This site offers many simple-to-use calculators and wire ampacity charts to aide you in properly sizing wire and conduit in compliance with the NEC. Visit the Calculators and Tables pages for a complete list of resources.

Wire Size Calculator

Enter the information below to calculate the appropriate wire size.

Wire Size -
Voltage at Max Drop -

Voltage - Enter the voltage at the source of the circuit. Single-phase voltages are usually 115V or 120V, while three-phase voltages are typically 208V, 230V or 480V.

Amperes - Enter the maximum current in amps that will flow through the circuit. For motors, it is recommended to multiply the nameplate FLA by 1.25 for wire sizing.

Phases - Select the number of phases in the circuit. This is typically single-phase or three-phase. For single-phase circuits, three wires are required. For three-phase circuits, four wires are required. One of these wires is a ground wire which can be sized down. To calculate ground wire size, use the Ground Wire Size Calculator.

Insulation - Select the thermal rating of the insulation on the wire.

Conductor - Choose the material used as a conductor in the wire. Common conductors are copper and aluminum.

Installation - Choose the installation method for the circuit. This is typically in a raceway (cable-tray or conduit), in a cable, burried in the Earth, or in open air.

Voltage Drop - Choose the maximum percentage of the source voltage drop. It is recommended not to exceed a voltage drop of 5%.

Distance - Enter the one-way length of the wires in the circuit in feet.

Note: It's recommended to check the ampacity of a wire after doing a voltage drop calculation. Always use the total length of the circuit for calculations. Consult with an engineer if your application requires more complex calculations.

Source: NFPA 70, National Electrical Code, Table 310.15(B)(16-17)

Conductor Sizing

The National Electrical Code provides requirements for sizing electrical wire to prevent overheating, fire and other dangerous conditions. Properly sizing wire for many different applications can become complex and overwhelming. Amperage is the measure of electrical current flowing through a circuit. The ampacity rating of a wire determines the amperage that a wire can safely handle. In order to properly size a wire for your application, the ampacity ratings for a wire must be understood. However, many different external factors such as ambient temperature and conductor insulation play a part in determining the ampacity of a wire.

Wire ampacity is calculated in such a way as to not exceed a certain temperature rise at a particular electrical load. The heating of a conductor can be directly attributed from its I2R losses in the circuit. The length of a conductor is directly proportional to its resistance. However, the cross-sectional area of a conductor can also be changed to alter the conductor's resistance. By increasing the cross-section of the conductor (or increasing the size of the wire), the resistance decreases, and the allowable ampacity increases. Good judgement should be used when sizing conductors because large conductors can become costly and difficult to install, while small conductors can cause potential danger. Use the calculator above to size wire for basic applications, or view some of the wire ampacity charts for wire ampacity values.

Voltage Drop

Voltage drop can become an issue for engineers and electricians when sizing wire for long conductor runs. Voltage drop in a circuit can occur by using a wire gauge that is too small, or the length of the condcutor is too long. For long conductor runs where voltage drop may be an issue, use the Voltage Drop Calculator to determine voltage drop and the Circuit Distance Calculator to determine maximum circuit length.

Electric Motors

There are many different types of electric motors ranging from single-phase to three-phase AC motors, low and high voltage DC motors, synchronous and asynchronous motors. When designing a feeder or branch circuit with one or more electric motors, there are several important things to account for. The inrush current of a motor can sometimes reach up to 7 times the full load amps of the motor. The motor wire size should be designed to handle this in rush current, as well as handle a continuous full-load current of the motor. There are also motor winding protection and thermal considerations to design for when designing motor feeder and branch circuits. View the Motor Wire Size Calculator or the Motor Wire Size Chart for information on sizing wire and circuit protection devices for motors.

This site has many wire size calculators and wire size charts to aide you in properly sizing wire according to code. Visit the Terms of Use and Privacy Policy for this site. Your feedback is greatly appreciated. Let us know how we can improve.