Pull Box Sizing Calculator

What This Calculator Does

This tool is a screening-level worksheet for estimating the minimum interior dimension of a pull box, junction box, or conduit body under NEC 314.28 — Pull and Junction Boxes and Conduit Bodies. It applies the two governing formulas (straight pull and angle/U pull) to the raceway sizes you enter and returns the larger, controlling value as a starting reference. It does not select a final box, does not check box fill under 314.16, and does not account for splices, conductor bending space, gutter space, manufacturer listing limitations, or your local Authority Having Jurisdiction’s (AHJ) interpretation — all of which must be verified separately before specifying a box.

Code Basis and Edition

The 8× and 6× multipliers used here come from NEC Article 314.28(A)(1) and (A)(2). As of mid-2026, NEC 2023 remains the edition adopted by most U.S. jurisdictions, while a smaller number of states (e.g., Massachusetts, Texas) have moved to NEC 2026. The numeric multipliers (8× for straight pulls, 6× plus row sum for angle/U pulls and splices) are unchanged between the 2023 and 2026 editions. Confirm which edition your local AHJ has adopted before finalizing any box selection, since section numbering, exceptions, and enforcement practices can still vary by jurisdiction.

Straight-Pull Formula — NEC 314.28(A)(1)

For a straight pull, where conductors enter one wall of the box and exit through the opposite wall without changing direction, the box length in the direction of the pull must be at least 8 times the trade size of the largest raceway entering that wall:

\(L_{straight} = 8 \times D_{largest}\)

Worked example (matches calculator inputs): Largest raceway = 2 in.

\(L_{straight} = 8 \times 2\text{ in} = 16\text{ in}\) This is the value shown as “Straight-pull length reference: 16 in.”

Angle, U-Pull, and Splice Formula — NEC 314.28(A)(2)

For angle pulls, U pulls, or where splices are made, the distance from each raceway entry to the opposite wall of the box must be at least 6 times the trade size of the largest raceway in that row, increased by the sum of the trade sizes of all other raceways entering on the same wall and same row:

\(L_{angle/U} = \(6 \times D_{largest}\) + \sum D_{other\ raceways\ on\ same\ wall}\)

Each wall and row must be evaluated separately, and the single row producing the largest result governs.

Worked example (matches calculator inputs): Largest raceway = 2 in; sum of other same-wall raceway sizes = 3 in.

\(L_{angle/U} = \(6 \times 2\text{ in}\) + 3\text{ in} = 12\text{ in} + 3\text{ in} = 15\text{ in}\) This is the value shown as “Angle-pull length reference: 15 in.”

Controlling Dimension

The calculator compares both results and returns the larger one as the recommended minimum screening length:

\(L_{controlling} = \max(L_{straight},\ L_{angle/U})\)

With the inputs above (16 in vs. 15 in), the straight-pull value of 16 in controls, matching the “Recommended minimum screening length: 16 in” and “Controlling screening reference: 16 in” outputs. If your layout includes both straight and angle/U pulls at different walls, calculate each wall independently and size the box to satisfy the largest requirement across all walls.

Additional Requirements Not Captured by This Screening Tool

NEC 314.28 and related sections impose several requirements this worksheet does not evaluate:

• Conductor-enclosing raceways: the distance between raceway entries enclosing the same conductor must be at least 6 times the trade size of the larger raceway.
• Box-fill limits under 314.16 for any splices, taps, or devices housed in the enclosure.
• Bending space and gutter space for conductors, per manufacturer instructions and 312.6 where applicable.
• Exception allowances for conductors entering the back of an enclosure opposite a removable cover, which use conductor-size-based minimums (e.g., 3 in for 1 AWG, 3½ in for 1/0–2/0 AWG, 4 in for 3/0–4/0 AWG) rather than the 6× rule.
• Manufacturer catalog dimensions, listing constraints, and any local amendments enforced by your AHJ.