Amps, Volts & Watts Calculator (Ohm’s Law)
Calculate electric current (amperes), breaker sizing, and wire gauge from watts and voltage.
How to Use the Amps, Volts & Watts Calculator (Ohm’s Law)
Understanding the mathematical relationship between amperes (current), volts (pressure), and watts (power) is essential for electrical safety, home renovations, and sizing generators. Current, measured in amperes, is the volume of electrons flowing through a conductor per second. When an appliance demands power, current increases proportionally as voltage decreases.
Formula and Calculation Methodology
- Amps (I): Electrical current flow in amperes
- Watts (P): Real power consumed by the electrical appliance
- Volts (V): Potential difference across the circuit
- NEC 125% Rule: National Electrical Code requirement that continuous loads must not exceed 80% of breaker rating
Under the National Electrical Code (NEC Article 210.19), circuits must be sized for 125% of their continuous load (any load operating for three hours or more). For example, a 1,800-watt space heater on a standard 120-volt circuit pulls 15.0 amperes. Because 15.0A × 1.25 = 18.75A, running this appliance continuously requires a dedicated 20-amp circuit with 12 AWG copper wire to prevent circuit breaker tripping and fire hazards.
National Electrical Code (NEC) Breaker & Wire Gauge Standards
| Circuit Breaker Rating | Maximum Continuous Load (80%) | Minimum Copper Wire Gauge (AWG) | Typical Household Usage |
|---|---|---|---|
| 15 Amp | 12.0 Amps (1,440W @ 120V) | 14 AWG | General lighting, bedroom outlets |
| 20 Amp | 16.0 Amps (1,920W @ 120V) | 12 AWG | Kitchen counters, bathroom GFCI, laundry |
| 30 Amp | 24.0 Amps (5,760W @ 240V) | 10 AWG | Electric clothes dryers, RV hookups |
| 40 Amp | 32.0 Amps (7,680W @ 240V) | 8 AWG | Electric ranges, cooktops, heat pumps |
| 50 Amp | 40.0 Amps (9,600W @ 240V) | 6 AWG | Level 2 EV chargers, central air conditioning |
Frequently Asked Questions
How do I convert Watts to Amps?
Divide the wattage of the device by the operating voltage. For example: 1,200 Watts ÷ 120 Volts = 10 Amps.
What is the difference between 15A and 20A household outlets?
A 20-amp outlet has a perpendicular T-shaped neutral slot and requires 12 AWG wire, whereas a standard 15-amp outlet uses 14 AWG wire.
What is Power Factor (PF)?
Power factor is the ratio of working power (kW) to apparent power (kVA). Resistive devices like incandescent bulbs have a PF of 1.0, while inductive motors typically range from 0.80 to 0.90.