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# Transformer Secondary Wire Size Calculator Online

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.

### Formula

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

Where:

• 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.

### Example

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.