Watts to Amps Calculator

About the Watts to Amps Calculator

A watts to amps calculator converts power (watts) to electrical current (amperes) at a specified voltage — the reverse calculation from amps to watts. This conversion is essential when you know how many watts a device or load consumes and need to determine what circuit ampacity, wire gauge, fuse size, or power supply current rating is required to support it safely. Common scenarios: selecting the correct circuit for a new 5,500W electric water heater, verifying that a 20-amp kitchen circuit can handle a combination of appliances totalling 2,800 watts, sizing a 60A circuit for a level 2 EV charger, or designing a DC power supply to run electronics consuming a known wattage. Our calculator includes the NEC 125% continuous load rule for circuit breaker sizing and handles DC, single-phase AC, and three-phase AC calculations.

How It Works

I = P / V for DC and single-phase AC. I = P / (V × √3 × PF) for three-phase AC. Examples: 2,400W toaster oven at 120V: I = 2,400 / 120 = 20A — requires a dedicated 20A circuit at minimum, or 25A to follow the NEC 80% rule for continuous loads. Electric water heater 4,500W at 240V: I = 4,500 / 240 = 18.75A → requires 20A circuit (and 10 AWG wire). EV Level 2 charger 11.5 kW at 240V: I = 11,500 / 240 = 47.9A → requires 50A circuit and 6 AWG copper wire. Three-phase industrial motor 30 kW at 460V (PF = 0.9): I = 30,000 / (460 × 1.732 × 0.9) = 30,000 / 717 = 41.8A → requires 60A breaker per NEC sizing rules.

Tips & Best Practices

• ✓NEC sizing rule for continuous loads: circuit breaker ampacity ≥ 1.25 × calculated load current. A 20A load (continuous) requires a 25A breaker — but since 25A is not standard, use a 30A breaker.
• ✓Standard US breaker sizes: 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 amps. Always select the next standard size above the calculated requirement.
• ✓EV Level 2 charger sizing: most EVSE chargers draw 30-48A continuous. 48A EVSE × 1.25 = 60A circuit breaker. Wire with 6 AWG copper (60A ampacity) or 4 AWG aluminium.
• ✓Wire gauge follows amperage: once you know the required amperage, select wire gauge from NEC Table 310.15(B)(16). 15A → 14 AWG; 20A → 12 AWG; 30A → 10 AWG; 40A → 8 AWG; 60A → 6 AWG (copper).
• ✓Motor starting current: electric motors draw 6-7× their running current during start-up (locked rotor current). Size the circuit breaker using NEC Article 430 motor circuit rules, not just running amps.
• ✓AC versus DC current: at the same wattage, DC systems carry the same current as single-phase AC. However, DC systems avoid the power factor losses of AC inductive loads — making DC attractive for battery and solar applications.
• ✓Low-voltage systems: in 12V or 24V systems (RVs, boats, solar), watts-to-amps gives much higher current for the same power than 120V or 240V systems. 100W at 12V = 8.33A; at 120V = 0.83A. Low-voltage systems require heavier wire gauge for the same wattage.
• ✓Inverter sizing: to power AC loads from a battery via an inverter, total AC watts divided by inverter efficiency (typically 85-95%) gives DC watts needed. Divide by battery voltage to get DC amps for battery cable sizing.

Who Uses This Calculator

Electricians selecting wire gauge and circuit breaker sizes for new circuits. Homeowners planning electrical upgrades and verifying whether their panel has capacity. EV charger installers sizing dedicated circuits and wiring. RV and boat owners designing 12V/24V electrical systems. Solar system designers sizing charge controllers and battery cables. Industrial engineers specifying motor protection equipment. IT managers sizing power distribution units for server rack loads.