Wire Sizing
Wire sizing ensures safe current capacity (ampacity), limits voltage drop, and meets NEC requirements. Factors: load current, distance, voltage drop %, ambient temperature, conduit fill, and insulation rating.
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NEC ampacity tables vary by insulation (60°C, 75°C, 90°C). Voltage drop limit often 3% for branch, 5% total. Derating: 80% for 4-6 conductors, 70% for 7-9. Conduit fill limits prevent overheating in raceways.
Ready to run the numbers?
Why: Undersized wire causes overheating and fire. NEC mandates minimum sizes. Voltage drop affects equipment performance. Derating accounts for ambient heat and conduit fill.
How: Find ampacity from NEC tables. Check voltage drop: ΔV = 2IRL for round-trip. Apply derating factors for temperature and number of conductors. Select smallest gauge meeting all constraints.
Run the calculator when you are ready.
🏠 Residential Branch Circuit
Standard 20A branch circuit for kitchen outlets, 50 feet run
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⚙️ Motor Feeder Circuit
3-phase motor feeder, 30HP motor, 200 feet run
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☀️ Solar PV Array
Solar panel array DC circuit, 40A current, 150 feet to inverter
Click to use this example
🔌 EV Charger Circuit
Level 2 EV charger, 32A continuous load, 75 feet run
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🏭 Industrial Feeder
Large industrial feeder, 200A load, 300 feet run, multiple conductors
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🔩 Aluminum Service Entrance
Service entrance using aluminum wire, 150A load, 100 feet
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Enter Circuit Parameters
Basic Parameters
Voltage Drop Requirements
Derating Factors
Application Settings
❓ Frequently Asked Questions
What is wire ampacity and why is it important?
Ampacity is the maximum current a wire can safely carry without exceeding its temperature rating. It's critical for electrical safety - undersized wires can overheat, causing fires, while oversized wires waste money. NEC ampacity tables specify safe current limits based on wire size, insulation type, and installation conditions.
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage along a circuit due to wire resistance. Excessive voltage drop causes equipment to operate inefficiently, motors to run hot, and lights to dim. NEC recommends maximum 3% drop for branch circuits and 5% total system drop. This calculator ensures your wire size meets these requirements.
What are derating factors and when do they apply?
Derating factors reduce wire ampacity based on installation conditions. Temperature derating applies when ambient temperature exceeds 30°C (86°F). Conduit fill derating applies when multiple current-carrying conductors share a conduit. These factors ensure wires don't overheat in real-world conditions.
What is the 125% rule for continuous loads?
NEC requires wire and breaker sizing for continuous loads (operating 3+ hours continuously) to be sized at 125% of the load current. This accounts for sustained heating effects. Examples include EV chargers, water heaters, and HVAC equipment. Non-continuous loads don't require this multiplier.
What's the difference between copper and aluminum wire?
Copper has higher ampacity and better conductivity but costs more. Aluminum has lower ampacity (about 61% of copper for same size) but is lighter and cheaper. Aluminum requires larger wire sizes for the same current. Both are NEC-approved, but copper is preferred for smaller circuits due to reliability.
What insulation temperature ratings are available?
Common ratings are 60°C (THW, TW), 75°C (THWN, THHN), and 90°C (THHN, XHHW). Higher ratings allow higher ampacity but cost more. The rating must match the lowest-rated component (wire, terminals, breaker). Most modern installations use 75°C or 90°C rated wire.
How do I choose between single-phase and three-phase calculations?
Single-phase is used for residential circuits (120V, 240V) and small commercial. Three-phase is used for industrial and large commercial (208V, 480V). Three-phase has lower voltage drop per amp due to √3 factor (1.732). Always match your calculation to your actual system configuration.
What safety margin should I have?
A safety margin of 10-20% above required current is recommended for normal applications. This accounts for future expansion, measurement uncertainty, and provides a buffer. For critical circuits or high ambient temperatures, larger margins may be appropriate. The calculator shows your actual safety margin.
📚 Official Data Sources
NEC (NFPA 70)
National Electrical Code - US standard for electrical installations
Last Updated: 2026-01-01
UL (Underwriters Laboratories)
Safety certification and standards for electrical products
Last Updated: 2026-01-20
⚠️ Disclaimer: This calculator provides wire sizing estimates based on NEC ampacity tables, voltage drop formulas, and standard derating factors. Actual wire sizing must comply with local electrical codes, which may vary by jurisdiction. Always consult a licensed electrician and verify calculations with NEC tables for critical applications. Wire sizing depends on many factors including installation method, ambient conditions, load characteristics, and local code requirements. This calculator is for educational and planning purposes only and is not a substitute for professional electrical engineering services or code compliance verification.
For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
NEC Table 310.16 lists ampacity by wire size and insulation.
— NEC
Voltage drop ΔV = 2 × I × R × L for single-phase.
— Engineering Toolbox
Ambient >30°C requires ampacity derating.
— NEC
Conduit fill: max 40% for 3+ conductors.
— NEC
What is Wire Size Calculation?
Wire size calculation is a critical electrical engineering process that determines the appropriate conductor size (AWG or MCM) for an electrical circuit. Proper wire sizing ensures safe operation, prevents overheating, minimizes voltage drop, and ensures compliance with electrical codes such as the National Electrical Code (NEC).
The calculation considers multiple factors including current carrying capacity (ampacity), voltage drop limits, ambient temperature, conduit fill, and load characteristics. Selecting the correct wire size is essential for electrical safety, energy efficiency, and equipment performance.
How Does Wire Size Calculation Work?
Our calculator employs NEC ampacity tables, voltage drop formulas, and derating factors to determine the optimal wire size. The process involves:
- Current Requirement Analysis: Determine the load current, applying 125% multiplier for continuous loads per NEC requirements.
- Ampacity Selection: Select wire size from NEC tables based on insulation type (60°C, 75°C, or 90°C) and conductor material (copper or aluminum).
- Derating Application: Apply temperature and conduit fill derating factors to account for installation conditions.
- Voltage Drop Verification: Calculate voltage drop using resistance and circuit length, ensuring it meets branch circuit (3%) or total system (5%) limits.
- Safety Margin Assessment: Verify adequate capacity margin for safe operation and future expansion.
When to Use Wire Size Calculator
This calculator is essential for electrical engineers, electricians, and designers working on various electrical installations:
Residential Wiring
Size branch circuits, feeders, and service entrances for homes and apartments.
Applications:
- Kitchen circuits
- Bathroom circuits
- EV charger circuits
Industrial Applications
Size motor feeders, distribution panels, and large industrial circuits.
Applications:
- Motor circuits
- Distribution feeders
- High-current loads
Solar PV Systems
Size DC circuits for solar panel arrays and inverter connections.
Applications:
- PV array wiring
- DC combiner circuits
- Inverter connections
Wire Size Calculation Formulas
Our calculator employs multiple electrical engineering formulas based on NEC requirements and industry standards. Understanding these calculations helps ensure proper wire sizing for safe and efficient electrical installations.
📊 Core Calculation Formulas
Voltage Drop (Single-Phase)
Where VD is voltage drop (V), I is current (A), R is resistance per 1000ft (Ω), L is length (ft). Factor of 2 accounts for round-trip current path.
Voltage Drop (Three-Phase)
Where √3 (1.732) accounts for three-phase system geometry, other variables same as single-phase.
Derated Ampacity
Where I_base is base ampacity from NEC tables, F_temp is temperature derating factor, F_conduit is conduit fill derating factor.
Continuous Load Adjustment
NEC Article 210.19(A)(1) requires 125% multiplier for continuous loads (operating 3+ hours continuously).
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