resistor to scale 20mA to exactly 3.3V, or use an active op-amp circuit to shift and scale the 1-5V signal down to 0.66V-3.3V.
Frequently Asked Questions
Can I convert amps to volts without knowing watts or ohms?
No. Current (amps) is the flow rate, and voltage (volts) is the pressure. Without knowing the size of the pipe (resistance/ohms) or the total work being done (power/watts), the mathematical relationship cannot be solved. It is like trying to calculate water pressure knowing only the gallons-per-minute flow rate without knowing the pipe diameter.
Why does my 3-phase voltage calculation look wrong?
The most common mistake DIYers make is forgetting the square root of 3 (≈1.732) in 3-phase formulas. If you have a 10,000W load drawing 15A on a 3-phase system, the voltage is 10,000 / (1.732 × 15 × PF). If you omit the 1.732, your calculated voltage will be artificially high by 73%, leading to incorrect breaker and wire sizing.
What size wire do I need for a 240V, 30A circuit?
For a 30A, 240V single-phase circuit (like a dryer or EV charger), NEC-style guidance dictates using 10 AWG copper wire (rated for 30A in the 60°C column, or 35A in the 75°C column if terminals are rated for it). Always pair this with a 30A double-pole breaker. If the run exceeds 50 feet, calculate voltage drop; you may need to upsize to 8 AWG to keep the drop under 3%.
