Wire Size & AWG Calculator
Engineering Specification Sheet
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Generated: August 22, 2026
Wire Size & AWG Calculator
Determine the correct wire size and AWG for your electrical project based on NEC Table 310.16 ampacity ratings and rules.
Design Parameters
Per NEC 110.14(C), ampacity is capped at the breaker/lug rating even if the wire itself is rated hotter. Most circuits ≤100A use 60°C or 75°C terminations — check equipment labeling.
Wire Size Results
50.0% of capacity utilized
Derating Breakdown
Calculations based on NEC 2023 guidelines. Results are for estimation purposes — verify against local code before installation.
Verified against NEC 2023 · Table 310.16, 310.15(B)(1), 310.15(C)(1)
How this works
Wire size comes down to how much current the wire has to carry safely, not how much you want it to carry. NEC Table 310.16 sets that ampacity by conductor size, material, and insulation temperature rating, and this calculator reads straight off that table for whatever combination you enter.
Two rules push the number around. A continuous load, anything running three hours or more, has to be sized at 125% under NEC's continuous-load rule, so a 16 amp continuous load needs a wire rated for at least 20 amps. And your terminals usually cap you at 60°C or 75°C under NEC 110.14(C) even when you're pulling 90°C wire, because the ampacity that matters is whatever the lowest-rated point in the circuit can handle.
The 90°C column still matters even when you can't use its full ampacity. It's the starting point before temperature correction and conductor-bundling derates come off it under NEC 310.15(B)(1) and 310.15(C)(1).
Worked example
A 20A load, 120V, 10 feet one-way, THHN copper into a 75°C-rated panel, 30°C ambient, 3 current-carrying conductors.
#14 has enough ampacity for 20A on paper, but its 15A OCPD cap rules it out; #12 is the smallest wire that actually works here.
Frequently Asked Questions
How do I choose the correct wire size for a circuit?↓
Wire size is primarily determined by the circuit's overcurrent protection device (breaker) rating and the load it will carry. You must also consider temperature ratings (60°C, 75°C, 90°C), continuous loads (125% rule), and voltage drop over long distances.
What is the 125% rule for continuous loads?↓
The NEC requires that the wire size and overcurrent device for a continuous load (a load running for 3 hours or more) must be sized at 125% of the continuous load, plus 100% of the non-continuous load.
Can I use a smaller wire if it has a 90°C insulation rating?↓
While 90°C wire (like THHN) has a higher ampacity, you generally must size the wire based on the lowest temperature rating of the connected equipment or terminals (often 75°C or 60°C) per NEC 110.14(C). The 90°C ampacity is mainly used as a starting point for derating.
Learn more
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NEC Conduit Fill Calculation: How to Read Chapter 9 Table 1
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How to Choose the Right Wire Size: NEC Table 310.16 Guide
A practical guide to selecting wire size using NEC Table 310.16 ampacity ratings, temperature corrections, and conductor bundling derating factors.
Understanding Voltage Drop: NEC Requirements & How to Calculate It
How to calculate voltage drop per NEC 210.19(A) and why keeping it under 3% matters for branch circuits, with formulas and a worked example.
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