At a standard US 120V supply, 30 amps equals 3,600 watts. At a 240V split-phase supply, it equals 7,200 watts, and at a European 230V supply, it equals 6,900 watts. These baseline figures assume a purely resistive load with a Power Factor (PF) of 1.0. The foundational formula is Watts = Amps × Volts. However, because amperage measures current flow and wattage measures actual work done, the exact conversion shifts dramatically based on your system's voltage, phase configuration, and power factor. Below is the exact breakdown of how to calculate this across different global standards and load types.
The Core Formulas and Global Voltage Variations
The conversion from amps to watts is never a single universal number; it is entirely dependent on three fixing assumptions: system voltage, phase configuration (single vs. three-phase), and power factor (the efficiency of the load). If you change any of these variables, the wattage changes proportionally.
1-Phase Formula: Watts = Amps × Volts × Power Factor
3-Phase Formula: Watts = Amps × Volts × √3 (1.732) × Power Factor
For a purely resistive load like a baseboard heater or an incandescent lighting array, the Power Factor is 1.0, meaning all the current is doing real work. For inductive loads like air conditioners, welders, or well pumps, the Power Factor drops (typically between 0.7 and 0.9), meaning the actual wattage is lower than the raw Amps × Volts multiplication suggests.
| System Type | Nominal Voltage | Phase | Power Factor | Calculated Watts (at 30A) |
|---|---|---|---|---|
| US Standard Receptacle | 120V | 1-Phase | 1.0 (Resistive) | 3,600 W |
| US Dryer / RV / Welder | 240V | 1-Phase | 1.0 (Resistive) | 7,200 W |
| EU / UK Standard | 230V | 1-Phase | 1.0 (Resistive) | 6,900 W |
| US Commercial HVAC | 208V | 3-Phase | 0.85 (Inductive) | 9,179 W |
| US Industrial Motor | 480V | 3-Phase | 0.80 (Inductive) | 19,953 W |
Calculation example for 3-Phase 208V: 30A × 208V × 1.732 × 0.85 = 9,179 Watts. As noted by Fluke's guide on Power Factor, ignoring the PF on inductive 3-phase loads will cause you to drastically oversize your upstream transformers and wiring.
Neighboring Values and the NEC 80% Continuous Load Rule
When sizing wire and breakers for a 30A circuit, you rarely operate at the absolute maximum limit. According to the NFPA 70 National Electrical Code (NEC), if a load is expected to run continuously for three hours or more, you must derate the circuit to 80% of its maximum capacity. Therefore, a 30A breaker should only carry a continuous load of 24A.
Here is how the wattage scales across a ±20% range of a 30A baseline at a standard 120V supply. This table is critical for determining if your specific appliance will trip the breaker during sustained use.
| Current (Amps) | Watts at 120V (PF=1.0) | Load Classification & Wire Sizing Note |
|---|---|---|
| 24A | 2,880 W | NEC 80% Continuous Limit. Max safe continuous draw on a 30A breaker. Requires 10 AWG copper. |
| 27A | 3,240 W | Non-continuous load. Safe for short bursts (e.g., microwave, toaster oven). |
| 30A | 3,600 W | Breaker Trip Threshold. Absolute maximum non-continuous limit before thermal trip. |
| 33A | 3,960 W | Overload condition. Breaker will trip in seconds to minutes depending on ambient heat. |
| 36A | 4,320 W | Severe overload. Breaker will trip almost instantly to protect 10 AWG wire from melting. |
Wire Sizing Caveat: For a 30A circuit, you must use a minimum of 10 AWG copper wire. If you are using NM-B (Romex) cable, the NEC requires you to use the 60°C column of the ampacity table (NEC 310.16), which perfectly aligns with 10 AWG for 30A. If you are pulling individual THHN wires in conduit, you can use the 90°C column for derating purposes, but the termination points at your breaker and receptacle are almost always rated for 75°C or 60°C, keeping the 30A limit firm.
When the Conversion is Meaningless: Power Factor and Apparent Power
The direct conversion of 30 amps to watts becomes entirely meaningless if you are measuring an inductive load with a clamp meter and you do not know the Power Factor (PF).
When you clamp a meter around the feed wire of a running 5HP air compressor and read 30A, multiplying that by 240V to get 7,200W is technically incorrect. You have calculated the Apparent Power, measured in Volt-Amps (VA), not the Real Power (Watts). The magnetic fields inside the motor's stator require current to sustain themselves, but this current does no actual mechanical work. If the motor has a PF of 0.75, the real wattage is actually 30A × 240V × 0.75 = 5,400 Watts.
If you are sizing a solar inverter or a UPS system, you must size it based on the Volt-Amps (VA), not just the Watts, because the wiring and the inverter's internal MOSFETs must still physically carry the full 30A of current, regardless of whether it is doing real work or just sustaining a magnetic field.
Frequently Asked Questions
Can I plug a 3,600W appliance into a standard 15A household outlet?
No. A standard US 15A outlet at 120V can only safely deliver 1,800W continuously (15A × 120V × 0.80 NEC rule). Plugging a 3,600W (30A) load into a 15A circuit will instantly trip the breaker and could melt the receptacle contacts if the breaker fails. You need a dedicated 30A circuit with a NEMA L5-30 or TT-30 receptacle.
How many watts can a 30A RV hookup provide?
A standard RV 30A hookup (NEMA TT-30) is a 120V single-phase system. It provides a maximum of 3,600 watts. However, applying the 80% continuous load rule, you should plan to draw no more than 2,880 watts continuously from your RV's AC units and appliances to avoid tripping the pedestal breaker at the campground.
Does 30 amps at 12V DC equal the same watts as 30 amps at 120V AC?
No. Watts are the product of current and voltage. 30A at 12V DC (like in a car or solar battery bank) equals only 360 watts. To get 3,600 watts from a 12V battery bank, you would need to pull a massive 300 amps, which requires extremely thick 4/0 AWG battery cables to prevent voltage drop and fire hazards.
