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Transformer Secondary Wire Size Calculator Online

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This calculator assists in determining the appropriate wire size needed for the secondary side of a transformer based on the electrical load and distance. Correct wire sizing is crucial to prevent overheating, voltage drop, and potential failures in electrical systems.


To calculate the secondary wire size for a transformer, the following formula is used:

Transformer Secondary Wire Size


  • A is the cross-sectional area of the wire in circular mils (CM).
  • I is the current in amperes.
  • L is the length of the wire in feet.
  • K is a constant (12 for copper, 19 for aluminum).
  • E is the voltage drop per 1000 feet (typically set to 3 percent of the operating voltage).
  • D is the permissible voltage drop in volts.

To find the current (I), use the formula:

I = P / V

Where P is the power in watts and V is the secondary voltage in volts.

Measure the length of the wire run (L) from the transformer to the load. Use the constant K based on the wire material (12 for copper, 19 for aluminum). Calculate the permissible voltage drop (E). For a 3 percent drop in a system with voltage V, E can be calculated as:

E = 0.03 * V

Plug these values into the first formula to find the cross-sectional area (A) in circular mils.

Table for General Terms

CMCircular Mils – A unit of measure for the cross-sectional area of a wire.
AmpereThe unit of electric current.
Voltage DropThe reduction in voltage in the wire as electricity flows through it.


Consider a scenario where you need to install a wire for a 480-watt system with a secondary voltage of 120 volts, over a distance of 50 feet using copper wires. Using the formulas provided:

I = 480 / 120 = 4 amperes

E = 0.03 * 120 = 3.6 volts

Plugging these values into the wire size formula:

A = (4 * 50) / (12 * 3.6 * 3.6) = approximately 115 circular mils

Most Common FAQs

Q1: What does “K” represent in the formula?

A1: “K” is a constant that represents the resistivity of the material; 12 for copper and 19 for aluminum.

Q2: How do I determine the ‘E’ value for my project?

A2: “E” is calculated as 3% of your system’s operating voltage, representing the acceptable voltage drop per 1000 feet.

Q3: Can I use this calculator for any transformer size?

A3: Yes, this calculator is versatile and can be used for a wide range of transformer sizes and applications.

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