How to Convert Watts to Amps: Formulas for DC, Single-Phase, and Three-Phase
Learn how to convert watts to amps using Ohm's Law and power formulas for DC circuits, single-phase AC, and three-phase AC with practical worked examples.
Related Calculators
- Generator Sizing CalculatorEstimate generator running and starting capacity from total running watts, the largest additional surge load and an editable reserve margin.
- Watts to Amps CalculatorConvert watts to amps for DC, single-phase AC or three-phase AC circuits using voltage, power factor and efficiency.
- 3 Phase Power CalculatorCalculate three-phase real power, apparent power and reactive power from line voltage, current and power factor.
- Air Changes per Hour CalculatorCalculate air changes per hour from room volume and supplied or exhausted airflow.
- Air Conditioning Installation Cost CalculatorEstimate central air conditioning installation cost from equipment, labor and project extras.
- Amps to kW CalculatorConvert amps to kilowatts for single-phase and three-phase electrical loads.
The Watts to Amps Conversion Formula
Current (amps) depends on power (watts) and voltage — the relationship differs by circuit type.
DC Circuits
I (amps) = P (watts) / V (volts)
Example: A 60W DC device at 12V: I = 60 / 12 = 5 amps
Example: 1,200W at 120V: I = 1,200 / 120 = 10 amps
Single-Phase AC Circuits
Single-phase AC introduces power factor (PF), which accounts for reactive loads (motors, transformers):
I = P / (V × PF)
For purely resistive loads (heaters, incandescent bulbs): PF = 1 I = P / V (same as DC formula)
For motors and electronics (typical PF = 0.8–0.95):
Example: 1,500W motor at 120V, PF = 0.85: I = 1,500 / (120 × 0.85) = 1,500 / 102 = 14.7 amps
Compare to PF=1: I = 1,500/120 = 12.5 amps — ignoring power factor undersizes the wire.
Three-Phase AC Circuits
Line current formula: I = P / (V_LL × √3 × PF)
Where V_LL = line-to-line voltage, √3 ≈ 1.732
Example: 10,000W three-phase motor at 480V, PF = 0.90: I = 10,000 / (480 × 1.732 × 0.90) = 10,000 / 748.2 = 13.4 amps per phase
Common three-phase voltages: - 208V (US low voltage three-phase) - 240V (delta) - 480V (US industrial standard) - 400V (European standard)
Watts to Amps Reference Table (Single-Phase, PF=1)
VA vs. Watts: Why Power Factor Matters
Watts (W) = real power (does actual work) VA (volt-amperes) = apparent power (what the source must supply) VA = W / PF
For a 2,000W UPS with 0.8 PF load: apparent power = 2,000 / 0.8 = 2,500 VA Actual current = 2,500 VA / 120V = 20.8 amps (not the 16.7 you'd calculate ignoring PF)
Wire Sizing Implications
NEC requires wire capacity to be sized to actual current: - 15A circuit: 14 AWG (maximum 12A continuous = 80% of 15A) - 20A circuit: 12 AWG (maximum 16A continuous) - 30A circuit: 10 AWG - 40A circuit: 8 AWG - 60A circuit: 6 AWG
Always calculate actual amps including power factor before selecting wire and breaker size.
Related Guides
- Air Changes Per Hour: What ACH Means and How to Calculate ItUnderstand air changes per hour (ACH): what it measures, ASHRAE targets for different room types, how to calculate the airflow needed, and why it matters for indoor air quality.
- How to Use the Air Conditioning Installation Cost CalculatorLearn how to use the Air Conditioning Installation Cost Calculator, check its formula and interpret the result.
- Breaker Size Guide: How to Size Circuit Breakers for Any LoadLearn how to size circuit breakers correctly using NEC rules. Covers the 80% continuous load rule, motor circuit sizing at 250%, AFCI/GFCI requirements, and common household circuits.
- How to Convert Amp Hours to Watt HoursLearn how to use the Amp Hour to Watt Hour Calculator, understand its formula and interpret the result.
- Electrical Load Calculator: How to Estimate Home Panel and Circuit LoadsLearn to calculate your home's electrical load for panel sizing, service entry, and circuit planning. Covers NEC Standard Method, demand factors, and what drives panel capacity.
Frequently Asked Questions
- How do I convert watts to amps?
- Amps = Watts / Volts for DC or resistive AC loads. For AC with motors or electronics, include power factor: Amps = Watts / (Volts × Power Factor). Example: 1,000W at 120V AC with PF 0.85: Amps = 1,000 / (120 × 0.85) = 9.8 amps. At PF=1 (resistive): 1,000/120 = 8.33 amps.
- How many amps is 1500 watts at 120 volts?
- 1,500W ÷ 120V = 12.5 amps (assuming resistive load, PF=1). For a motor with PF=0.85: 1,500 / (120 × 0.85) = 14.7 amps. This is why electric heaters (which are purely resistive) draw 12.5A, but a motor of similar wattage draws more current from the same circuit.
- What is power factor and why does it matter for amps calculation?
- Power factor (PF) is the ratio of real power (watts) to apparent power (VA). Motors, transformers, and switching power supplies draw more current than their wattage suggests because some current is used to magnetize coils (reactive power) rather than doing work. Ignoring PF when calculating amps can undersize circuits — always use actual PF from the device nameplate.
- How many amps is 10000 watts at 240 volts?
- 10,000W ÷ 240V = 41.7 amps (resistive load). For three-phase at 240V with PF=0.90: 10,000 / (240 × 1.732 × 0.90) = 10,000 / 374.1 = 26.7 amps per phase. Single-phase 240V loads (electric ranges, dryers) use the simple formula; three-phase motor loads require the √3 multiplier.
Last updated 7/28/2026