Amperage Calculator | Free Construction Calculators
Calculate electrical current (amperage) for your electrical projects using Ohm's Law and power formulas. Our professional amperage calculator helps electricians, contractors, and DIY enthusiasts accurately determine current draw for circuit design, wire sizing, and breaker selection. Works with both single-phase and three-phase systems at 120V, 240V, 208V, 277V, and 480V. Simply enter watts, voltage, resistance, or power factor to get instant amperage results for proper electrical system planning and NEC code compliance.
Multiple Formulas
Calculate amperage from watts & voltage, voltage & resistance, or watts & resistance
Phase Support
Works with single-phase residential and three-phase commercial/industrial systems
Power Factor
Adjust for inductive loads like motors and transformers for accurate current calculations
How to Use This Calculator
- Select your calculation method (Watts & Voltage is most common for appliances)
- Enter the first value (watts, voltage, or resistance based on your method)
- Enter the second value (voltage or resistance)
- Set the power factor (use 1.0 for resistive loads, 0.7-0.9 for motors)
- Choose phase type (single-phase for residential, three-phase for industrial)
- Click Calculate to see the current draw in amps
- Use the result to size wire gauge and circuit breakers per NEC requirements
Calculation Formulas
Ohm's Law: I = V ÷ R (Current = Voltage ÷ Resistance)
Single-Phase Power: I = P ÷ (V × PF) (Current = Watts ÷ Voltage × Power Factor)
Three-Phase Power: I = P ÷ (√3 × V × PF) (1.732 × Voltage × Power Factor)
Watts & Resistance: I = √(P ÷ R) (Square root of Watts ÷ Resistance)
About Amperage Calculations
Accurate amperage calculations are fundamental to electrical system design and safety. Understanding current draw is essential for selecting proper wire gauge (14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG), circuit breaker sizing (15A, 20A, 30A, 40A, 50A), and ensuring NEC code compliance. Whether you're installing a new circuit, sizing a generator, or troubleshooting an overloaded system, knowing the exact amperage prevents electrical fires, equipment damage, and code violations.
The National Electrical Code (NEC) requires that continuous loads (operating for 3 or more hours) not exceed 80% of the circuit breaker rating. This means a 20-amp breaker should only carry 16 amps continuously. Our calculator helps you determine actual current draw so you can apply the 125% multiplier (1 ÷ 0.8 = 1.25) when selecting breakers. For example, a 1500-watt space heater on 120V draws 12.5 amps, requiring a 20A breaker (12.5 × 1.25 = 15.6A, rounded up to 20A).
Power factor significantly affects amperage calculations for inductive loads. Motors, transformers, and fluorescent ballasts have power factors between 0.7-0.9, meaning they draw more current than their wattage suggests. A 5-horsepower motor rated at 3,730 watts with a 0.8 power factor draws 19.4 amps on 240V, not the 15.5 amps you'd calculate without power factor correction. Always check equipment nameplates for actual power factor and full-load amperage ratings.
Common Appliance Amperage (120V Circuits)
- 60W Light Bulb: 0.5 amps (general lighting circuits)
- Microwave (1000W): 8.3 amps (dedicated 15A or 20A circuit)
- Coffee Maker (1200W): 10 amps (kitchen small appliance circuit)
- Space Heater (1500W): 12.5 amps (requires 20A circuit for continuous use)
- Vacuum Cleaner: 8-12 amps (standard 15A circuit adequate)
- Hair Dryer (1875W): 15.6 amps (maximum for 20A bathroom circuit)
Common Appliance Amperage (240V Circuits)
- Electric Water Heater (4500W): 18.75 amps (requires 30A breaker, 10 AWG wire)
- Electric Dryer (5000W): 20.8 amps (requires 30A breaker, 10 AWG wire)
- Electric Range (8000W): 33.3 amps (requires 40A breaker, 8 AWG wire)
- Central AC Unit (3 Ton): 15-18 amps (requires 30A breaker, 10 AWG wire)
- EV Charger Level 2 (7.2kW): 30 amps (requires 40A breaker, 8 AWG wire)
- Well Pump (1.5 HP): 12-15 amps (requires 20-30A breaker depending on motor)
Frequently Asked Questions
How do I calculate amperage from watts and voltage?
For single-phase circuits, divide watts by voltage to get amps: I = W ÷ V. For example, a 1500-watt heater on 120V draws 12.5 amps (1500 ÷ 120 = 12.5A). For three-phase systems, divide watts by 1.732 times voltage times power factor: I = W ÷ (1.732 × V × PF). This calculator handles both formulas automatically based on your phase selection.
What wire size do I need for different amperage ratings?
