Amperage Calculator
Use our free amperage calculator to convert watts, kilowatts, kVA, or resistance into amps for single-phase and three-phase electrical circuits...
Amperage Calculator
Whether you are sizing a circuit breaker, selecting wire gauge, or planning a new electrical installation, knowing the exact amperage of a circuit is one of the most fundamental skills in electrical work. This free amperage calculator helps electricians, engineers, contractors, and DIY homeowners quickly convert watts, kilowatts, kVA, or resistance values into amperes — for both single-phase and three-phase systems.
What Is Amperage?
Amperage, measured in amperes (A) or amps, is the quantity of electrical current flowing through a conductor at any given moment. Current is driven by voltage and limited by resistance, and together these three values obey Ohm's Law: I = V ÷ R. In power-based calculations, current can also be derived from the formula I = P ÷ V, where P is power in watts and V is voltage in volts. Understanding amperage is critical for safe circuit design because every wire, breaker, and device is rated for a maximum current load.
How to Use This Amperage Calculator
Start by selecting your preferred calculation method from the dropdown menu. The calculator supports four common methods:
- Watts & Volts: Enter the load power in watts and the supply voltage. This is the most common method for everyday appliances and lighting circuits.
- Kilowatts & Volts: Ideal for larger loads such as HVAC equipment, electric water heaters, and industrial machinery. Enter power in kW and the line voltage.
- kVA & Volts: Used for transformer and generator sizing, where the apparent power rating is given in kilovolt-amperes rather than true watts.
- Ohms & Volts (Ohm's Law): Enter the supply voltage and the circuit resistance in ohms to calculate current directly using Ohm's Law.
Next, choose whether your system is Single Phase or Three Phase. Three-phase calculations divide total power by the square root of 3 (approximately 1.732) multiplied by the line voltage, which reflects how power is distributed across three conductors. Once you have entered all values, click Calculate Amperage to see the result instantly.
Understanding the Results
The results panel displays the calculated amperage to four decimal places for precision, along with the voltage and power values you entered, and a plain-language description of the method used. These outputs help you:
- Select the correct wire gauge based on ampacity tables in the NEC (National Electrical Code).
- Choose the appropriately rated circuit breaker or fuse — typically sized at 125% of the continuous load per NEC 210.20.
- Verify that an existing circuit is not overloaded by comparing the calculated amps to the breaker rating.
- Cross-check equipment nameplate data against actual operating conditions.
Amperage Formulas Used
The calculator applies industry-standard electrical formulas:
- Single Phase — Watts/Volts: I (A) = P (W) ÷ V (V)
- Three Phase — Watts/Volts: I (A) = P (W) ÷ (√3 × V)
- Single Phase — kW: I (A) = (kW × 1000) ÷ V
- Three Phase — kW: I (A) = (kW × 1000) ÷ (√3 × V)
- kVA: I (A) = (kVA × 1000) ÷ V (single) or (kVA × 1000) ÷ (√3 × V) (three phase)
- Ohm's Law: I (A) = V (V) ÷ R (Ω)
Frequently Asked Questions
What is the difference between amps, volts, and watts?
Volts (V) measure electrical pressure or potential difference — the force pushing current through a circuit. Amps (A) measure the flow rate of electrical charge through a conductor. Watts (W) measure real power, which is the product of voltage and current (P = V × I) in a purely resistive circuit. Think of voltage as water pressure, current as flow rate, and wattage as the total work being done by that flow.
How many amps does a 1,500-watt space heater draw on a 120V circuit?
Using the formula I = P ÷ V, a 1,500-watt heater on a 120-volt single-phase circuit draws 1,500 ÷ 120 = 12.5 amps. Since the NEC requires continuous loads to not exceed 80% of breaker capacity, you would need at least a 15-amp or preferably a 20-amp dedicated circuit to run this heater safely for extended periods.
Why does three-phase amperage differ from single-phase for the same wattage?
In a three-phase system, power is shared across three conductors simultaneously. The effective voltage seen across the load accounts for the phase relationship between conductors, which introduces a factor of √3 (≈1.732). This means that for the same wattage and line voltage, a three-phase circuit draws less current per phase than a single-phase circuit — making three-phase distribution more efficient for high-power applications.
What is power factor and when does it matter in amperage calculations?
Power factor (PF) is the ratio of real power (watts) to apparent power (VA) in an AC circuit containing inductive or capacitive loads such as motors, transformers, and variable frequency drives. A PF of 1.0 means all power is being used productively; a lower PF means some current is reactive and does not perform useful work. When using the kVA method, the apparent power already accounts for reactive current, so a separate PF adjustment is not needed in that formula.
How do I use amperage results to pick the right wire size?
Once you know the circuit amperage, refer to NEC Table 310.16 (or your local code equivalent) to find an appropriate wire gauge based on the conductor material (copper or aluminum), insulation temperature rating (60°C, 75°C, or 90°C), and installation conditions. For example, 20 amps typically requires 12 AWG copper at 60°C, while 30 amps requires 10 AWG copper. Always apply derating factors for conduit fill, ambient temperature, and continuous loads as required by code.
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