QuestionSeptember 18, 2025

You need to install a pull box in a run of conduit.The conduit contains twenty 18 AWG wires . What is the minimum size 4'' square box you could use? Use the following table to determine the MINIMUM size 4'' square box required.

You need to install a pull box in a run of conduit.The conduit contains twenty 18 AWG wires . What is the minimum size 4'' square box you could use? Use the following table to determine the MINIMUM size 4'' square box required.
You need to install a pull box in a run of conduit.The conduit contains twenty 18 AWG wires . What is the minimum size 4'' square box you could use? Use the following table to determine the MINIMUM size 4'' square box required.

Solution
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Answer

4'' \times 4'' \times 3\frac{1}{2}'' (54 in³) box Explanation 1. Find box fill allowance for 18 AWG wire NEC Table 314.16(B) gives 2.0\ \text{in}^3 per 18 AWG conductor. 2. Calculate total volume required 20 \times 2.0\ \text{in}^3 = 40\ \text{in}^3 3. Check standard 4'' square box sizes Common 4'' square boxes have depths of 1\frac{1}{2}'', 2\frac{1}{8}'', 2\frac{1}{2}'', and 3\frac{1}{2}''. Their volumes are: - 1\frac{1}{2}'': 21\ \text{in}^3 - 2\frac{1}{8}'': 30.3\ \text{in}^3 - 2\frac{1}{2}'': 35\ \text{in}^3 - 3\frac{1}{2}'': 54\ \text{in}^3 4. Select the minimum size meeting requirement Only the 3\frac{1}{2}'' deep box (54\ \text{in}^3) meets or exceeds 40\ \text{in}^3.

Explanation

1. Find box fill allowance for 18 AWG wire<br /> NEC Table 314.16(B) gives $2.0\ \text{in}^3$ per 18 AWG conductor.<br />2. Calculate total volume required<br /> $20 \times 2.0\ \text{in}^3 = 40\ \text{in}^3$<br />3. Check standard $4''$ square box sizes<br /> Common $4''$ square boxes have depths of $1\frac{1}{2}''$, $2\frac{1}{8}''$, $2\frac{1}{2}''$, and $3\frac{1}{2}''$. Their volumes are:<br />- $1\frac{1}{2}''$: $21\ \text{in}^3$<br />- $2\frac{1}{8}''$: $30.3\ \text{in}^3$<br />- $2\frac{1}{2}''$: $35\ \text{in}^3$<br />- $3\frac{1}{2}''$: $54\ \text{in}^3$<br />4. Select the minimum size meeting requirement<br /> Only the $3\frac{1}{2}''$ deep box ($54\ \text{in}^3$) meets or exceeds $40\ \text{in}^3$.
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