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Wire Size Calculator
Find the right copper or aluminum wire gauge (AWG/kcmil) for your amps, run length and voltage, checking NEC ampacity and keeping voltage drop under 3%.
How to use it
- Enter the load in amps (for continuous loads such as EV chargers or heaters, choose "Yes" to size at 125%).
- Enter the one-way distance from the panel to the load.
- Pick the voltage and conductor material; 3% is the common voltage-drop target for branch circuits.
How it works
The calculator picks the smallest wire whose 75 °C ampacity (NEC Table 310.16) covers the load, applies the small-conductor limits of 240.4(D) (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A copper), then increases the size until voltage drop = 2 × length × amps × resistance per foot stays under your limit.
Many residential terminals and NM (Romex) cable are limited to the 60 °C column; check the cable type and equipment labels.
FAQ
What size wire for 30 amps?
10 AWG copper for a typical 30 A circuit; long runs may need 8 AWG to keep voltage drop under 3%.
Does distance change wire size?
Yes. Ampacity sets the minimum size; on long runs voltage drop usually forces a larger wire.
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