At a standard US residential voltage of 120V, 30 amps equals exactly 3,600 watts. At 240V, 30 amps equals 7,200 watts. The governing formula for single-phase DC or AC resistive loads is Watts = Volts × Amps × Power Factor. Substituting the standard values for a 120V circuit with a purely resistive load (Power Factor = 1.0): 120V × 30A × 1.0 = 3,600W. However, treating this single number as a universal truth is a fast track to tripped breakers or undersized feeders. The actual wattage shifts dramatically based on your supply voltage, phase configuration, and the power factor of the load.
The Core Conversion Matrix: 30A and Neighboring Values
Because real-world loads rarely sit at a perfect, static 30.0A draw, the table below maps the ±20% operating range (24A to 36A) across the most common single-phase and three-phase voltages you will encounter in residential and light commercial panels. All values below assume a Power Factor (PF) of 1.0.
| Current (Amps) | 120V (1-Phase) | 240V (1-Phase) | 208V (3-Phase) |
|---|---|---|---|
| 24A (-20%) | 2,880 W | 5,760 W | 8,646 W |
| 27A (-10%) | 3,240 W | 6,480 W | 9,727 W |
| 30A (Nominal) | 3,600 W | 7,200 W | 10,808 W |
| 33A (+10%) | 3,960 W | 7,920 W | 11,888 W |
| 36A (+20%) | 4,320 W | 8,640 W | 12,969 W |
How Voltage, Phase, and Power Factor Shift the Math
The conversion from amps to watts is not a fixed constant; it is entirely dependent on three fixing assumptions: voltage, phase count, and power factor.
120V vs. 240V Single-Phase
In North American split-phase systems, a 30A double-pole breaker supplies 240V across two hot legs, yielding 7,200W. If you are only measuring one hot leg to neutral (120V), that single leg is delivering 3,600W. A common mistake on the jobsite is measuring 30A on one leg of a 240V baseboard heater circuit and assuming the total load is 3,600W, when in reality, the heater is pulling 30A across both legs simultaneously, consuming 7,200W total.
The 3-Phase Multiplier
For three-phase systems (common in commercial workshops or EV fast-charging stations), the formula changes to: Watts = √3 × Volts × Amps × PF.
At 208V 3-phase, 30 amps equals 10,808 watts (1.732 × 208 × 30 × 1.0). At 480V 3-phase, that same 30A draw translates to a massive 24,944 watts. This is why industrial machinery uses higher voltages: it delivers more wattage without requiring thicker, more expensive copper conductors.
When the Conversion is Meaningless: The Power Factor Trap
If you are dealing with inductive loads—like a 30A air compressor motor, a welding transformer, or an HVAC condenser—converting amps directly to watts without knowing the Power Factor (PF) is electrically meaningless.
Motors draw reactive power to build magnetic fields. If your 30A, 240V compressor has a PF of 0.80, the apparent power is 7,200 Volt-Amps (VA), but the real power (the actual watts doing mechanical work and generating heat) is only 5,760 watts (240 × 30 × 0.80). As detailed in All About Circuits' guide to AC power, sizing a generator or inverter based purely on the wattage of an inductive load will result in severe voltage sag because the generator must supply the full 7,200 VA, not just the 5,760W.
The 80% Rule: Why a 30A Breaker Doesn't Mean 3,600W Continuous
The most critical code restriction for this conversion is the NEC 80% continuous load rule. According to NFPA 70 (National Electrical Code) Article 210.20(A), if a load is expected to run for three hours or more, the branch circuit must be derated to 80% of the breaker's rating.
For a 30-amp breaker, the maximum continuous current is 24 amps. Therefore, your maximum continuous wattage limits are:
- 120V Circuit: 2,880 continuous watts (not 3,600W)
- 240V Circuit: 5,760 continuous watts (not 7,200W)
If you plug a 3,500W portable heater (which draws ~29.1A at 120V) into a 30A RV receptacle and leave it running overnight, the breaker will eventually trip due to thermal accumulation, even though 29.1A is technically below the 30A absolute limit.
Common 30A Receptacles and Their Actual Wattage Capacities
| Receptacle Type | Voltage / Phase | Max Peak Watts | Max Continuous Watts (80%) | Typical Application |
|---|---|---|---|---|
| NEMA TT-30 | 120V / 1-Phase | 3,600 W | 2,880 W | RV Park Pedestals, Travel Trailers |
| NEMA 14-30 | 240V / 1-Phase | 7,200 W | 5,760 W | Modern Electric Dryers, EV Level 2 Chargers |
| NEMA L6-30 | 250V / 1-Phase | 7,500 W | 6,000 W | Locking Generator Inlets, Workshop Welders |
| NEMA L14-30 | 125/250V / 1-Ph | 7,500 W | 6,000 W | Portable Generator Transfer Switches |
Note: Older NEMA 10-30 dryer receptacles lack a dedicated equipment grounding conductor. If you are upgrading a dryer circuit, NEC-style guidance requires replacing the 10-30 with a 14-30 and running a 4-wire feed (two hots, neutral, ground) using 10 AWG copper THHN or NM-B cable.
Frequently Asked Questions
Can I run a 3,000-watt inverter on a 30-amp breaker?
Yes, but it depends on the inverter's input voltage. If it is a 120V input inverter, 3,000W requires 25 amps (3000W / 120V = 25A). Because 25A exceeds the 24A continuous limit of a 30A breaker, the breaker will likely trip after an hour of heavy use. You would need a 40A breaker and 8 AWG wire for a continuous 3,000W 120V load.
How many watts is 30 amps at 12 volts DC?
At 12V DC (such as an automotive or solar battery system), 30 amps equals exactly 360 watts (12V × 30A = 360W). This is why high-wattage DC loads like winches or large off-grid inverters require massive 2/0 AWG battery cables to carry the hundreds of amps required to produce usable wattage at low voltages.
Does wire length change the wattage conversion?
No, wire length does not change the source wattage calculation (Volts × Amps), but it introduces voltage drop. If you run 10 AWG wire 150 feet to a 30A load, you might lose 5% of your voltage at the terminal. The load will still attempt to draw 30A, but the actual wattage delivered to the appliance will drop, and the remaining wattage will be dissipated as heat in the copper wire.
