Pull Box Size Calculator

The two NEC 314.28(A) rules

Trade size (metric designator) is used in inches directly, so a 3" conduit contributes 3 to the multiplier. Article 314.28 applies only once a box carries conductors 4 AWG or larger; boxes whose conductors are all 6 AWG and smaller are sized by 314.16(B) box fill instead.

Worked example: an angle pull with three conduits

Conductors enter the left wall of a junction box through one 3" and two 2" conduits and turn 90 degrees to leave the bottom wall through matching raceways.

• Left wall (angle pull): 6 × 3 + 2 + 2 = 22 in to the opposite wall (314.28(A)(2)).
• Bottom wall (angle pull): the same three raceways give 6 × 3 + 2 + 2 = 22 in.
• Same conductor: the entering and leaving raceway of one conductor must be at least 6 × 3 = 18 in apart.

So the box must be at least 22 in × 22 in in the plane of the pulls, and the corner-to-corner geometry must keep paired raceways 18 in apart. Compare that to a straight pull of the same 3" conduit, which needs only 8 × 3 = 24 in of length and no minimum width from 314.28: the 90 degree turn is what drives the box larger, because conductors need room to make the bend without damage.

Details that trip people up

1. Straight pulls use only the largest raceway. Four 2" conduits in a straight pull still need just 8 × 2 = 16 in; the other three do not add. Angle pulls are the opposite: the other raceways in the row do add.
2. Each wall is its own row. Calculate every wall that receives entries separately and take each result as that box dimension. The single row giving the largest distance governs its axis.
3. Trade size, not conductor count. 314.28 never looks at how many wires are in the pipe; Chapter 9 conduit fill already limits that. Only the raceway trade sizes and the geometry matter here.
4. Removable-cover reduction is not applied here. Where a raceway enters directly opposite a removable cover, 314.28(A)(2) Exception lets the wall-to-cover distance follow the Table 312.6(A) bending space instead of 6x. This calculator reports the full 6x distance, which is always compliant.

Frequently asked questions

What is the difference between a pull box and a junction box?

A pull box gives conductors a place to be pulled through a long or complex raceway run without splices; a junction box holds splices or taps. NEC 314.28(A) sizes both the same way from the raceway trade sizes, and the angle/U-pull rule explicitly covers boxes that contain splices.

Why is an angle pull box bigger than a straight pull box?

Conductors 4 AWG and larger are stiff and cannot bend sharply without damaging the insulation. A straight pull only needs enough length to feed the conductors through (8x), but an angle pull needs room across the box for the conductor to make the turn (6x from each entry to the far wall, plus the other raceways), which usually produces a larger box.

Does 314.28 set the box depth?

314.28(A) sets the in-plane dimensions from the pulls. Depth is driven by the raceway entries and fittings on the back or sides and by conductor bending space; a box deep enough to land the locknuts and bushings and clear the conductors satisfies the pull geometry. This tool reports the pull dimensions the 8x and 6x rules require.

What conductors require 314.28 sizing?

Any box or conduit body containing conductors 4 AWG or larger. Below that, through 6 AWG and smaller, boxes are sized by the 314.16(B) volume box-fill method. Feeder and service pulls almost always land in 314.28 territory.

Related calculators

• Box Fill Calculator: NEC 314.16(B) volume fill for boxes with 18 to 6 AWG conductors.
• Conduit Fill Calculator: conductor fill for EMT, RMC, IMC, and PVC per NEC Chapter 9, Table 1.
• Conduit Size Calculator: minimum conduit trade size for a conductor bundle.
• Wire Ampacity Calculator: derated ampacity of a specific conductor.