Amp Calculator

How to Convert Watts to Amps: Complete Guide

Understanding the relationship between watts, amps, and volts is essential for any electrical work — from sizing breakers to choosing the right wire gauge. Here's everything you need to know.

The Basic Formulas

For single-phase AC circuits:

Amps = Watts ÷ (Volts × Power Factor)

Watts = Amps × Volts × Power Factor

For three-phase AC circuits:

Amps = Watts ÷ (Volts × √3 × Power Factor)

Watts = Amps × Volts × √3 × Power Factor

For DC circuits (no power factor):

Amps = Watts ÷ Volts

Common Watts to Amps Conversions (120V)

Watts	Amps (120V)	Typical Appliance
100W	0.83A	LED light fixtures
500W	4.17A	Window fan
1,000W	8.33A	Toaster, iron
1,500W	12.5A	Space heater, hair dryer
1,800W	15.0A	Max for 15A circuit (continuous)
2,400W	20.0A	Max for 20A circuit

Common Watts to Amps Conversions (240V)

Watts	Amps (240V)	Typical Appliance
3,000W	12.5A	Water heater (small)
4,500W	18.75A	Water heater (standard)
5,000W	20.83A	Clothes dryer
7,200W	30.0A	EV charger (Level 2)
10,000W	41.67A	Electric range
24,000W	100A	Central AC (5-ton)

Wire Sizing by Amperage

Amps	Wire Gauge (AWG)	Breaker Size	Common Use
15A	14 AWG	15A	Lighting, general outlets
20A	12 AWG	20A	Kitchen, bathroom, shop outlets
30A	10 AWG	30A	Dryers, water heaters
40A	8 AWG	40A	Electric ranges, cooktops
50A	6 AWG	50A	EV chargers, sub-panels
60A	6 AWG	60A	Large appliances, sub-panels
100A	3 AWG	100A	Sub-panels, main service
200A	2/0 AWG	200A	Main service panel

Understanding Power Factor

Power factor (PF) is the ratio of real power to apparent power. Resistive loads like heaters, incandescent lights, and electric ranges have a PF of 1.0. Inductive loads draw more current:

• PF 1.0: Heaters, incandescent lights, toasters
• PF 0.95: LED drivers, modern electronics
• PF 0.85–0.90: Motors (HVAC compressors, pumps)
• PF 0.80: Fluorescent lights, older motors
• PF 0.60–0.70: Unloaded motors, cheap power supplies

Why it matters: A 1,000W motor with PF 0.85 at 120V actually draws 9.8 amps, not 8.3 amps. That extra current matters for wire sizing and breaker selection.

5 Electrical Mistakes That Trip Breakers

1. Overloading a circuit. A 15-amp circuit maxes out at 1,440W continuous (80% rule). Two space heaters on one circuit = guaranteed trip.
2. Ignoring power factor for motors. Motors draw more amps than their wattage suggests. Always use the nameplate amps, not calculated amps from watts.
3. Using 14-gauge wire on a 20-amp breaker. This is a code violation and fire hazard. 20-amp circuits require 12-gauge minimum.
4. Not accounting for voltage drop. Long wire runs (50+ feet) lose voltage. Use the next size up wire gauge for runs over 50 feet.
5. Confusing 120V and 240V calculations. A 240V appliance draws half the amps of the same wattage at 120V. Always check the nameplate voltage.

Breaker Sizing Guide: How Many Amps Do You Need?

Circuit	Breaker	Wire	Max Watts (Continuous)
General lighting	15A / 120V	14 AWG	1,440W
Kitchen / Bath outlets	20A / 120V	12 AWG	1,920W
Garage / Workshop	20A / 120V	12 AWG	1,920W
Electric dryer	30A / 240V	10 AWG	5,760W
Electric range	50A / 240V	6 AWG	9,600W
EV charger (L2)	50A / 240V	6 AWG	9,600W
Central AC (3-ton)	30A / 240V	10 AWG	5,760W
Central AC (5-ton)	60A / 240V	6 AWG	11,520W

Remember the 80% rule: For continuous loads (running 3+ hours), multiply the breaker rating by 0.8 for the maximum safe load. A 20-amp breaker should carry no more than 16 amps continuously. The values above already reflect this rule.