π Wire Gauge Calculator
Correct AWG size by ampacity and voltage drop.
Recommended Copper Wire Size
For planning only. Consult a licensed electrician and local electrical code (NEC).
How to Use This Calculator
Enter the circuit current in amps, the one-way cable length in feet (not the round trip), the circuit voltage, and your allowable voltage drop percentage. The calculator finds the smallest copper AWG size that satisfies both the NEC ampacity table and the voltage drop limit, and shows which of the two requirements drove the answer.
Enter the circuit amps. For continuous loads (running 3+ hours), size for 125 percent of the load per NEC: a 16 amp continuous load needs a 20 amp circuit calculation.
Enter the one-way length from panel to load in feet. Measure the actual cable route, including up, over, and around, not the straight-line distance.
Pick 120V or 240V and set the allowable drop, defaulting to 3 percent per NEC recommendation for branch circuits.
Read the recommended AWG and the limiting factor. If voltage drop drove the size, a shorter route or higher voltage would let you use smaller wire.
The Formulas
Circular mils are the standard unit of wire cross-section in the US: one circular mil is the area of a circle 0.001 inch in diameter. The factor of 2 covers both conductors of the round trip. K = 12.9 is the resistivity constant for copper at operating temperature.
The AWG scale is counterintuitive: smaller numbers mean bigger wire. Going from 14 AWG to 12 AWG roughly doubles nothing but adds about 59 percent more copper area, and each 3-gauge step doubles the area. That is why long runs jump sizes faster than people expect.
Worked Example
A 20 amp, 120 volt circuit with a 50-foot one-way run and 3 percent allowable drop. Required CM = (2 x 12.9 x 20 x 50) / (120 x 0.03) = 25,800 / 3.6 = 7,167 circular mils. By ampacity, 12 AWG (20A) would do, and by area, 12 AWG has 6,530 CM, which is short of 7,167. So 10 AWG (10,380 CM, 30A) is the answer, driven by voltage drop. Actual drop with 10 AWG = (2 x 12.9 x 20 x 50) / (10380 x 120) x 100 = 2.07 percent, comfortably under 3 percent.
Tips
When in doubt, go one size larger: the extra copper cost is small compared to a rewire, and larger wire runs cooler and leaves headroom for future loads. Derate ampacity for hot attics (over 86 F ambient) and for more than 3 current-carrying conductors in one conduit, per NEC 310. Use the 60 C column ampacity for older panels and terminations unless the equipment is listed for 75 C. Never use this calculator as a substitute for a permit and inspection.
For planning only. Consult a licensed electrician and local electrical code (NEC). Incorrect wiring can cause fire, shock, and death.
Frequently Asked Questions
How do you calculate wire size for voltage drop?
The formula is: required circular mils = (2 x K x amps x one-way length) / (voltage x allowable drop as a decimal), where K is 12.9 for copper. The factor of 2 accounts for the round trip, since current travels out on one conductor and back on the other. You then pick the smallest AWG size whose circular mil area meets or exceeds that number.
What is the difference between ampacity and voltage drop sizing?
Ampacity is the maximum current a wire can safely carry without overheating, set by the wire size and insulation. Voltage drop is the power lost as heat along a long run, which can starve motors and dim lights. A circuit must satisfy both: this calculator finds the smallest AWG that passes the ampacity table and the voltage drop calculation, because long runs often need a bigger wire than ampacity alone suggests.
What is an acceptable voltage drop for a circuit?
The NEC recommends keeping voltage drop under 3 percent for branch circuits and under 5 percent total from service to the farthest outlet. This calculator defaults to 3 percent. Long runs at 120 volts are the worst case, because the same wattage draws twice the current as 240 volts and voltage drop scales with current.
Can I use aluminum wire instead of copper?
Aluminum is allowed by the NEC for larger feeders and is cheaper, but it has higher resistance (K = 21.2 instead of 12.9), so you need a larger size for the same circuit, and terminations require anti-oxidant compound and AL-rated lugs. This calculator sizes for copper only. For small branch circuits, copper is the standard and safer choice.
What size wire do I need for a 30 amp circuit?
A 30 amp circuit needs at least 10 AWG copper by ampacity, but distance matters. At 100 feet one-way on 120 volts with a 3 percent drop limit, the voltage drop calculation calls for about 8 AWG. Enter your actual length in the calculator, because the ampacity answer alone can be wrong for long runs.