About
Ohm's power law: P (watts) = V (volts) × I (amps) for DC. AC adds power factor for non-resistive loads. AC three-phase: P = √3 × V × I × pf. Always size wires + breakers for max current.
Formula
Frequently asked questions
How do you convert watts to amps?
For a DC circuit, divide power by voltage: amps = watts / volts. This comes from the power law P = V x I. So a 1000 W device on a 120 V supply draws 1000 / 120 = 8.33 amps. For AC you also divide by the power factor, and for three-phase you divide by the square root of 3 times voltage times power factor.
What is the formula behind watts, volts, and amps?
The base relationship is P = V x I, where P is power in watts, V is voltage in volts, and I is current in amps. Rearranged: amps = watts / volts, watts = volts x amps, volts = watts / amps. For AC loads that are not purely resistive, real power adds a power factor: P = V x I x PF (single phase) or P = sqrt(3) x V x I x PF (three phase).
What is power factor and when does it matter?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amps), between 0 and 1. Purely resistive loads like heaters and incandescent bulbs have PF = 1, so you can ignore it. Motors, transformers, and many electronics have PF below 1, which means they draw more current than P / V alone suggests. Use the device's rated PF (often 0.8 for motors) in AC calculations.
Why does a US 120 V circuit draw more amps than a 240 V circuit?
Because amps = watts / volts, halving the voltage doubles the current for the same power. A 1500 W appliance pulls 12.5 A at 120 V but only 6.25 A at 240 V. This is why high-power appliances (dryers, ovens, EV chargers) use 240 V circuits: lower current means thinner wire and less heat loss for the same wattage.
How do I use amps to size a wire or breaker?
Calculate the maximum current the load will draw, then choose a breaker rated above it and wire rated for that breaker, following local electrical code and continuous-load derating (often 125 percent for loads running over 3 hours). For example a 12.5 A continuous load needs 12.5 x 1.25 = 15.6 A capacity, so a 20 A breaker with 12 AWG copper. Always have a licensed electrician verify safety-critical wiring.
About watts, amps, and volts
Watts, volts, and amps describe three different things about electricity, and confusing them is the root of most wiring mistakes. Volts measure electrical pressure (the push), amps measure current (the flow rate), and watts measure power (the rate of energy use). The plumbing analogy: voltage is water pressure, current is how much water flows per second, and power is the total work the water does.
Converting watts to amps matters whenever you need to know how much current a device will pull, which determines what wire gauge, breaker, fuse, extension cord, generator, or power supply you need. A device's nameplate usually lists watts and volts, but the safety-critical number is the current in amps, because that is what heats up conductors and trips breakers.
How the conversion works
Everything follows from the power law P = V x I. Rearranged to solve for current, with adjustments for AC:
DC: I = P / V AC single-phase: I = P / (V x PF) AC three-phase: I = P / (sqrt(3) x V x PF)
- P is power in watts, V is voltage in volts, I is the current in amps you are solving for.
- PF (power factor) is 1 for resistive loads (heaters, incandescent bulbs) and below 1 for motors and electronics. It accounts for current that does no real work.
- sqrt(3) (about 1.732) appears in three-phase because the three phases are 120 degrees out of step, changing how line current relates to power.
Worked example
A 1000 W appliance on a standard US 120 V DC-equivalent (resistive) circuit:
I = P / V = 1000 / 120 = 8.33 amps
Now run the same 1000 W as an AC motor with a 0.85 power factor on single phase:
Common appliance current reference
Approximate current draw at common voltages (resistive, PF = 1):
| Device | Power | Voltage | Current |
|---|---|---|---|
| LED bulb | 10 W | 120 V | 0.08 A |
| Laptop charger | 65 W | 120 V | 0.54 A |
| Microwave | 1100 W | 120 V | 9.2 A |
| Hair dryer | 1500 W | 120 V | 12.5 A |
| Electric oven | 3000 W | 240 V | 12.5 A |
| EV charger (L2) | 7680 W | 240 V | 32 A |
Common pitfalls
- Ignoring power factor on AC. Using I = P / V for a motor underestimates current. Always divide by PF for inductive loads.
- Mixing up the voltage. A 1500 W device draws 12.5 A at 120 V but 6.25 A at 240 V. Use the actual supply voltage, not a guess.
- Forgetting continuous-load derating. Electrical code typically requires sizing a breaker at 125 percent of a load that runs over 3 hours, so a 16 A continuous load needs a 20 A circuit.
- Confusing apparent and real power. Volt-amps (VA) and watts differ whenever PF is below 1. Generators and UPS units are often rated in VA, not watts.
- Treating this as a substitute for an electrician. The math sizes the load, but code, wire derating, and safety require a qualified professional for permanent wiring.