Wire Size Calculator

Enter the continuous load current the wire needs to carry in amperes. This is the current draw of the connected equipment, fixtures, or circuit under normal operating conditions.

How do I determine the correct load current?

Load current can be calculated by dividing the total wattage of the connected load by the system voltage, or by referencing the amperage listed on the equipment's specification sheet. For lighting circuits, add the wattage of all fixtures on the circuit, then divide by the operating voltage to determine the total amperage. Per NEC Article 210.19, continuous loads must be calculated no less than 125% of the actual load current for branch circuit sizing, where non-continuous loads must be calculated at no less than 100% of the non-continuous load.

For mixed loads, motor loads, or specialty applications, consult a licensed electrician to verify load calculations against your project's electrical drawings.

Choose the system voltage that matches your project's electrical configuration. Common system voltages for commercial and industrial applications include 120V, 208V, 240V, 277V, 480V, and 600V.

The system voltage affects voltage drop calculations and determines which wire sizes are appropriate for the application. Higher voltage systems can typically use smaller wire sizes for the same load due to reduced amperage, while lower voltage systems may require larger conductors over longer distances.

Voltages above 50V present serious shock hazards. Working on energized electrical systems requires proper training, personal protective equipment (PPE), and adherence to OSHA and NFPA 70E standards. Verify the actual system voltage at the source before specifying conductors.

Determine the one-way distance from the power source to the load and enter the measurement in feet. The one-way distance refers to the linear length the wire must travel in a single direction, not the total length of conductor required.

Why does distance matter for wire sizing?

Voltage drop increases proportionally with distance. Longer runs require larger conductors to maintain acceptable voltage at the load. Accurate distance measurements help ensure the wire size selected will deliver the required voltage to the equipment without exceeding voltage drop limits. Field-measured distances should account for vertical rises, bends, and routing obstacles, as actual conductor length often exceeds the straight-line distance.

Enter the maximum acceptable voltage drop as a percentage. The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% combined for feeders and branch circuits, as referenced in NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2.

For lighting applications, lower voltage drop percentages help maintain consistent fixture performance, color temperature, and lumen output across the system. Sensitive equipment, motor loads, or critical systems may require tighter voltage drop tolerances.

Consult the equipment manufacturer's specifications for minimum operating voltage requirements before finalizing voltage drop limits.

Select the conductor material for your application:

• Copper: Higher conductivity, smaller wire size for the same load, more common in interior and branch circuit applications
• Aluminum: Lower cost and lighter weight, often used for service entrance, feeders, and long outdoor runs

Wire material affects ampacity, voltage drop, and the physical wire size required for the application. Copper has approximately 61% greater conductivity than aluminum, meaning aluminum conductors typically need to be sized one or two AWG sizes larger than copper for equivalent performance.

Aluminum conductors require terminations rated for aluminum (CO/ALR or AL/CU), antioxidant compound at connections, and proper torque specifications to prevent loosening over time. Mixing copper and aluminum at terminations without listed connectors can cause galvanic corrosion and connection failure. Material selection should be reviewed by a qualified electrical professional familiar with the application and local code requirements.

Choose the installation method that matches how the wire will be routed:

• Conduit / Raceway: Conductors installed inside metallic or non-metallic conduit, EMT, or raceway systems
• Cable: Multi-conductor cable assemblies such as MC, AC, or NM cable
• Free Air: Conductors installed in open air with adequate spacing for heat dissipation

The installation method affects the wire's ampacity rating because it determines how heat is dissipated from the conductor. Wires installed in conduit or bundled in cable assemblies have lower ampacity ratings than those installed in free air due to reduced heat dissipation. Per NEC Table 310.15(C)(1), ampacity adjustment factors apply when more than three current-carrying conductors are installed in a raceway or cable.

This calculator does not account for all NEC adjustment and correction factors, including conduit fill, mutual heating from adjacent circuits, or specialized installation conditions such as direct burial or wet locations. Final installation methods must comply with NEC Chapter 3 and applicable local codes.

Enter the ambient temperature of the environment where the wire will be installed. Standard NEC ampacity ratings are based on an ambient temperature of 86°F (30°C), as referenced in NEC Table 310.16.

How does ambient temperature affect wire sizing?

Higher ambient temperatures reduce a conductor's ability to dissipate heat, lowering its effective ampacity. Installations in hot environments, such as mechanical rooms, refineries, or outdoor locations in warm climates, may require derating factors that increase the required wire size. Conversely, cooler environments can support higher ampacities for the same conductor.

Click calculate to determine the estimated wire size for your application. The result accounts for the load current, voltage drop limits, installation conditions, and ambient temperature inputs provided.

This tool provides an estimate based on standard NEC ampacity tables and voltage drop formulas. It does not replace a stamped engineering design, electrical permit drawings, or the judgment of a licensed electrical professional. The calculator does not evaluate:

• Short-circuit current ratings or fault current calculations
• Overcurrent protection device coordination
• Grounding and bonding requirements
• Conduit fill calculations
• Special occupancy or hazardous location requirements (NEC Articles 500–517)
• State, county, or municipal code amendments

Final wire sizing must be verified against the current adopted edition of the National Electrical Code, local jurisdictional requirements, the authority having jurisdiction (AHJ), and the specific equipment manufacturer's installation instructions.