Wire Gauge Calculator
Enter the circuit amperage, wire run distance, system voltage, and allowable voltage drop to determine the correct wire gauge. Uses the NEC circular mil method for copper conductors.
What is Wire Gauge?
A wire gauge calculator determines the minimum conductor size (AWG — American Wire Gauge) needed for an electrical circuit based on current, distance, voltage, and the maximum allowable voltage drop. Selecting the correct wire gauge is critical for both safety and performance: undersized wire overheats and creates fire hazards, while excessive voltage drop causes equipment malfunction, dimming lights, and wasted energy. The National Electrical Code (NEC) sets ampacity limits for each wire gauge — the maximum current a conductor can carry safely based on its insulation type and installation conditions. However, ampacity alone does not account for voltage drop over long wire runs. The NEC recommends a maximum voltage drop of 3 percent for branch circuits and 5 percent total for the combination of feeder and branch circuit. Many electricians design to 3 percent as standard practice. This calculator uses the circular mil method, which is the industry-standard approach for voltage drop calculations. The formula calculates the minimum circular mil area needed: CM = (2 x K x I x D) / VD, where K is the resistivity constant (12.9 for copper at 75 degrees Celsius), I is the current in amps, D is the one-way distance in feet, and VD is the allowable voltage drop in volts. The factor of 2 accounts for the round-trip distance (hot and neutral conductors). Standard copper wire sizes and their circular mil areas are: 14 AWG = 4,110 CM, 12 AWG = 6,530 CM, 10 AWG = 10,380 CM, 8 AWG = 16,510 CM, 6 AWG = 26,240 CM, 4 AWG = 41,740 CM, 2 AWG = 66,360 CM, and 1/0 AWG = 105,600 CM. The calculator selects the smallest wire gauge whose circular mil area meets or exceeds the calculated requirement and whose ampacity rating meets or exceeds the load. For 240-volt circuits, the higher voltage allows longer runs at the same wire gauge because the percentage voltage drop is calculated against the higher base voltage. This is why 240-volt appliance circuits and subfeeder runs can use smaller wire for equivalent distances compared to 120-volt circuits.
How to Calculate
- Determine the circuit amperage (breaker size or load calculation)
- Measure the one-way distance from the electrical panel to the device or outlet in feet
- Enter the system voltage (120V for standard outlets, 240V for appliances, 208V or 480V for commercial)
- Set the maximum voltage drop percentage (3% is the NEC recommendation for branch circuits)
- The calculator recommends the minimum wire gauge and shows the actual voltage drop
- Always verify with local electrical code and a licensed electrician
Formula
Required Circular Mils (CM) = (2 x K x I x D) / VD Where: - K = 12.9 (copper resistivity constant at 75°C) - I = current in amps - D = one-way distance in feet - VD = allowable voltage drop in volts = Voltage x (Max Drop % / 100) The calculator selects the smallest standard AWG wire whose circular mil area is greater than or equal to the required CM and whose ampacity meets the load. Actual Voltage Drop = (2 x K x I x D) / selected wire CM Wire Resistance = (2 x K x D) / selected wire CM
Example Calculation
A 20-amp circuit at 120V, 50 ft one-way distance, 3% max drop: Allowable Drop = 120 x 0.03 = 3.6 volts Required CM = (2 x 12.9 x 20 x 50) / 3.6 = 7,167 CM Smallest qualifying gauge: 10 AWG (10,380 CM, rated 30A) Note: 12 AWG (6,530 CM) is too small since 6,530 < 7,167 Actual Drop = (2 x 12.9 x 20 x 50) / 10,380 = 2.49 volts Voltage Drop % = 2.49 / 120 x 100 = 2.07% Wire Resistance = (2 x 12.9 x 50) / 10,380 = 0.1243 ohms Use 10 AWG copper wire for this circuit.
Frequently Asked Questions
What is the maximum allowable voltage drop per the NEC?
The NEC recommends (not mandates) a maximum of 3% voltage drop for branch circuits and 5% total for the combination of branch circuit and feeder. While these are recommendations rather than hard requirements in most jurisdictions, exceeding 5% total drop causes noticeable performance issues with motors, lighting, and sensitive electronics.
Does this calculator work for aluminum wire?
No. This calculator uses K=12.9, the resistivity constant for copper at 75 degrees Celsius. For aluminum wire, use K=21.2. Aluminum requires larger gauge wire for the same amperage and distance. Aluminum wire also has different ampacity ratings and requires special connectors rated for aluminum.
Why does the recommended gauge sometimes jump two sizes?
The calculator must satisfy two constraints: sufficient circular mil area for voltage drop AND adequate ampacity for the load current. Sometimes a wire meets the voltage drop requirement but not the ampacity requirement, causing the calculator to recommend the next size up. For example, 14 AWG is rated for only 15 amps, so a 20-amp circuit requires at least 12 AWG regardless of distance.
Should I use the breaker size or actual load for amperage?
Use the circuit breaker rating for wire sizing, not the actual device load. NEC requires conductors to be sized for the overcurrent protection device (breaker) on the circuit. A 20-amp breaker requires wire rated for at least 20 amps even if the connected device only draws 12 amps.
How does voltage affect wire size?
Higher voltage allows smaller wire for the same wattage load and distance. A 240V circuit carries the same power as a 120V circuit at half the amperage, which halves the voltage drop in absolute terms. This is why 240V is used for high-power appliances and long subfeeder runs — it permits smaller, less expensive conductors.