The length of the pull box must be at least how many times the outer diameter of the largest shielded or lead-covered conductor entering the box?

The correct answer states that the length of the pull box must be at least 48 times the outer diameter of the largest shielded or lead-covered conductor entering the box. This guideline is established to ensure sufficient space within the pull box for the proper bending radius of the conductors and to facilitate their installation and maintenance.

When dealing with electrical installations, especially with larger or shielded conductors, it's critical to adhere to specific ratios for pull box dimensions. The requirement of 48 times the diameter helps prevent damage to the insulation of the conductors due to excessive bending and allows for easier manipulation during installation and future modifications.

Using this ratio not only helps maintain the integrity of the conductors but also adheres to safety standards set forth in the National Electrical Code (NEC), which governs the safe installation of electrical wiring and equipment. This standard ensures that conductors are not stressed beyond their capacity, which could lead to failures or hazardous situations.

In contrast, the other options suggest incorrect ratios that do not meet the installation requirements for pull boxes as specified in the relevant codes. Thus, the standardized approach of using 48 times the diameter is critical for ensuring safety and compliance in electrical installations.