Free electrical planning tool

Electrical wire size calculator

Find the smallest modeled copper or aluminum conductor with enough adjusted ampacity for your design load.

Ampacity inputs

Start with the design load

This result sizes for ampacity within the modeled range. Check voltage drop separately when route length matters.

Adjustment assumptions30°C ambient · 3 current-carrying conductors · THHN/THWN-2

Uses a 90°C THHN/THWN-2 correction basis, then limits the result by the selected terminal rating and small-conductor rules.

Preliminary result

8 AWG

8 AWG is the smallest modeled conductor with enough adjusted ampacity.

Required ampacity
40.0 A
Available ampacity
50.0 A
Ambient factor
100%
Conductor adjustment
100%

Preliminary design aid: verify load calculations, continuous and nonlinear loads, terminals, equipment instructions, overcurrent protection, voltage drop, adopted code, and field conditions.

How to read the result

The recommended size is the first conductor in the modeled range whose available ampacity meets the required ampacity. Required ampacity starts with the entered load and applies a 125% basis when continuous load is selected. Available ampacity reflects the terminal rating, ambient correction, current-carrying conductor adjustment, and supported small-conductor limits.

This is an ampacity result. It does not prove that voltage drop, overcurrent protection, equipment terminals, or every installation-specific rule is satisfied.

A practical guide

How preliminary wire sizing works

Conductor sizing starts with the load, but the table value is only the beginning. Installation temperature, conductor grouping, terminal limits, and load duration can reduce or increase the ampacity basis that controls the final selection.

Available ampacityThe lowest applicable terminal, adjusted insulation, and small-conductor limit

The controlling limit, not the highest table value, governs the modeled result.

Continuous load changes the basis

A load expected to run at maximum current for three hours or more can require a 125% sizing basis under applicable rules. A 40 A continuous load therefore creates a 50 A required-ampacity input in this calculator.

Terminals can cap ampacity

Even when conductor insulation carries a 90°C rating, equipment termination provisions can require use of a lower ampacity column. The 90°C value may support correction and adjustment calculations without becoming the final terminal ampacity.

Heat and grouping matter

Higher ambient temperature reduces the conductor's ability to shed heat. More than three current-carrying conductors in a raceway or cable can also require adjustment. Both effects can move the recommendation to a larger size.

A useful sizing sequence

  1. Establish the design load. Complete the load calculation for the equipment and circuit type.
  2. Apply the continuous-load basis. Separate continuous and noncontinuous portions when the installation calls for it.
  3. Choose the temperature basis. Confirm conductor insulation and equipment terminal ratings.
  4. Apply correction and adjustment. Account for ambient temperature and current-carrying conductors.
  5. Check the rest of the system. Review overcurrent protection, voltage drop, raceway fill, equipment instructions, and applicable special rules.

Modeled scope and source

The ampacity data comes from NEC 2023 Table 310.16. Ambient correction and current-carrying conductor adjustment use Table 310.15(B)(1) and Table 310.15(C)(1). Supported small-conductor limits are included. Review the official NFPA 70, 2023 edition (opens in a new tab) and the rules that apply to the installation.

  • Supported conductors are copper and aluminum, 14 AWG through 600 kcmil, on a THHN/THWN-2 correction basis.
  • Parallel conductors, dwelling service allowances, motor circuits, transformers, welders, nonlinear loads, rooftop temperature adders, and CEC behavior are not modeled here.
  • Aluminum sizes without a supported table value are skipped rather than estimated.

Sizing is one part of the route

Keep conductor choices connected to length, raceway, and material.

Draw the route once, then review preliminary ampacity, voltage drop, conduit fill, and material quantities in the same planning workflow.

Open ZapSketch and plan a home run