Voltage drop, ampacity, and why a circuit needs to pass both
Two completely separate limits decide how big a conductor has to be, and the larger of the two wins. The first is ampacity: a wire heats up as current flows through it, and every gauge has a maximum current it can carry without cooking its own insulation. That is a safety limit, it is set by code tables, and it is not negotiable. The second is voltage drop: resistance in a long run eats supply voltage before it reaches the load. That is a performance limit — motors run hot and start badly on low voltage, heaters put out less heat, LED drivers buzz or flicker, and low-voltage DC runs suffer worst of all because a couple of volts lost on a 12 V system is a huge fraction of the supply.
The standard voltage-drop formula for a single-phase or DC run is VD = 2 × K × I × L ÷ CM, where I is the current in amps, L is the one-way length in feet, CM is the conductor's cross-sectional area in circular mils, and K is the resistivity constant — approximately 12.9 for copper and 21.2 for aluminium. The 2 accounts for current travelling out and back along two conductors. Rearranged, the circular mils you need is 2 × K × I × L ÷ the volts you are willing to lose, and you then step up to the next real gauge above that number. Aluminium's higher K is why an aluminium conductor generally has to be roughly two gauge sizes larger than copper for the same job.
The common design target is 3% drop on a branch circuit and no more than 5% total from the service to the furthest outlet. Those are recommendations for good performance rather than hard prohibitions in most cases, but treating 3% as your ceiling is the sensible habit. The minimum size shown by this calculator is the larger of the voltage-drop requirement and a basic ampacity check, and ampacity here uses the 75°C column before any adjustment. Real installations often need a bigger conductor still: high ambient temperature, several current-carrying conductors bundled in one raceway, continuous loads that must be sized at 125%, and conduit fill all push the required size up.
Estimate for planning and shopping only — not a design document. Conductor sizing, overcurrent protection, insulation type, temperature and bundling adjustments, grounding and termination ratings must all follow the NEC (or CEC) and your local amendments. Electrical work must be performed or verified by a licensed electrician and inspected where required. Do not energise a circuit sized from this page without professional review.