How to use the wire size calculator
- Enter the maximum current in amps the circuit will carry.
- Set the one-way distance in feet from the power source to the load.
- Enter the system voltage (for example 120 V or 240 V).
- Choose the allowable voltage drop percentage and conductor material.
- Read the recommended minimum AWG wire gauge and supporting values below.
How the wire gauge is calculated
The wire size calculator uses the DC resistance formula rearranged to solve for the required cross-sectional area of the conductor. For a single-phase circuit the round-trip length is twice the one-way distance:
A = (2 x K x I x D) / V_drop
Where A is the area in circular mils, K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75 degrees C), I is current in amps, D is one-way distance in feet and V_drop is the maximum allowed voltage drop in volts.
For example, a 20 A load at 50 feet on a 120 V circuit with a 3 percent drop allows 3.6 V of loss. The required area is (2 x 12.9 x 20 x 50) / 3.6 = 7,167 circular mils. Looking up the standard AWG table, 12 AWG provides 6,530 circular mils which falls short, so the calculator selects 10 AWG (10,380 circular mils) as the minimum safe size.
Standard AWG wire sizes reference
The American Wire Gauge system assigns smaller numbers to thicker wires. Below is a reference table of common gauges used in building wiring, showing their approximate area in circular mils and typical maximum ampacity for copper conductors with 60 degree C insulation per NEC Table 310.16.
| AWG Size | Area (circular mils) | Copper Ampacity (60 C) |
|---|---|---|
| 14 | 4,107 | 15 A |
| 12 | 6,530 | 20 A |
| 10 | 10,380 | 30 A |
| 8 | 16,510 | 40 A |
| 6 | 26,240 | 55 A |
| 4 | 41,740 | 70 A |
| 2 | 66,360 | 95 A |
| 1/0 | 105,600 | 125 A |
| 2/0 | 133,100 | 145 A |
| 4/0 | 211,600 | 195 A |
Always verify the final wire gauge against the applicable electrical code for your jurisdiction, as ambient temperature, conduit fill and insulation type all affect safe ampacity.
Copper versus aluminum conductors
Copper has lower electrical resistance than aluminum, so a copper wire can carry more current at a given gauge. The resistivity constant K for copper is approximately 12.9 ohms per circular mil-foot at 75 degrees C, while for aluminum it is about 21.2. This means an aluminum conductor needs roughly 1.6 times the cross-sectional area of a copper conductor to achieve the same voltage drop over the same distance.
Aluminum wire is lighter and less expensive, making it common for service entrance cables and long feeder runs where weight and cost matter. However, aluminum connections require special terminals rated for aluminum and often an antioxidant compound to prevent oxidation. When this wire size calculator recommends an aluminum gauge, it accounts for the higher resistivity automatically.
Why voltage drop matters
Voltage drop is the reduction in voltage between the source and the load caused by the resistance of the wire. Excessive voltage drop causes motors to run hot, lights to dim and electronic equipment to malfunction or shut down. The National Electrical Code recommends that branch circuit voltage drop not exceed 3 percent, and that the combined feeder and branch drop stay under 5 percent for reasonable efficiency.
Lower-voltage systems are far more sensitive to wire resistance. A 12 V lighting circuit carrying 10 A over 30 feet loses a much larger percentage of its supply voltage than a 240 V circuit carrying the same current over the same distance. That is why low-voltage installations such as landscape lighting or automotive wiring often require surprisingly thick cable. Use the wire size calculator to check these scenarios before purchasing materials.
Common mistakes when sizing wire
One frequent error is entering the total round-trip length instead of the one-way distance. The formula already multiplies by two for single-phase circuits, so doubling the input produces an oversized recommendation. Another mistake is ignoring the starting surge current of motors. An induction motor may draw five to eight times its running current for a few seconds, and if the wire is sized only for the steady-state load the momentary voltage drop can stall the motor.
People also sometimes confuse three-phase and single-phase calculations. For balanced three-phase circuits the multiplier changes from 2 to approximately 1.732 (the square root of 3), which reduces the required wire area slightly. This tool uses the single-phase formula, which gives a conservative result for most residential and light commercial applications. For critical industrial work, consult a licensed electrician and the latest edition of the NEC or your local equivalent.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| 20 A branch circuit, 50 ft, 120 V | Current (amps): 20; One-way distance (feet): 50; System voltage (volts): 120; Max voltage drop (%): 3; Conductor material: cu | Recommended AWG: 10 AWG; Actual voltage drop: 2.49 V; Required area: 7,167 CM; Selected area: 10,380 CM; Material: Copper |
| 30 A RV outlet, 75 ft, 120 V | Current (amps): 30; One-way distance (feet): 75; System voltage (volts): 120; Max voltage drop (%): 3; Conductor material: cu | Recommended AWG: 8 AWG; Actual voltage drop: 3.52 V; Required area: 16,125 CM; Selected area: 16,510 CM; Material: Copper |
| 60 A subpanel, 100 ft, 240 V | Current (amps): 60; One-way distance (feet): 100; System voltage (volts): 240; Max voltage drop (%): 3; Conductor material: cu | Recommended AWG: 6 AWG; Actual voltage drop: 5.90 V; Required area: 21,500 CM; Selected area: 26,240 CM; Material: Copper |
| 100 A service, 150 ft, 240 V aluminum | Current (amps): 100; One-way distance (feet): 150; System voltage (volts): 240; Max voltage drop (%): 3; Conductor material: al | Recommended AWG: 1/0 AWG; Actual voltage drop: 6.02 V; Required area: 88,333 CM; Selected area: 105,600 CM; Material: Aluminum |
FAQ
What does AWG stand for?
AWG stands for American Wire Gauge. It is a standardized system used primarily in North America to specify the diameter of round, solid, nonferrous, electrically conducting wire. Lower AWG numbers indicate thicker wire with greater current-carrying capacity.
Does this wire size calculator account for temperature?
The calculator uses resistivity constants at 75 degrees Celsius (167 degrees Fahrenheit), which matches typical operating conditions for THHN and similar building wire. Higher ambient temperatures increase resistance further. For runs in very hot environments, consider selecting one gauge size larger than the result shown.
Should I enter the one-way or round-trip distance?
Enter the one-way distance from the power source to the load. The formula automatically doubles this value to account for the return path in a single-phase circuit.
What voltage drop percentage should I use?
The NEC recommends a maximum of 3 percent for branch circuits and 5 percent for the total of feeder and branch combined. Use 3 percent for most general-purpose calculations to ensure good performance.
Can I use this for three-phase power?
This tool applies the single-phase formula, which multiplies the one-way distance by 2. For three-phase circuits the multiplier is the square root of 3 (about 1.732), so the single-phase result is slightly conservative and still safe to follow.
Is the recommended wire size legally sufficient?
This wire size calculator provides guidance based on voltage drop alone. Local electrical codes also set minimum ampacity requirements, derating factors for conduit fill and ambient temperature rules. Always verify the final selection against the NEC or your local authority having jurisdiction.