Amp Calculator
Calculate electrical current (amps) from power, voltage, and circuit type—or use Ohm's Law directly.
What Is an Amp?
An ampere (amp) is a unit of electric current. In practical terms, it tells you how much electrical flow is moving through a wire or circuit at any moment. Knowing current is essential for selecting the right breaker size, wire gauge, power supply, and protective devices.
This amp calculator is designed for everyday electrical planning. Whether you are checking a home appliance, sizing a solar inverter load, or estimating motor current, you can quickly convert power and voltage into current using standard electrical formulas.
Core Formulas Used by the Calculator
1) DC Circuits
I = P / (V × η)
- I = current in amps
- P = power in watts
- V = voltage in volts
- η = efficiency (as a decimal, e.g., 95% = 0.95)
2) AC Single-Phase Circuits
I = P / (V × PF × η)
- PF is power factor, typically between 0.8 and 1.0
3) AC Three-Phase Circuits
I = P / (√3 × V × PF × η)
Three-phase systems distribute power efficiently, so for the same power level they usually carry lower current per conductor than equivalent single-phase setups.
4) Ohm's Law
I = V / R
Use this when you know voltage and resistance directly, such as in electronics, battery projects, and simple DC circuits.
How to Use This Amp Calculator
- Select a method: Power & Voltage or Ohm's Law.
- If using Power & Voltage, choose the circuit type (DC, single-phase AC, or three-phase AC).
- Enter your known values carefully, including power factor and efficiency when applicable.
- Click Calculate Amps to see the current and the exact formula path used.
Practical Examples
Example 1: Space Heater on 120V
A 1500W heater on 120V (assuming near 100% efficiency and PF = 1) draws about: 1500 / 120 = 12.5A.
Example 2: Single-Phase AC Load
A 5kW single-phase load on 240V with PF = 0.9 and efficiency = 95%: I ≈ 5000 / (240 × 0.9 × 0.95) = 24.37A.
Example 3: Three-Phase Motor
A 12kW motor on 400V three-phase with PF = 0.85 and efficiency = 92%: I ≈ 12000 / (1.732 × 400 × 0.85 × 0.92) = 22.15A.
Why Amp Calculations Matter for Safety
Undersized wiring can overheat. Oversized breakers can fail to trip when needed. Calculating current correctly is the first step to safe electrical design. Always verify against your local code requirements (NEC, IEC, or regional standards), ambient temperature corrections, conductor bundling factors, and duty cycle.
- Use proper wire ampacity charts.
- Size breakers based on continuous vs non-continuous loads.
- Include startup/inrush current for motors and compressors.
- Consult a licensed electrician for final installation decisions.
Common Mistakes to Avoid
- Forgetting to convert kW to W (1 kW = 1000 W).
- Using PF = 1 for inductive equipment that actually has lower power factor.
- Ignoring efficiency losses in motors and power electronics.
- Confusing line-to-line and line-to-neutral voltage in three-phase systems.
Final Takeaway
A reliable amp calculator saves time and improves accuracy. Use it to estimate current quickly, then apply electrical code rules for conductor sizing, breaker selection, and safe real-world implementation. Numbers are only the start—safety margins and compliance finish the job.