Convert watts to amps for DC, single-phase, and three-phase AC circuits. Includes power factor, NEC breaker sizing, wire gauge recommendation, voltage drop check, and appliance reference table.
Converting watts to amps is one of the most common electrical calculations. For DC circuits it is simply I = P/V (current equals power divided by voltage). For AC circuits, the power factor must be included: I = P/(V × PF) for single-phase or I = P/(√3 × V × PF) for three-phase. This conversion is essential for selecting the correct wire gauge, breaker size, and ensuring safe circuit loading.
The National Electrical Code (NEC) requires that continuous loads (operating for 3+ hours) not exceed 80% of the breaker rating. A 1,500 W heater on a 120 V circuit draws 12.5 A, which requires a 20 A breaker (12.5/0.8 = 15.6 A minimum). The wire must be rated for the breaker size, not just the load current — so 12 AWG copper (rated 20 A) is required.
This calculator converts watts to amps for DC, single-phase AC, and three-phase AC systems. It automatically recommends the minimum breaker size (NEC 80% rule), wire gauge (NEC 310.16 ampacity), and calculates voltage drop for the specified wire length. A breaker loading visual shows how much of the breaker capacity is utilized.
Choosing the wrong breaker or wire gauge can cause nuisance tripping, overheating, or fire. This calculator applies NEC-style sizing logic automatically, including continuous-load derating, wire ampacity matching, and a basic voltage-drop check, so you can move from watts to a practical circuit estimate without doing multiple manual lookups. It is useful for quick appliance checks, load planning, and comparing the effect of voltage and power factor on current draw.
DC: I = P / V Single-phase AC: I = P / (V × PF) Three-phase AC: I = P / (√3 × V × PF) Continuous derating: I_min = I_actual / 0.8 Breaker ≥ I_min (next standard size) Wire gauge: per NEC Table 310.16 for breaker size Voltage drop: V_drop = I × R_wire × 2 × Length
Result: 12.5 A, 20 A breaker, 12 AWG wire
I = 1500/(120 × 1) = 12.5 A. Continuous derating: 12.5/0.8 = 15.6 A minimum breaker, so a 20 A breaker is needed. NEC 310.16 requires 12 AWG copper for a 20 A circuit. At 50 ft one-way, voltage drop with 12 AWG is about 0.32%.
The NEC (National Electrical Code, NFPA 70) governs conductor sizing in the US. Table 310.16 lists ampacity by wire gauge, insulation temperature rating, and conductor material. The key rule: circuit wire must have ampacity ≥ breaker rating, and breaker must be ≥ 125% of continuous load current. For example, a 16 A continuous load needs a 20 A breaker with 12 AWG copper (60°C insulation) or better.
Long wire runs suffer significant voltage drop (I × R × 2L), reducing the voltage at the load. NEC recommends no more than 3% drop on branch circuits and 5% total. For a 120 V circuit, 3% is only 3.6 V. Solutions include using larger wire, higher voltage (240V), or shorter runs. Voltage drop is proportional to current and length, so converting to watts and running the calculation before installation prevents problems.
Three-phase systems dominate industrial settings because they deliver more power per conductor kilogram and produce smoother torque in motors. When converting watts to amps for three-phase, use the line-to-line voltage (208V, 240V, 480V) with the √3 factor. The per-phase current is I_L = P/(√3 × V_LL × PF). Higher voltages dramatically reduce current and allow smaller conductors.
NEC requires that continuous loads (3+ hours) not exceed 80% of the breaker rating. This provides a safety margin for conductor heating. A 12.5 A continuous load needs a 12.5/0.8 = 15.6 A minimum, rounded up to a 20 A standard breaker.
Power factor is the ratio of real power to apparent power. Resistive loads (heaters, incandescent bulbs) have PF ≈ 1. Motors typically have PF 0.8-0.9. Low PF means more current for the same watts, requiring larger wire and breaker.
No. NEC requires the wire ampacity to match or exceed the breaker rating. 14 AWG is rated 15 A and can only be used on 15 A breakers. 20 A circuits require 12 AWG minimum.
Add up the wattages of all loads on the circuit. Enter the total watts in this calculator. NEC limits most general-purpose circuits to 80% of breaker rating for any combination of continuous and non-continuous loads.
VA (volt-amps) = V × I, which determines wire heating and breaker loading. Watts = VA × PF, which is the real power. For circuit sizing, always use VA (or equivalently, use watts with the actual power factor). This calculator accounts for this.
Three-phase distributes power across three conductors. For the same total power, each phase carries 1/√3 of the current that a single-phase system would need. This means smaller conductors, less copper, and lower I²R losses.