Electrical Calculators | Free Construction Calculators
Calculate circuit load, wire size, voltage drop, and amperage
Circuit Load Calculator
Calculate circuit current and breaker size
Wire Size Calculator
Calculate required wire size
Voltage Drop Calculator
Calculate voltage drop in circuits
Amperage Calculator
Calculate current using Ohm's Law
About Electrical Calculations
Accurate electrical calculations are essential for safe circuit design, proper equipment protection, and code compliance. Whether you're installing a new circuit, sizing conductors, or analyzing voltage drop, professional electrical calculators help ensure your wiring meets NEC (National Electrical Code) requirements and industry standards. Our tools cover circuit load analysis, wire ampacity determination, voltage drop verification, and amperage calculations for both single-phase and three-phase systems.
Why Use Our Electrical Calculators?
Code Compliance
Ensure your electrical designs meet NEC requirements for circuit protection, wire sizing, voltage drop limits, and ampacity ratings to pass inspections.
Safety Verification
Verify that wire sizes match breaker ratings, voltage drop is within acceptable limits, and all components are properly coordinated to prevent hazards.
Cost Optimization
Determine the minimum acceptable wire size to reduce material costs while maintaining safety and code compliance for your project.
Equipment Performance
Prevent voltage drop issues that cause lighting dimming, motor torque loss, and equipment malfunction through proper conductor sizing.
Professional Documentation
Maintain accurate calculations and documentation for permit submissions, inspections, insurance claims, and future system modifications.
Multiple System Support
Calculate for single-phase 120V/240V residential systems and three-phase 208V/277V/480V commercial and industrial systems.
Common Electrical Calculations
Wire Size Determination
Select wire gauge (AWG) based on amperage requirements, distance from breaker to load, voltage drop limits, and insulation temperature rating. Use larger wire sizes for longer runs to minimize voltage drop.
Ampacity Verification
Verify that selected conductor ampacity matches or exceeds circuit amperage. Account for temperature derating when multiple wires are bundled in conduit or exposed to elevated ambient temperatures.
Voltage Drop Analysis
Calculate voltage drop for branch circuits (3% maximum) and feeders (2% maximum) to ensure adequate voltage reaches end-use equipment and devices operate properly.
Breaker Size Calculation
Select breaker rating based on conductor ampacity for non-continuous loads, or apply 125% multiplier for continuous loads (loads running 3+ hours) per NEC 210.19.
Load Analysis
Calculate circuit amperage from voltage and power (watts) using Ohm's Law and power formulas. Sum connected loads and apply demand factors for service panel sizing.
Three-Phase Calculations
Calculate power and current for three-phase systems using specialized formulas (P = √3 × V × I × PF) for commercial and industrial equipment and distribution systems.
Wire Ampacity Reference (75°C Copper Conductors)
| Wire Size (AWG) | Ampacity (A) | Common Use | Typical Breaker |
|---|---|---|---|
| 18 AWG | 14 A | Thermostat, control circuits | N/A (not used) |
| 16 AWG | 18 A | Fixture wiring, low power | N/A (not used) |
| 14 AWG | 20 A | Lighting circuits | 15 A max |
| 12 AWG | 25 A | Receptacles, kitchen, bath | 20 A max |
| 10 AWG | 35 A | Dryer, AC, 30A circuits | 30 A max |
| 8 AWG | 50 A | Range, 40-50A circuits | 40-50 A |
| 6 AWG | 65 A | 50-60A circuits, subpanels | 50-60 A |
| 4 AWG | 85 A | 70-80A circuits | 70-80 A |
| 2 AWG | 115 A | 100A service, feeders | 100 A |
| 1 AWG | 130 A | 125A service | 125 A |
Note: Ampacity values shown are for copper conductors in free air (ambient 30°C). Reduce ampacity by 10-30% when wires are bundled in conduit or installed in high-temperature environments. Always consult NEC Table 310.15(B)(16) for comprehensive ampacity ratings.
