Pull Box & Junction Box Sizing: NEC Rules with Chart

Undersizing a pull box is one of the most common violations on commercial and industrial installations. Too small and conductors get kinked on installation, insulation gets damaged pulling around tight corners, and the inspector writes it up. Get the calculation wrong on an exam and you lose easy points on a topic that has a clear, repeatable formula.

This guide covers the complete NEC sizing rules for pull boxes and junction boxes — when NEC 314.16 applies versus NEC 314.28, the straight-pull formula, the angle-pull formula, and the U-pull rule — with step-by-step worked examples and a quick-reference sizing chart.

• When to use NEC 314.16 vs. NEC 314.28 — the conductor size threshold
• Straight-pull sizing: the 8× rule
• Angle-pull and U-pull sizing: the 6× rule with row addition
• Distance between raceways on the same wall
• Accessibility requirement — NEC 314.29
• Step-by-step examples with mixed raceway sizes

Quick Reference: Which NEC Rule Applies?

Table 1: NEC Box Sizing Rule Selection — Conductor Size Determines Which Section Applies (NEC 2023)

Conductor Size in Box	Applicable NEC Section	Sizing Basis
18 AWG through 6 AWG	NEC 314.16	Volume — cubic inch capacity based on conductor count, device yokes, clamps, and EGCs
4 AWG and larger	NEC 314.28	Dimension — minimum box size based on raceway trade size and pull type

When a box contains both conductors 6 AWG and smaller AND conductors 4 AWG and larger, NEC 314.28 governs the overall box dimensions. The box-fill rules of NEC 314.16 do not apply to boxes sized under NEC 314.28.

The Three Pull Types — Definitions

Before calculating box size, you must identify what type of pull occurs at each raceway. The pull type is determined by where conductors enter and exit the box.

Table 2: Pull Types Defined — NEC 314.28 (2023 Edition)

Pull Type	Definition	Sizing Formula
Straight Pull	Conductor enters one wall and exits through the opposite wall	Box length ≥ 8 × trade size of largest raceway
Angle Pull	Conductor enters one wall and exits through an adjacent (perpendicular) wall	Distance from entry wall to opposite wall ≥ (6 × largest raceway in row) + sum of remaining raceways on same wall/row
U-Pull	Conductor enters and exits from the same wall	Same formula as angle pull — treated identically under NEC 314.28(A)(2)

Important: Raceways do not need to be directly opposite each other to qualify as a straight pull. Any two raceways on opposite walls — regardless of their position along that wall — constitute a straight pull for those conductors. Offset entries on opposite walls are still straight pulls.

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

For straight pulls, the minimum length of the box in the direction of the pull must be at least 8 times the trade size of the largest raceway entering or exiting on that wall.

Minimum Length = 8 × (Largest Raceway Trade Size)

Straight-Pull Example

Scenario: A pull box has three raceways entering the left wall: one 4-inch, one 3-inch, and one 2-inch conduit. Three matching raceways exit the right wall (straight pulls). What is the minimum box dimension in the horizontal direction?

Table 3: Straight-Pull Sizing Example — NEC 314.28(A)(1)

Step	Action	Value
1	Identify the largest raceway on the left/right axis	4-inch conduit
2	Apply formula: 8 × largest raceway	8 × 4 = 32 inches minimum

The box must be at least 32 inches long in the horizontal direction. The 3-inch and 2-inch raceways do not affect the straight-pull dimension — only the largest raceway on that axis matters for a straight pull.

Angle-Pull and U-Pull Sizing — NEC 314.28(A)(2)

For angle pulls and U-pulls, the calculation is more involved. For each wall where conductors make an angle pull, the minimum distance from that entry wall to the opposite wall must be:

Minimum Distance = (6 × Largest Raceway in That Row) + (Sum of All Other Raceway Trade Sizes in the Same Row on That Wall)

The formula applies independently for each wall that has angle-pull raceways. The box must satisfy the minimum dimension for every wall involved.

Angle-Pull Example — Single Wall

Scenario: A junction box has three raceways entering the left wall: one 3-inch and two 2-inch conduits. These conductors exit through the bottom wall (angle pulls). What is the minimum distance from the left wall to the right wall?

Table 4: Angle-Pull Sizing Example — NEC 314.28(A)(2), Single Wall

Step	Action	Value
1	Identify the largest raceway on the left wall	3-inch conduit
2	Calculate 6 × largest raceway	6 × 3 = 18 inches
3	Sum all other raceways on the same wall/row	2 + 2 = 4 inches
4	Add: 18 + 4	22 inches minimum (left wall to right wall)

Angle-Pull Example — Two Walls (Most Common Field Scenario)

Scenario: A pull box has the following raceways:

• Left wall: one 4-inch and one 2-inch conduit (angle pulls to bottom)
• Bottom wall: one 3-inch conduit (angle pull to right wall)

Find the minimum horizontal dimension (left wall to right wall) and minimum vertical dimension (top wall to bottom wall).

Table 5: Angle-Pull Sizing Example — NEC 314.28(A)(2), Two Walls

Dimension	Entry Wall	Calculation	Minimum
Horizontal (left → right)	Left wall: 4-inch + 2-inch	(6 × 4) + 2 = 24 + 2	26 inches
Vertical (bottom → top)	Bottom wall: 3-inch	(6 × 3) + 0 = 18 + 0	18 inches

The minimum box size for this scenario is 26 inches wide × 18 inches tall.

Distance Between Raceways on the Same Wall — NEC 314.28(A)(2)

A second requirement applies when multiple raceways on the same wall make angle pulls to different walls. The distance between any two raceways on the same wall must not be less than 6 times the trade size of the larger of the two raceways.

