Wire Size Calculator
This tool is for informational and educational purposes only and does not constitute professional, legal, financial, or code-compliance advice. Figures, rates, codes, and requirements change over time and vary by jurisdiction, and may not reflect the latest local regulations. Results are estimates — always verify with an official source or a qualified professional before making decisions.

How to Use the Wire Size Calculator

Last updated:

This guide explains what each input means, how the calculator works, and what the results tell you. It is not a substitute for the National Electrical Code or a licensed electrician. All calculations are estimates — see the disclaimer above.

What This Calculator Does

The Wire Size Calculator determines the minimum AWG (American Wire Gauge) conductor size for a circuit by applying two independent constraints from NEC 2023 (NFPA 70):

  1. Ampacity — the conductor must be large enough to safely carry the load current without overheating, per NEC Table 310.16 (75°C column).
  2. Voltage drop — the conductor must be large enough that the resistive voltage loss over the run length stays within your specified percentage.

The recommended wire size is the larger of the two results — the conservative, safe choice that satisfies both constraints.

Understanding the Inputs

  • Load (amps) — The actual current draw of the load in amperes. For motors or other equipment, use the nameplate full-load current (FLC), not the breaker size.
  • One-way run length (ft) — Distance from the overcurrent protective device (breaker or fuse) to the load, measured along the conductor path. The voltage-drop formula uses this one-way distance; the round-trip factor is built into the formula.
  • Conductor material — Copper or aluminum. Aluminum has higher resistivity (K ≈ 21.2 vs. 12.9 for copper) and lower ampacity at the same gauge. Aluminum is not recommended below 6 AWG for most applications per NEC 310.3(B).
  • System voltage — The nominal circuit voltage. Voltage drop is expressed as a percentage of this value. Choose 120 V for standard branch circuits, 240 V for two-pole circuits, 208 V or 480 V for applicable systems.
  • Phase — Single-phase uses a factor of 2 in the voltage-drop formula (one outgoing, one return conductor). Three-phase uses √3 (≈ 1.732).
  • Allowable voltage drop (%) — The maximum acceptable percentage voltage loss from source to load. NEC 210.19(A) informational note recommends 3% on branch circuits. Sensitive equipment may require 2% or less.
  • Continuous load toggle — Check this if the load will be energized for 3 or more continuous hours (NEC Article 100 definition). Per NEC 210.20(A), the wire and breaker must be rated at 125% of the continuous load current. EV charger circuits are always continuous per NEC Article 625.42.

Understanding the Results

  • Min AWG by Ampacity — The smallest standard AWG size whose 75°C ampacity meets or exceeds the design current (after applying 125% if continuous), per NEC 2023 Table 310.16.
  • Min AWG for Voltage Drop — The smallest standard AWG size at which the calculated voltage drop stays at or below your allowable percentage.
  • Recommended AWG — The larger of the two sizes above. Satisfies both ampacity and voltage-drop requirements.
  • Actual Voltage Drop — The calculated percentage voltage drop at the recommended wire size and your design current.

The Voltage-Drop Formula

This calculator uses the standard circular-mils voltage-drop formula published in NEC training materials and Ugly's Electrical Reference:

Vd = phaseFactor × K × I × L / CM

  • Vd = voltage drop in volts
  • phaseFactor = 2 (single-phase) or √3 (three-phase)
  • K = resistivity constant: 12.9 ohm·cmil/ft (copper) or 21.2 (aluminum)
  • I = design current in amps
  • L = one-way run length in feet
  • CM = conductor cross-sectional area in circular mils

Percentage voltage drop = (Vd / system voltage) × 100.

NEC Table 310.16 and the 75°C Column

NEC 2023 Table 310.16 lists allowable ampacities for insulated conductors in raceway or cable with not more than 3 current-carrying conductors at 30°C (86°F) ambient. Three temperature columns exist: 60°C, 75°C, and 90°C.

This calculator uses the 75°C column, which is the most commonly applied in practice. NEC 110.14(C) limits most terminations (breaker lugs, wire connectors) to 75°C regardless of conductor insulation rating. Using the 90°C column requires that all terminations on the circuit be rated for 90°C — which is uncommon in residential and light-commercial work.

Limitations of This Tool

This calculator provides estimates under standard NEC conditions only. It does not account for:

  • Ambient temperature correction — If ambient exceeds 30°C (86°F), ampacity must be derated per NEC Table 310.15(B)(1).
  • Conductor bundling / conduit fill — More than 3 current-carrying conductors in a raceway requires adjustment factors per NEC 310.15(C)(1).
  • Motor circuits — Motor circuits follow NEC Article 430 rules (FLC tables, service-factor adjustments, locked-rotor current) that differ from simple ampacity lookup.
  • Parallel conductors — Loads requiring sizes above 4/0 AWG typically use parallel conductor sets. This tool tops out at 4/0 AWG.
  • Local AHJ amendments — Your local authority having jurisdiction (AHJ) may have adopted amendments to the NEC or local ordinances that change these requirements.
  • Exact conductor resistance — The K-factor formula is an approximation. Actual resistance varies with conductor temperature, stranding, and installation method.

Frequently Asked Questions

See the FAQ section on the main calculator page for common questions about ampacity, voltage drop, the continuous-load rule, and aluminum conductors.

Related Calculators