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.

### Table for General Terms

Term | Definition |
---|---|

CM | Circular Mils – A unit of measure for the cross-sectional area of a wire. |

Ampere | The unit of electric current. |

Voltage Drop | The reduction in voltage in the wire as electricity flows through it. |

### 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.