Example: Two raceways on the left wall — a 3-inch and a 2-inch conduit — both making angle pulls. The minimum distance between those two raceways on the left wall is:

6 × 3 (larger raceway) = 18 inches minimum between the two conduits on the left wall

This spacing requirement ensures that conductors pulled through adjacent raceways have enough room to bend without crossing or damaging each other’s insulation.

Pull Box Sizing Quick-Reference Chart

Table 6: Pull Box Minimum Dimensions by Raceway Size — NEC 314.28 (Straight Pull: 8× | Angle Pull: 6× + others)

Largest Raceway (Trade Size)	Straight Pull Minimum	Angle Pull Minimum (No Other Raceways on Wall)
1 inch	8 inches	6 inches
1¼ inch	10 inches	7.5 inches
1½ inch	12 inches	9 inches
2 inch	16 inches	12 inches
2½ inch	20 inches	15 inches
3 inch	24 inches	18 inches
3½ inch	28 inches	21 inches
4 inch	32 inches	24 inches
5 inch	40 inches	30 inches
6 inch	48 inches	36 inches

Note: Angle pull minimums above assume a single raceway on the wall. Add the trade size of every additional raceway on the same wall to the angle-pull figure.

Additional NEC 314.28 Requirements

Removable Covers — NEC 314.28(A)(2) Exception

For enclosures with a removable cover (such as a conduit body or wireway section), the minimum distance from the raceway entry to the removable cover is the bending space listed in NEC Table 312.6(A) for one conductor per terminal. This exception provides a practical alternative for certain enclosure types where the full 6× dimension would be impractical.

Accessibility — NEC 314.29

All pull boxes, junction boxes, and conduit bodies must be installed so that the wiring contained in them is accessible without removing any part of the building structure or finish. This is a non-negotiable requirement. Burying a pull box in a poured concrete wall, inside a finished ceiling with no access panel, or behind a permanently fixed fixture violates NEC 314.29. Access panels, removable ceiling tiles, and hinged covers all satisfy this requirement — as long as they can be opened without tools or building demolition.

Depth Requirements

NEC 314.28 governs the length and width of pull boxes based on raceway trade size, but does not prescribe box depth specifically (beyond the general requirement that conductors must fit without damage). The depth must accommodate the raceway fittings, locknuts, and conductor bending radius. In practice, a depth of at least the largest raceway trade size is the typical minimum used in the field, though the AHJ may have additional requirements.

Common Mistakes on Pull Box Sizing

Using the Wrong Formula for the Pull Type

Applying the 8× straight-pull formula to an angle-pull box — or forgetting to add the remaining raceway sizes to the 6× angle-pull calculation — are the two most common errors. Always identify the pull type for every raceway before calculating. Draw the box layout first.

Calculating Only One Axis

When a box has both horizontal and vertical angle pulls, both axes require independent calculations. The box must satisfy both minimums simultaneously. Calculating only the more obvious axis and ignoring the other is a common field and exam mistake.

Forgetting the Between-Raceway Spacing Rule

NEC 314.28(A)(2) requires minimum spacing between raceways on the same wall, not just minimum box size from wall to opposite wall. On an exam, a question may present a box that satisfies the wall-to-wall dimension but violates the between-raceway spacing requirement.

Applying 314.28 to Small-Conductor Boxes

NEC 314.28 applies only to boxes containing conductors 4 AWG and larger. A junction box containing only 12 AWG and 14 AWG conductors must be sized under NEC 314.16 (volume / box fill) — not NEC 314.28.

Conclusion

Pull box sizing under NEC 314.28 follows two straightforward formulas: straight pulls use 8 times the largest raceway, and angle or U-pulls use 6 times the largest raceway plus the sum of all other raceways on the same wall and row. Every box must also remain accessible per NEC 314.29.

For small-conductor box fill calculations under NEC 314.16, see our NEC Box Fill Guide. For raceway sizing with upsized pull-box conductors, use the VoltageLab Conduit Fill Calculator.

Frequently Asked Questions

What NEC section covers pull box sizing?

NEC 314.28 covers sizing requirements for pull boxes and junction boxes containing conductors 4 AWG and larger. For boxes containing conductors 6 AWG and smaller, NEC 314.16 applies, and sizing is based on cubic inch volume and conductor count (box fill).

What is the formula for a straight pull in a pull box?

Per NEC 314.28(A)(1), the minimum box length in the direction of the straight pull must be at least 8 times the trade size of the largest raceway entering that wall. For a 3-inch raceway, the minimum straight-pull dimension is 24 inches.

What is the formula for an angle pull in a pull box?

Per NEC 314.28(A)(2), the minimum distance from the entry wall to the opposite wall must be at least 6 times the trade size of the largest raceway on that wall, plus the sum of the trade sizes of all other raceways on the same wall and row. For example, a left wall with a 3-inch and a 2-inch conduit (both making angle pulls) requires: (6 × 3) + 2 = 20 inches from the left wall to the right wall.

Does a pull box need to be accessible?

Yes — NEC 314.29 requires all pull boxes, junction boxes, and conduit bodies to be installed so the wiring inside is accessible without removing any part of the building structure or finish. This means pull boxes cannot be buried in walls, poured into concrete, or installed behind permanently fixed building elements without an accessible cover or access panel.

Can one pull box have both straight and angle pulls?

Yes, and this is common in practice. When a box contains both straight and angle pulls, you must calculate each dimension independently. The straight-pull formula applies to one axis, and the angle-pull formula applies to another. The box must satisfy both calculated minimums simultaneously.