About the Wire Calculator
Choosing the right wire size for an electrical circuit means passing two tests. First, the wire must carry the current safely without overheating, which is its ampacity. Second, on longer runs, it must not waste too much voltage along the way, which is the voltage drop. A wire that is fine for a short run to a nearby outlet can be too small for the same current at the end of a long run to a shed, well pump or electric vehicle charger.
This wire calculator checks both. Enter the load current, the one-way length of the run, the voltage and the conductor material, and it finds the smallest American Wire Gauge size that carries the current and keeps the voltage drop within your limit. It shows which test decided the size, the actual voltage drop, and the circular mils required.
How to Use the Wire Calculator
Enter the load current in amps.
Enter the one-way run length from the panel to the load, in feet.
Choose the voltage and conductor material, and set the maximum voltage drop.
The Formulas
allowed drop = voltage × max drop %
circular mils = 2 × K × amps × length ÷ allowed drop
K = 12.9 for copper, 21.2 for aluminum
ampacity size = smallest wire whose 75 °C rating ≥ amps
drop size = smallest wire with at least the circular mils
wire size = the larger of the twoStep-by-Step Example
20 amps at 120 volts over a 100 ft run, copper, 3% maximum drop.
allowed drop 120 × 3% = 3.6 V
circular mils 2 × 12.9 × 20 × 100 ÷ 3.6 = 14,333
ampacity 12 AWG copper carries 20 A
voltage drop 8 AWG (16,510 cmil) is the smallest that meets 14,333
result 8 AWG copper, dropping 3.13 V (2.6%)Over 25 feet, the same circuit needs only 3,583 circular mils, so 12 AWG, sized by ampacity, is enough.
Ampacity
Ampacity is the current a wire can carry continuously without its insulation getting too hot. The calculator uses the 75 °C column of NEC Table 310.16 for copper and aluminum conductors, with the small conductor limits that cap 14 AWG copper at 15 amps, 12 AWG at 20 amps and 10 AWG at 30 amps. Wires bundled together, run through hot attics, or with 60 °C terminations may need to be larger. The breaker protecting the circuit must also match the wire.
Voltage Drop
All wire has resistance, so current flowing through it loses some voltage as heat. The longer the run and the higher the current, the bigger the loss. The formula counts the length twice because current flows out on one wire and back on the other. The National Electrical Code recommends a maximum of 3% on a branch circuit and 5% total including the feeder. Excessive voltage drop makes lights dim, motors run hot and struggle to start, and electronics misbehave.
Copper and Aluminum
Copper conducts better and is the usual choice for branch circuits in homes. Aluminum is lighter and cheaper, and modern alloy aluminum cable is widely used for service entrances and feeders to subpanels, in larger sizes. Because it has higher resistance, aluminum needs a larger size than copper for the same current and run, typically about two gauge sizes larger. Aluminum must be used with connectors and devices rated for it, and terminations torqued to the maker's specification.
Circular Mils and AWG
Wire cross-sections are measured in circular mils, the square of the diameter in thousandths of an inch. American Wire Gauge numbers run backwards: smaller numbers are thicker wires. Each step of three gauge sizes roughly doubles the area. Above 1 AWG the sizes continue as 1/0, 2/0, 3/0 and 4/0. Larger conductors are sized in thousands of circular mils (kcmil), beyond the range of this calculator.
Safety
Electrical work can cause fires and fatal shocks. This calculator is a planning tool. The legal wire size depends on the code adopted where you live, the insulation type, the breaker, the installation method, conduit fill and temperature corrections. Permits and inspections are usually required. Have a licensed electrician design or check any new circuit.
Common Circuits
For typical household circuits the result follows familiar patterns. A 15-amp lighting circuit uses 14 AWG copper and a 20-amp kitchen or bathroom circuit 12 AWG, when runs are short. A 30-amp dryer circuit uses 10 AWG, a 40 to 50-amp range or EV charger 8 or 6 AWG. Detached garages, workshops and outbuildings often sit far from the panel, which is where voltage drop pushes the wire up one or two sizes. Running a feeder at 240 volts instead of 120 halves the current for the same power, which cuts the percentage drop to about a quarter.
Upsizing the Ground and Neutral
When wire is upsized for voltage drop, the code generally requires the equipment grounding conductor to be increased in proportion. The neutral is normally the same size as the hot conductors on branch circuits.
Understanding Your Result
The headline gives the wire size.
The why line shows the sizes needed for current and for voltage drop.
The voltage drop line gives the drop in volts and percent.
The circular mils line shows the area needed and the area of the chosen wire.
When Should You Use This Calculator?
Use it when planning circuits to garages, sheds, pumps or chargers.
Use it to see whether a long run needs a larger wire.
Use it to compare copper and aluminum for a feeder.
Common Mistakes
Sizing by current only. Long runs need bigger wire for voltage drop.
Using round-trip length. Enter the one-way distance.
Mixing aluminum with copper-only devices. Use rated connectors.
Skipping the permit. New circuits usually need inspection.