Wire Size Calculator
Find the correct wire gauge for any electrical circuit based on current load, run length, and allowable voltage drop. Use this when wiring outlets, appliances, or sub-panels to avoid overheating and energy loss.
About this calculator
Wire gauge determines how much current a conductor can carry safely over a given distance. As current flows through wire, resistance causes a voltage drop proportional to the wire's length and inversely proportional to its cross-sectional area. The formula used here calculates the minimum AWG gauge number: AWG = ceil(log(2 × 12.9 × I × d / (V × Vdrop%/100)) / log(1.26)), where I is current in amps, d is one-way distance in feet, V is system voltage, and Vdrop% is the maximum allowable voltage drop percentage. The constant 12.9 represents the resistivity of copper in circular-mil-ohms per foot. The factor 2 accounts for the round-trip wire path. Lower AWG numbers mean thicker wire, which reduces resistance and voltage drop over long runs.
How to use
Suppose you are wiring a 20-amp circuit to a garage 50 feet away on a 120 V system, allowing a 3% voltage drop. Plug in: I = 20 A, d = 50 ft, V = 120 V, Vdrop = 3%. Compute the argument: 2 × 12.9 × 20 × 50 / (120 × 0.03) = 25,800 / 3.6 = 7,166.7. Take log(7166.7)/log(1.26) ≈ 8.875/0.2311 ≈ 38.4, then ceiling to 39 — but practical AWG tables map this to #10 AWG (the nearest standard gauge). Always verify against local electrical code tables.
Frequently asked questions
What wire gauge should I use for a 30-amp, 240-volt circuit?
For a 30-amp, 240-volt circuit such as a dryer or HVAC unit, #10 AWG copper wire is the standard minimum for short runs. If the circuit run exceeds 50–75 feet, voltage drop becomes significant and you may need to step up to #8 AWG. Always calculate the actual voltage drop using your specific one-way distance to confirm the wire size before installation.
How does one-way distance affect wire gauge selection?
Longer wire runs increase total circuit resistance, which causes more voltage to be lost before it reaches the load. The NEC and most electrical codes recommend keeping voltage drop at or below 3% for branch circuits. Doubling the distance roughly doubles the voltage drop, which can require jumping up one or two wire gauges to compensate. That is why a 100-foot run to an outbuilding needs heavier wire than an identical 20-foot run inside the home.
Why does voltage drop matter when choosing wire size?
Excessive voltage drop means the appliance or motor at the end of the circuit receives less voltage than it was designed for. Motors running under-voltage draw higher current, generate extra heat, and wear out prematurely. Lighting may appear dim, and sensitive electronics can malfunction. Staying within a 3% drop on branch circuits (5% total including feeder) protects both equipment and safety, and is required by NEC Article 210.