How to use this wire size and voltage drop calculator
This calculator gives electricians and designers a fast planning check for branch circuits, feeders, machinery connections and control power runs. Enter the nominal supply voltage, anticipated load current, one-way conductor distance and conductor material. The result identifies a practical AWG size based on ampacity and a three-percent voltage-drop target, then reports voltage available at the load and the watts dissipated in the conductor.
Voltage drop matters because every conductor has resistance. Long runs, high currents and smaller conductors raise resistance loss. A motor may struggle to start, a contactor coil may chatter, or sensitive equipment may operate outside its rated voltage when a circuit is undersized.
Calculation method
The voltage-drop model is Vd = I × R × L × 2 for single phase, and Vd = I × R × L × √3 for a balanced three-phase run. R is published conductor resistance and L is one-way length. Percentage drop equals Vd divided by nominal voltage times 100. The calculator walks a hard-coded AWG reference table to find a conductor meeting ampacity and the three-percent planning guideline.
Frequently asked questions
Is 3% always required? No. It is a common design recommendation, not a universal rule. Does this calculate conduit fill? No. Conduit fill, bundling, ambient temperature, termination ratings and derating need separate checks. Can I use aluminum? Yes, but its lower conductivity normally requires a larger conductor.
Safety note
Verify all final conductor, overcurrent protection and grounding selections against the current NEC, IEC or local authority requirements, equipment nameplates, installation method and a qualified electrician or engineer.