Copper vs Aluminum Conductors
Copper Advantages
- Better conductivity (61% higher than aluminum)
- Smaller wire sizes for same ampacity
- Lower voltage drop over distance
- Better corrosion resistance
- More flexible, easier to work with
- Standard terminations compatible
- Preferred for branch circuits
Aluminum Advantages
- Lighter weight (1/3 of copper)
- Lower material cost
- Suitable for long distances
- Common in service entrance
- Used for large feeders
- Economic choice for 4 AWG+
- Requires special terminations
Voltage Systems Overview
Single-Phase (Residential)
- 120V/240V split-phase (most common)
- 120V outlets, 240V large appliances
- Service sizes: 100A, 150A, 200A
- Power formula: P = V × I
- Used for homes, small shops
Three-Phase (Commercial/Industrial)
- 208V, 277V, 480V common voltages
- 73% more power capacity than single-phase
- Power formula: P = √3 × V × I × PF
- Used for motors, HVAC, industrial
- Better load distribution
NEC Code Requirements
Article 210 - Branch Circuits
- 15A circuits: 14 AWG minimum
- 20A circuits: 12 AWG minimum
- 30A circuits: 10 AWG minimum
- GFCI in wet locations
- AFCI in sleeping areas
- 80% rule for continuous loads
- Ground fault protection for outlets
Article 310 - Wire Ampacity
- Ampacity based on insulation rating
- 60°C, 75°C, or 90°C rating
- Temperature derating factors
- Conduit fill adjustment
- Ambient temperature correction
- 125% for continuous loads
- Multiple conductor bundles
Article 215 - Feeders
- Feeder ampacity 125% for continuous
- Sizing based on demand load
- Disconnecting means required
- Ground fault protection
- Voltage drop 2% maximum
- Proper bonding and grounding
- Protection from overcurrent
Article 220 - Load Calculation
- Demand factors for loads
- Diversity for multiple circuits
- Future load allowance
- Three-phase power calculations
- Service and feeder sizing
- Heating and AC interaction
- Nameplate rating application
Frequently Asked Questions
How do I determine the correct wire size (AWG) for my circuit?
Wire size is determined by the amperage requirement and the distance from the breaker to the load. First, calculate the circuit amperage using Ohm's Law (I = P/V). Then use a wire ampacity chart based on the insulation temperature rating (usually 60°C or 75°C). The wire must safely carry the circuit amperage with appropriate derating for temperature and conduit fill. For example, a 20-amp circuit at 120V typically requires 12 AWG copper wire. For longer runs, you may need a larger wire size to account for voltage drop. Always check local electrical codes and the NEC (National Electrical Code) Article 310 for ampacity tables. Add 5-10% to calculated amperage for future load growth.
What is ampacity and how does it relate to breaker selection?
Ampacity is the maximum current an electrical conductor can safely carry without exceeding its insulation temperature rating. For example, 12 AWG copper wire at 75°C has an ampacity of 20 amps. Breaker selection is based on the wire's ampacity rating, not on what the load requires. A 20-amp breaker must be paired with 12 AWG wire (20A ampacity). Using undersized wire with a breaker creates a fire hazard. The conductor ampacity must always equal or exceed the breaker rating. Continuous loads (loads running 3+ hours) require special consideration: multiply the continuous load by 1.25 (125%) when selecting the breaker, but the wire size is still based on actual ampacity needed. Temperature derating applies when multiple wires are in the same conduit (reduce ampacity by 10-30% depending on number of wires).
What are the NEC voltage drop limits for branch circuits and feeders?
