*Question G5C05:
Why is the primary winding wire of a voltage step-up transformer usually a larger size than that of the secondary winding?*  

*Correct Answer is  B: To accommodate the higher current of the primary*  

### AI Response: (may contain errors)  

The correct answer to the question is **B: To accommodate the higher current of the primary**. 

### Explanation of the Correct Answer:
- In a step-up transformer, the primary winding is connected to the input voltage source and typically carries a higher current than the secondary winding. This is due to the principle of conservation of energy, where the power input to the transformer must equal the power output (ignoring losses). 
- The relationship between voltage, current, and power can be expressed as:
  
  $$P = V \times I$$

- For a step-up transformer, if the voltage increases in the secondary winding, the current must decrease proportionally. Therefore, the primary winding must be designed to handle the higher current, necessitating a larger wire size to prevent overheating and ensure safe operation.

### Explanation of Incorrect Answers:
- **A: To improve the coupling between the primary and secondary**  
  - Coupling is primarily affected by the design and placement of the windings, not the wire size. Larger wire does not inherently improve magnetic coupling.

- **C: To prevent parasitic oscillations due to resistive losses in the primary**  
  - Parasitic oscillations are more related to circuit design and component values rather than wire size. While larger wire can reduce resistive losses, it is not the primary reason for the size difference in transformer windings.

- **D: To ensure that the volume of the primary winding is equal to the volume of the secondary winding**  
  - The volume of the windings is not a design requirement. The wire size is determined by the current-carrying capacity, not by a need for equal volume.

### Summary:
- The primary winding of a step-up transformer is larger to handle the higher current, ensuring safe operation and efficiency. Understanding the relationship between voltage, current, and power is crucial for grasping transformer operation.