30 amps at 120 volts is exactly 3,600 watts, assuming a direct current (DC) circuit or an alternating current (AC) circuit with a Power Factor (PF) of 1.0. The foundational formula used for this conversion is Watts = Amps × Volts. Substituting your specific values into the equation: 30A × 120V = 3,600W. This is the maximum theoretical real power you can pull from a 120V source drawing 30 amps of current.
Continuous Safe Limit (NEC 80% Rule): 2,880 Watts.
The Core Formula and the Power Factor Catch
The calculation above assumes a purely resistive load—like an electric space heater or an incandescent light bulb—where voltage and current waveforms are perfectly in sync. In electrical terms, this means the Power Factor (PF) is exactly 1.0. For DC circuits, PF doesn't exist, so the 3,600W answer is absolute.
However, if you are running inductive AC loads like motors, compressors, or transformers, the current and voltage waveforms fall out of phase. This introduces a Power Factor of less than 1.0 (typically between 0.7 and 0.9 for common workshop motors). The true AC single-phase formula is:
Watts = Amps × Volts × Power Factor
If your 30-amp load is a large 120V air compressor with a PF of 0.8, the actual real power (watts) doing useful work drops to 2,880 watts (30 × 120 × 0.8). The remaining current is reactive power (measured in VARs), which doesn't do physical work but still generates heat in your wires and counts toward your breaker's thermal limit. For a deep dive into how phase angles affect real power, All About Circuits provides an excellent breakdown of AC power triangles.
Neighboring Amp Values at 120V (±20% Range)
When sizing wires or checking breaker headroom, you rarely sit at exactly 30.0 amps. Here is a spec-sheet-table showing the wattage for currents within a 20% margin of your 30A target, assuming a standard 120V nominal supply and a PF of 1.0.
| Current (Amps) | Voltage | Power Factor | Real Power (Watts) | Typical Application Context |
|---|---|---|---|---|
| 24A | 120V | 1.0 | 2,880W | NEC 80% continuous load limit for a 30A breaker |
| 27A | 120V | 1.0 | 3,240W | Heavy resistive load (e.g., large RV water heater) |
| 30A | 120V | 1.0 | 3,600W | Maximum breaker trip threshold (non-continuous) |
| 33A | 120V | 1.0 | 3,960W | Overload condition; breaker will trip on thermal curve |
| 36A | 120V | 1.0 | 4,320W | Severe overload; rapid thermal trip expected |
How the Math Shifts: 120V vs 230V vs 3-Phase
A common mistake on the workbench or jobsite is treating a 30-amp rating as a universal wattage limit. Wattage scales linearly with voltage, meaning the exact same 30-amp current delivers vastly different power depending on the supply architecture.
- 120V Single-Phase (North America standard): 30A × 120V = 3,600W. Common in RV TT-30 receptacles and heavy-duty window AC units.
- 230V Single-Phase (Europe/UK/AU standard): 30A × 230V = 6,900W. Common for European electric ovens and EV Level 2 chargers.
- 208V Three-Phase (US Commercial): 30A × 208V × √3 (1.732) = 10,807W. Used for commercial HVAC and industrial machinery.
- 240V Single-Phase (US Dryer/Range): 30A × 240V = 7,200W. Standard for NEMA 10-30 or 14-30 dryer receptacles.
If you are measuring 30 amps on a clamp meter around a 3-phase industrial motor and you do not know the Power Factor or the exact line-to-line voltage, calculating watts using basic multiplication is useless. You will only find the Apparent Power (measured in Volt-Amps, or VA). To find true watts on complex inductive loads, you must use a power analyzer or a multimeter with a true-watts measurement function, as noted by Fluke's power quality guidelines.
Frequently Asked Questions
How many watts can a 30-amp breaker handle continuously at 120V?
Under NEC Article 210.20(A), a breaker can only be loaded to 80% of its rated capacity for continuous loads (defined as running for 3 hours or more). Therefore, a 30-amp breaker can safely handle a continuous draw of 24 amps. At 120 volts, your continuous wattage limit is 2,880 watts (24A × 120V). If you are wiring a 30A circuit using 10 AWG THHN copper wire, ensure the load does not exceed this 2,880W threshold if it runs all day.
Is a 30-amp RV plug (TT-30) the same as a 30-amp dryer outlet?
No, and confusing them can destroy your RV's electrical system. An RV TT-30 receptacle is 120V, single-phase, yielding 3,600 watts. A standard residential dryer outlet (NEMA 10-30 or 14-30) is 240V, yielding 7,200 watts. The physical prongs are different to prevent cross-plugging, but DIYers using cheap adapter cables sometimes force 240V into a 120V RV panel, instantly frying the microwave, TV, and power converter. Always verify voltage with a multimeter before using an adapter.
Why does my 3600-watt generator trip when I start a 30-amp compressor?
While your compressor might draw 30 amps (3,600W) while running, AC motors require a massive surge of inrush current to overcome initial inertia and start spinning. This Locked Rotor Amps (LRA) surge can be 3 to 6 times the running current. A 30-amp compressor might briefly demand 120 amps (14,400W) for a fraction of a second upon startup. Your 3,600-watt generator's inverter or alternator cannot supply this surge, causing its internal overload protection to trip. To fix this, you need a generator with a higher peak surge rating or a soft-start device installed on the compressor motor.
