If you are punching numbers into an amp volt watt converter for a standard US 15-amp circuit at 120 volts, the maximum peak wattage is 1,800 watts, and the maximum continuous wattage is 1,440 watts. The base formula used is Watts = Amps × Volts × Power Factor. Substituting the exact values for a purely resistive load (Power Factor = 1.0): 15A × 120V × 1.0 = 1,800W. Applying the NEC 80% continuous load rule (required for loads running 3 hours or more) yields 1,800W × 0.8 = 1,440W. You do not need to guess; these numbers dictate your wire and breaker sizing.
The Core Assumptions That Fix Your Conversion
An amp volt watt converter is only as accurate as the assumptions you feed it. Three variables lock in your final number:
- Nominal Voltage: We calculate using 120V or 240V as the baseline. While a US breaker is technically rated for 125V/250V, utility delivery targets 120V/240V. Always use the nominal system voltage for load calculations, not the breaker's maximum insulation rating.
- Power Factor (PF): For resistive loads (space heaters, incandescent bulbs, toaster ovens), PF is 1.0. For inductive loads (compressors, drill presses, HVAC fans), PF drops to roughly 0.80 to 0.85. If you ignore PF on a motor, your wattage calculation will be dangerously high, leading to undersized wiring.
- Phase Configuration: Single-phase math is straightforward multiplication. Three-phase math requires multiplying by the square root of 3 (1.732).
Amp Volt Watt Converter Table (±20% Range)
Below is the reference table for a ±20% range around the standard 15A baseline (12A to 18A). This covers the exact footprint of standard 15A and 20A branch circuits in North America.
| Current (Amps) | 120V Peak (Watts) | 120V Continuous (80% Rule) | 240V Peak (Watts) | 240V Continuous (80% Rule) |
|---|---|---|---|---|
| 12A | 1,440 W | 1,152 W | 2,880 W | 2,304 W |
| 13A | 1,560 W | 1,248 W | 3,120 W | 2,496 W |
| 14A | 1,680 W | 1,344 W | 3,360 W | 2,688 W |
| 15A | 1,800 W | 1,440 W | 3,600 W | 2,880 W |
| 16A | 1,920 W | 1,536 W | 3,840 W | 3,072 W |
| 17A | 2,040 W | 1,632 W | 4,080 W | 3,264 W |
| 18A | 2,160 W | 1,728 W | 4,320 W | 3,456 W |
Note: Continuous ratings assume an ambient temperature of 30°C (86°F). If your conduit is in a hot attic, apply NEC 310.15 derating factors.
How the Math Shifts: 120V vs 230V vs 3-Phase
Do not treat single-voltage answers as universal. The moment you cross borders or move from residential to light commercial, the formula shifts.
- 120V / 240V (North America Split-Phase): Uses the standard W = A × V formula. 240V loads (like dryers or baseboard heaters) simply double the wattage output for the same amperage compared to 120V.
- 230V (Europe/UK/AU Single-Phase): The formula remains W = A × V, but the baseline voltage is 230V. A 15A circuit in the UK yields 3,450 watts peak. (Note: UK ring mains are typically 32A, yielding 7,360W).
- 3-Phase (Commercial/Industrial): The formula becomes W = √3 × VLine-to-Line × I × PF. For a 15A load on a 208V 3-phase wye system (common in US commercial buildings), the math is: 1.732 × 208V × 15A × 1.0 = 5,403 watts. For a 480V delta system, that same 15A yields 12,470 watts.
When This Conversion is Meaningless (and How to Fix It)
An amp volt watt converter becomes useless when you are dealing with inductive loads and the Power Factor is unknown. If you measure 10 amps on a table saw motor with a clamp meter and multiply by 120V, you get 1,200W. But because of the phase shift between voltage and current caused by the motor's inductance, the actual real work being done (Real Power) might only be 960W. The remaining 240W is Reactive Power bouncing back and forth.
When PF is unknown, your calculation yields Volt-Amps (VA), not Watts. According to Fluke Corporation's electrical guidelines, sizing wire based on VA (Apparent Power) is actually the safest fallback because the wire must carry the total current, regardless of whether it is doing real work. However, if you are sizing a solar inverter or a generator, you must use Real Power (Watts). To fix this, check the manufacturer's nameplate for the PF rating, or use a true-power meter (like a Kill-A-Watt or a Fluke power quality analyzer) to measure Watts directly.
Decision Tree: Sizing Your Breaker and Wire
Use this decision path to terminate your calculation and select the exact physical components for your panel. This assumes standard copper conductors in a residential setting (per NEC Article 210 and 310).
| Condition (Continuous Load) | Action / Rule | Concrete Part Pick |
|---|---|---|
| Load is ≤ 1,152W @ 120V (or ≤ 2,304W @ 240V) | Max 12A continuous. Use 15A breaker and 14 AWG wire. | 14 AWG NM-B + Square D QO115 (15A Breaker) |
| Load is 1,153W to 1,536W @ 120V (or up to 3,072W @ 240V) | Max 16A continuous. Upgrade to 20A breaker and 12 AWG wire. | 12 AWG NM-B + Square D QO120 (20A Breaker) |
| Load is 1,537W to 1,920W @ 120V | Max 16A continuous, but 120V 20A circuits are maxed. Move to 240V. | Rewire to 240V using 12 AWG NM-B + Square D QO220 |
| Load is 3,073W to 3,840W @ 240V | Max 16A-20A continuous at 240V. Requires 30A breaker and 10 AWG wire. | 10 AWG NM-B + Square D QO230 (30A Breaker) |
Frequently Asked Questions
Why does my appliance nameplate show higher amps than the wattage suggests?
Appliance nameplates often list the "Locked Rotor Amps" (LRA) or maximum startup surge, which can be 3 to 5 times higher than the running wattage. Always size your breaker for the continuous running watts, but ensure your wire can handle the brief startup voltage drop.
Can I use a 20A breaker on 14 AWG wire if my wattage calculation is under 1,440W?
No. NEC 240.4(D) strictly limits 14 AWG copper to a maximum 15A overcurrent device, regardless of the actual calculated load. The breaker protects the wire, not just the appliance.
