Amp to Wire Size Calculator

Your details

The continuous load current the wire must carry, in amperes.
A
The NEC limits most 60 C and 75 C termination equipment to the 60 C or 75 C ampacity column even if a higher-rated cable is used. For general branch circuits, 75 C is the practical maximum.
One-way distance from source to load. The calculator automatically doubles this for single-phase/DC voltage-drop (there and back).
ft
NEC 210.19 recommends 3% for branch circuits and 5% total for feeder+branch. Sensitive electronics may need 1-2%.
%
Temperature of the air or environment surrounding the conductor (not inside the wire). NEC derating applies above or below 30 C.
C
Number of current-carrying conductors sharing the same conduit or cable. Neutral counts only if it carries unbalanced current.
NEC 210.20(A) requires conductors and OCPD to be rated at 125% of continuous loads.
Minimum wire size (ampacity)Voltage drop OK
12 AWG

Smallest gauge that meets NEC ampacity after all derating factors.

Recommended wire size10 AWG
Cross-section5.26 mm²
Derated ampacity25A
Actual voltage drop2.07%
Voltage drop (volts)2.49V
Design current25A
2.07 %
Good (<3%)<3Marginal (3-5%)3-5Excessive (>5%)5+

Use 10 AWG for this circuit.

  • Ampacity alone needs 12 AWG, but voltage drop over your run length requires the larger 10 AWG.
  • After temperature and conduit-fill derating, this gauge carries 25.0 A, covering your design current of 25.0 A.
  • Voltage drop on 10 AWG is 2.07% (2.49 V), within the NEC-recommended 3%.

Next stepAlways verify the final wire size with a licensed electrician and local code amendments. The over-current protection device (breaker or fuse) must match or be smaller than the wire ampacity.

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