Wire Sizing Calculator - NEC Voltage Drop & Ampacity Tables

How It Works

1. Select Your Load

   Pick from common presets (EV charger, welder, subpanel, HVAC) or enter custom amps and voltage. The calculator applies local 125% planning multipliers for continuous, motor, EV-charger, and lighting rows; verify the governing NEC article before relying on them.

2. Set Load Type & Distance

   Tell the calculator what kind of load you are feeding and enter the one-way distance from your panel to the equipment. Motor, continuous, and EV-charger circuits may have additional code and equipment requirements beyond this calculator.

3. Configure Environment

   Set ambient temperature and conductor-count adjustment prompts. Hot attics, rooftops, and bundled conductors can require ampacity derating under NEC 310.15, but the actual correction depends on the adopted code and installation details.

4. Review Derating Impact

   See exactly how temperature and conduit fill derating affect your wire capacity. The calculator shows the combined derating factor so you understand why a wire rated for 100A might only carry 73A in your conditions.

5. Compare Copper vs Aluminum

   View side-by-side planning rows for both materials with wire size, voltage drop percentage, voltage at the load end, rough conduit sizing, and rough material cost.

6. Check the Circuit Diagram

   A visual circuit diagram shows your panel voltage, wire run, and the voltage actually arriving at your equipment. Color-coded voltage drop indicators make it obvious if your run is marginal.

Built For

• Licensed electricians screening branch-circuit and feeder inputs before adopted-code review
• Homeowners planning garage, workshop, or detached shop wiring with long runs
• Solar installers sizing DC and AC runs from panels to inverters and main panels
• EV charger planning where EVSE listing, Article 625, load calculation, and AHJ review still control
• Well pump circuits running 200+ feet from the house panel to a well house or pump
• Long-distance runs to barns, outbuildings, pole buildings, and detached garages
• Sub-panel feeder planning for additions, finished basements, or accessory dwelling units

Features & Capabilities

NEC 310.16 Ampacity Table Prompts

Built-in local ampacity rows from the NEC 310.16 75°C column for copper and aluminum conductors. Circuits 100A and smaller commonly require 60°C termination review under NEC 110.14(C).

Load Type Planning Multipliers

Applies local 125% planning multipliers for selected load types and keeps motor, EV, lighting, welder, equipment-listing, and Article-specific review gaps visible.

Temperature & Fill Derating Prompts

Local ambient-temperature and conductor-count adjustment prompts help frame review of NEC 310.15 correction and adjustment factors. Verify actual raceway and conductor count before design use.

Parallel Run Prompt

When a single run cannot satisfy the local calculator, the calculator can show parallel-run prompts (up to 4) and rough cost changes, with parallel termination and code-review warnings.

Circuit Diagram Visualization

Interactive SVG diagram shows your panel, wire run, and load with color-coded voltage drop. See exactly what voltage arrives at your equipment and whether your run is in the green, yellow, or red zone.

Total Material Cost Estimate

Calculates wire and conduit cost for both copper and aluminum. Choose between EMT, PVC Schedule 40, or rigid metallic conduit materials. See the total savings from aluminum at a glance.

Comparison

Assumptions

• Ampacity rows use the NEC Table 310.16 75°C column; NEC 110.14(C) termination-temperature verification remains required, especially for circuits 100A and smaller.
• Voltage drop uses K-factor approximations for copper and aluminum conductors with one-way distance.
• Ambient temperature assumed at 30°C (86°F) unless user specifies otherwise.
• Conduit-fill adjustment is represented by user-selected planning factors, not a complete Chapter 9 fill calculation.
• Single-phase or three-phase calculations based on user selection; power factor assumed at 1.0 for voltage drop unless specified
• Wire insulation/type, wet-location rating, cable assembly rules, and termination ratings are not verified.

Limitations

• Does not account for harmonic currents on shared neutrals in three-phase, four-wire systems
• Does not calculate short-circuit withstand ratings or fault current protection
• Temperature derating for bundled cables in cable trays follows NEC 392 - not covered by this tool
• Does not address medium-voltage conductor sizing (over 600V)
• Aluminum conductor recommendations do not account for specific alloy differences (AA-8000 series vs. older alloys)
• Voltage drop percentages are guidelines - NEC recommends but does not mandate the 3%/5% thresholds

References

1. NEC (NFPA 70) Article 310 - Conductors for General Wiring, Table 310.16
2. NEC Article 215.2(A)(4) and 210.19(A)(1) - Voltage Drop Recommendations (3% branch, 5% combined)
3. NEC 110.14(C) - Temperature Limitations at Terminations
4. IEEE Std 141 (Red Book) - Recommended Practice for Electric Power Distribution for Industrial Plants
5. Copper Development Association - Voltage Drop Calculator Methodology
6. NEC Table 310.15(C)(1) - Adjustment Factors for More Than 3 Current-Carrying Conductors

Frequently Asked Questions