How Many Watts on 240 Volt? Breaker Limits, Formulas, and Sizing

There is no single fixed wattage for 240 volts because voltage measures electrical pressure, not power. To find the wattage, you must multiply the voltage by the current (amps). However, if you are asking how many watts a standard residential 240V circuit can safely handle, the answer is dictated by the breaker size. For the most common 240V circuits: a 20-amp double-pole breaker supports 4,800 watts (3,840W continuous), a 30-amp breaker supports 7,200 watts, and a 50-amp breaker supports 12,000 watts.

The foundational assumption fixing these numbers is a single-phase, purely resistive AC load (Power Factor = 1.0) at 240V nominal. The core formula is P (Watts) = V (Volts) × I (Amps). Substituting our baseline values for a standard 20A circuit: P = 240V × 20A = 4,800W.

The 80% Continuous Load Rule

The 4,800W figure is the absolute maximum trip threshold for a 20A breaker. However, the National Electrical Code (NEC) mandates a derating for continuous loads—defined as any load expected to run for three hours or more, like a space heater or EV charger. Under NEC Article 210.20(A), you must size the overcurrent device at 125% of the continuous load, which effectively limits you to 80% of the breaker's rated capacity.

240V Wattage Capacity Table (±20% Range)

When designing a circuit, you rarely land on an exact standard breaker size. Below is a reference chart centered on the common 20A baseline, showing a ±20% range of current draw (16A to 24A) and the corresponding wattage limits at 240V.

Decision Path: Sizing Your 240V Circuit

Use this decision tree to select the correct breaker and wire gauge for your specific 240V appliance. Always calculate based on the continuous wattage limit if the device runs for extended periods.

Final Recommendation: If you are wiring a standard 5,000W 240V garage heater, do not use a 20A breaker. The continuous draw is 20.8A. Terminate your decision path here: purchase a 30A double-pole breaker and run 10 AWG copper NM-B cable (or 10 AWG THHN in conduit).

How the Math Shifts: 120V, 230V, and 3-Phase

The 240V calculations above assume North American single-phase split power. If your environment changes, the wattage shifts dramatically even if the ampacity remains identical.

• 120V Systems (North America): Wattage is halved for the same amp draw. A 20A breaker at 120V only yields 2,400W max (1,920W continuous). This is why high-draw appliances are wired for 240V—to keep current (and wire thickness) down.
• 230V Systems (EU/UK Nominal): European harmonized voltage is 230V, not 240V. A 20A MCB (Miniature Circuit Breaker) yields 4,600W. Always use the 230V baseline when sizing European imports or calculating solar inverter outputs in the EU.
• 3-Phase Power (Commercial/Industrial): In a 3-phase system, power delivery is smoother and denser. The formula shifts to P = V × I × √3 (assuming PF=1). According to All About Circuits, a 240V 3-phase system pulling 20A delivers roughly 8,313 watts—nearly double the single-phase equivalent.

When the Conversion is Meaningless: Power Factor

The formula P = V × I only works perfectly for resistive loads like incandescent heaters, toasters, and resistive wire. When you introduce inductive loads—such as AC compressor motors, well pumps, or large transformers—the current and voltage waveforms fall out of sync. This introduces Power Factor (PF).

The true AC power formula is P = V × I × PF. As noted by Fluke's power quality guides, a motor with a PF of 0.8 drawing 20A at 240V is consuming 4,800 Volt-Amps (VA) of apparent power, but only 3,840 actual Watts of real working power.