The National Electrical Code (NEC) recommends maximum voltage drop limits of 3% for branch circuits and 2% for feeders, with a combined maximum of 5% for the entire system. For example, on a 120V circuit, 3% voltage drop equals 3.6 volts, which is acceptable. A 5% drop (6 volts) is acceptable but not ideal. Voltage drop is critical for motor performance and lighting brightness: every 1% voltage drop reduces motor torque by approximately 2% and reduces light output by about 3%. To minimize voltage drop, use larger wire sizes (typically increase one AWG size per 50-100 feet of distance depending on load). Calculate voltage drop using the formula: Drop = (2 × K × I × D) / CM, where K is resistance constant (12 for copper), I is current, D is distance in feet, and CM is wire circular mil area. Professional installers verify voltage drop with multimeters on completion.
How is breaker size calculated for continuous loads according to NEC Article 210?
Breaker sizing for continuous loads requires applying the 125% continuous load rule (NEC 210.19). First, determine the circuit load in amperes. For continuous loads (loads running 3+ hours), multiply the load amperage by 1.25 (125%). This increased amperage is used to select both the breaker size and wire size. For example, a 16-amp continuous load requires: 16 × 1.25 = 20-amp breaker minimum, and the wire must have 20-amp ampacity (12 AWG at 75°C). Non-continuous loads don't require the 125% multiplier. Breakers are available in standard sizes: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, etc. Select the next standard size if the calculated load falls between sizes. The 125% rule protects against thermal aging of equipment and reduces nuisance breaker trips during heavy loads. Document continuous vs non-continuous classification during design.
How does Ohm's Law apply to electrical calculations?
Ohm's Law is the fundamental relationship between voltage (V), current (I), and resistance (R): V = I × R. Rearranged: I = V / R and R = V / I. For power calculations, use P = V × I (power in watts = voltage × current). These formulas are essential for electrical design. Example: a 120V circuit with a 10-amp load has: P = 120 × 10 = 1,200 watts. To find amperage from power: I = P / V. So a 2,400-watt dryer at 240V requires: I = 2,400 / 240 = 10 amps. For three-phase systems, power = √3 × V × I × PF (power factor, typically 0.85-0.95). Ohm's Law also explains voltage drop: higher current and longer distances increase voltage drop proportionally. Understanding these relationships allows proper circuit design, load balancing, and code compliance. Most electrical calculators use these formulas internally to convert between voltage, amperage, power, and wire sizing.
What's the difference between aluminum and copper wire for residential and commercial use?
Copper and aluminum conductors have significant differences affecting wire size selection and installation: Copper has superior conductivity (approximately 61% better than aluminum), allowing smaller wire sizes for the same ampacity. For example, 4 AWG copper and 2 AWG aluminum both carry 85 amps. Aluminum is lighter (approximately one-third the weight of copper) and less expensive per pound, but requires larger wire sizes. Aluminum requires at least two wire sizes larger than copper for equivalent ampacity. Copper is preferred for branch circuits and has superior corrosion resistance, durability, and flexibility. Aluminum is common for main service entrance conductors, subfeeders, and long-distance transmission due to cost savings. Both materials require proper termination techniques: aluminum connections need anti-oxidation compound and special terminals designed for aluminum. Copper terminations can be standard. Building codes often require copper for residential circuits and allow aluminum for service entrance and feeders. Cost analysis should include both material and installation labor when choosing between copper and aluminum. Voltage drop is higher with aluminum due to higher resistance, so larger sizes are needed for long runs.
Safety and Professional Guidance
All electrical work must comply with the National Electrical Code (NEC) and local electrical codes. Obtain proper permits for any electrical installations or modifications. Use licensed electricians for complex work or modifications to existing systems. Always verify circuits are de-energized with a voltage tester before working. These calculators provide estimates and guidance, but professional engineering review may be required for complex systems, commercial installations, or when local codes have specific requirements.
- Turn off power at the breaker before any electrical work
- Verify circuits are de-energized with a voltage tester
- Never exceed the ampacity rating of conductors
- Ensure proper grounding and bonding for safety
- Use appropriate personal protective equipment (PPE)
- Follow manufacturer specifications for all equipment
- Verify breaker rating matches wire ampacity
- Account for voltage drop in long runs
- Apply derating factors for temperature and bundling
- Consult local electrical inspector for code questions