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To determine how many #10 conductors can be legally installed in a 4-inch square box that is 2 1/8 inches deep, one must consider the volume allowance per conductor as specified in the National Electrical Code (NEC).

For conductors sized #10 AWG, each conductor requires a specific volume within the box. According to NEC Article 314, a #10 wire occupies 2.0 cubic inches of space.

Calculating the total volume of the box is essential. A standard 4-inch square box typically has a volume of 21 cubic inches. However, adjustments must be made for internal equipment such as clamps and devices which will occupy some of this volume. In a standard box, a portion of the volume is allocated for these items.

Once you account for these allowances, you can calculate how many #10 conductors can fit:

1. Start with the adjusted volume of the box after deducting for internal components.

2. Divide the usable volume by the volume required per #10 conductor.

Once calculations are performed correctly, the maximum number of #10 conductors that can be legally and safely installed in the box is determined to be 12. Therefore, the answer indicating 12 conductors