NEC Table 310.16 specifies minimum wire sizes: 14 AWG for 15A circuits (maximum 80% load = 12A continuous), 12 AWG for 20A circuits (16A continuous), 10 AWG for 30A circuits (24A continuous), 8 AWG for 40A circuits (32A continuous), and 6 AWG for 50-55A circuits. These ratings assume copper wire at 75°C. Always upsize wire for long runs to prevent voltage drop exceeding 3% per NEC recommendations.
Why is power factor important for amperage calculations?
Power factor represents the ratio of real power (watts) to apparent power (volt-amps). Inductive loads like motors and transformers have power factors below 1.0, meaning they draw more current than their wattage suggests. A motor with 0.8 power factor draws 25% more current than a resistive load of equal wattage. Ignoring power factor leads to undersized circuits, nuisance breaker trips, and overheated wires. Always use actual power factor from equipment nameplates.
What's the difference between single-phase and three-phase amperage?
Single-phase systems (residential 120V/240V) use the formula I = P ÷ V. Three-phase systems (commercial/industrial 208V/480V) distribute power across three conductors, using I = P ÷ (√3 × V) where √3 = 1.732. This makes three-phase more efficient - a three-phase motor draws 42% less current per conductor than single-phase for the same horsepower. A 10 HP three-phase motor at 480V draws 14 amps, while single-phase at 240V draws 28 amps.
How much amperage does a typical house use?
Average residential service is 200 amps at 240V (48,000 watts maximum capacity). Typical usage: HVAC system 15-30A, electric range 40-50A, electric dryer 30A, water heater 20-30A, general lighting and receptacles 30-50A combined. During peak usage (AC running, cooking dinner, dryer operating), homes commonly draw 80-120 amps. NEC requires 100A minimum service for homes under 3,000 square feet, 200A for larger homes or those with electric heat.
Related Electrical Calculators
NEC Code Requirements for Amperage
- NEC 210.19(A): Branch circuit conductors must have ampacity not less than the load
- NEC 210.20(A): Continuous loads limited to 80% of breaker rating
- NEC 240.4: Overcurrent protection required based on conductor ampacity
- NEC 430.6: Use motor nameplate current for amperage calculations
- NEC 430.22: Motor branch circuit conductors sized at 125% of motor full-load current
- Standard Breakers: 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A
Wire Ampacity Quick Reference (Copper, 75°C)
| Wire Size | Maximum Amps | Breaker Size | Continuous Load (80%) |
|---|---|---|---|
| 14 AWG | 20 A | 15 A | 12 A |
| 12 AWG | 25 A | 20 A | 16 A |
| 10 AWG | 35 A | 30 A | 24 A |
| 8 AWG | 50 A | 40 A | 32 A |
| 6 AWG | 65 A | 50-60 A | 40-48 A |
| 4 AWG | 85 A | 70-80 A | 56-64 A |
Professional Electrician Tips
- Always check nameplates: Use actual equipment ratings, not estimated wattage
- Account for starting current: Motors draw 5-8x running current during startup
- Apply 125% multiplier: Size circuits for continuous loads at 125% of calculated amperage
- Consider ambient temperature: Hot environments (attics, outdoor) require wire derating
- Plan for future expansion: Oversizing circuits by one size provides flexibility
- Use copper wire: Superior conductivity and easier to work with than aluminum
- Verify with clamp meter: Measure actual current draw to validate calculations
- Follow local codes: Some jurisdictions require larger wire sizes than NEC minimums
Real-World Amperage Calculation Examples
Example 1: Electric Water Heater (240V)
Given: 4500-watt water heater, 240V, resistive load (PF = 1.0)
Calculation: I = 4500W ÷ 240V = 18.75 amps
Continuous Load: 18.75A × 1.25 = 23.4 amps (requires 30A breaker)
Wire Size: 10 AWG copper (rated 35A at 75°C)
Example 2: Three-Phase Motor (480V)
Given: 10 HP motor, 480V three-phase, PF = 0.85
Power: 10 HP × 746 W/HP = 7460 watts
Calculation: I = 7460W ÷ (1.732 × 480V × 0.85) = 10.5 amps
NEC 430.22: 10.5A × 1.25 = 13.1 amps (minimum conductor ampacity)
Wire Size: 14 AWG copper (20A rating adequate with margin)
Example 3: Kitchen Circuit (120V)
Given: Microwave 1200W + toaster oven 1500W, 120V
Total Load: 1200W + 1500W = 2700 watts
Calculation: I = 2700W ÷ 120V = 22.5 amps
Required: Two separate 20A circuits (NEC requires two 20A small appliance circuits)
Wire Size: 12 AWG copper on each circuit