Wire Size Calculator
Estimate a minimum conductor size for a given amperage, wire material, and insulation temperature rating. For authoritative values, refer to NFPA 70 §310.16.
Recommended Wire Size
Wire Size (minimum)
12 AWG
Ampacity Rating
25A
Max Overcurrent Device
20A max
This conductor’s table ampacity is 25A, but NEC 240.4(D) limits overcurrent protection for 12 AWG copper to 20A.
Typical Use Case
Outlet circuits, lighting
How this calculator works
The recommendation above is a BuildSpec engineering reference, derived from the physical properties of building-wire conductors (cross-section, resistivity, and insulation temperature rating) for a single-phase 120/240 V circuit with up to three current-carrying conductors in a raceway at a 30 °C ambient. It is not a reproduction of the published code table.
Real-world projects regularly need adjustments that this tool does not apply automatically — temperature corrections, bundling (more than three current-carrying conductors), voltage-drop compensation for long runs, and termination-temperature limitations.
For the authoritative ampacity table, derating rules, and exceptions, refer to NFPA 70 (NEC) §310.16 at nfpa.org.
Code Reference
This BuildSpec ampacity reference is based on the conditions commonly described in NFPA 70 §310.16 (three current-carrying conductors, standard 30 °C ambient). For official values, temperature derating, adjustment factors, and exceptions, refer to the current NEC edition on nfpa.org.
Disclaimer: This calculator is for reference only. Always verify with your local building department.