Size Electrical Wire Calculator
Wire Size & Gauge Calculator works out Recommended conductor and Recommended conductor area from What to calculate, Load current and Continuous load (3 hours or more). The default figures on this page give recommended conductor of 8 AWG; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Required ampacity = 40 A.
- Smallest copper conductor whose corrected ampacity reaches 40 A: 8 AWG (55 A at 90 degC x 1.0000 ambient x 1 bundling = 55.00 A, held to 50.00 A by the terminations).
- Voltage drop at 8 AWG over 100 ft: 2.604 % against a 3 % allowance.
- Ampacity governs; the voltage-drop allowance is already satisfied at this size.
- Equipment grounding conductor for a 40 A device (NEC Table 250.122): 10 AWG.
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the Wire Size & Gauge Calculator calculates
Sizes copper or aluminium conductors from load current, run length and voltage-drop allowance, derates ampacity for ambient temperature and bundling, looks up equipment grounding conductors, and converts sheet-metal gauge numbers to thickness.
How to use this Wire Size & Gauge Calculator
Enter What to calculate, Load current, Continuous load (3 hours or more) and Conductor material. Recommended conductor updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Recommended conductor, Recommended conductor area and Required ampacity.
The method follows NFPA 70, National Electrical Code — Table 310.16, Allowable Ampacities of Insulated Conductors, NFPA 70 — 310.15(B)(2) ambient correction, Table 310.15(C)(1) adjustment, 110.14(C) termination ratings, 240.4(D) small-conductor limits, 210.19(A)/215.2(A)(2) voltage-drop informational notes, NFPA 70 — Table 250.122, Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment, ASTM B258, Standard Specification for Nominal Diameters and Cross-Sectional Areas of AWG Sizes and Manufacturers' Standard Gauge for Sheet Steel.
100 A feeder, copper, 100 ft at 240 V example
With What to calculate: wire-size, Load current: 100, Continuous load (3 hours or more): false, Conductor material: copper, the calculator returns Required ampacity: 100, Recommended conductor: 3 AWG, Voltage drop at the recommended size: 2.04294945. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Recommended conductor, Recommended conductor area and Required ampacity. The inputs you can vary are What to calculate, Load current, Continuous load (3 hours or more), Conductor material, Conductor size and Conductor insulation rating.
Understanding Recommended conductor, Recommended conductor area and Required ampacity
Advisory only. Conductor selection depends on the NFPA 70 edition adopted by your authority having jurisdiction, the wiring method, termination listings, conduit fill, fault duty and load classification. A licensed electrical professional must approve any installed design.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
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Frequently asked questions
What information do I need for the Wire Size & Gauge Calculator?
You need What to calculate, Load current, Continuous load (3 hours or more), Conductor material, Conductor size, Conductor insulation rating, Termination temperature rating and Ambient temperature. The page opens with default figures that give recommended conductor of 8 AWG, so you can see the scale before entering your own.
What does this Wire Size & Gauge Calculator show?
It shows Recommended conductor, Recommended conductor area, Required ampacity, Table 310.16 ampacity, Ambient correction factor, Conductor-count adjustment factor, Corrected ampacity and Usable ampacity after termination limits, with the working and any warnings for your entries.
Can I check how the Wire Size & Gauge Calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.