AWG wire size equation
The calculator checks each AWG size from small to large and selects the first conductor that passes both ampacity and voltage-drop limits.
Ireq = Iload × Fc / KbundleVd = 2 × Iload × Rft × LVd = √3 × Iload × Rft × LSelect first AWG where Iamp ≥ Ireq and Vd% ≤ Vlimitwhere:
- Ireq
- Required conductor ampacity after continuous-load and bundling adjustmentA
- Iload
- Load currentA
- Fc
- Continuous-load factor, 1.25 when selected
- Kbundle
- Bundled current-carrying conductor derating factor
- Vd
- Voltage dropV
- Rft
- AWG conductor resistance per foot after material adjustmentohm/ft
- L
- One-way conductor lengthft
- Iamp
- Reference ampacity for the AWG conductorA
- Vlimit
- Maximum allowed voltage drop%
The internal ampacity table is simplified for planning; final conductor sizing must follow the applicable NEC, IEC, BS, or local standard.
Assumptions
- Reference ampacity table is simplified for planning
- Bundling derating follows common 4-6, 7-9, and 10-20 conductor adjustment bands
- Voltage drop uses copper resistance with material factor
Important Warnings
- Final conductor sizing must follow the locally adopted NEC/IEC/BS standard, insulation rating, terminal temperature, ambient correction, and installation method.
- Small-signal conductors may have product-specific limits not captured by building-wire ampacity tables.
FAQ
Why can voltage drop force a larger wire than ampacity?
Long routes can pass current capacity but lose too much voltage before the load, especially on low-voltage circuits.
Why does continuous load change the result?
Continuous loads are commonly sized at 125%, so the conductor must have more ampacity headroom.