Pull Box Sizing Calculator

Pull Box Sizing Calculator results give the minimum inside length and width under NEC 314.28. Pick the pull type, the largest raceway in a row and how many raceways share that row.

Pull Box Sizing Calculator Rules From NEC 314.28

The NEC code for pull box sizing is section 314.28. It covers pull boxes, junction boxes and conduit bodies that hold conductors 4 AWG or larger. Mike Holt explains in his EC&M article on sizing junction boxes that the rule exists to keep large conductors from being bent so hard that the insulation is damaged.

The Pull Box Sizing Calculator works from raceway trade sizes, not from wire counts. A straight pull is the simplest case. The conductors enter one wall and leave through the opposite wall, and 314.28(A)(1) asks for a box length of eight times the trade size of the largest raceway.

$$L=8\times D$$

A 2 in conduit run pulled straight needs a box at least 16 in long. A 4 in run needs 32 in. Only the length is set this way, and the hero line reads Minimum Box Length. The width and depth just have to take the raceways and their fittings.

Conductors smaller than 4 AWG fall outside this rule. Choosing Smaller Than 4 AWG brings up the Rule Starts at 4 AWG note, because those boxes are sized by volume under 314.16. The Box Fill Calculator handles that count, and the Wire Size Calculator settles the conductor size first if it is still open.

Angle Pull Box Size

In an angle pull the conductors enter one wall and leave through an adjacent wall. To calculate the size of an angle pull box, 314.28(A)(2) measures from each raceway wall to the wall opposite. That distance is six times the largest raceway in a row, plus the trade sizes of the other raceways in the same row on the same wall.

$$L=6\times D+S$$

D is the largest raceway in the row and S is the sum of the others. Take two 3 in raceways in one row, leaving through a single 3 in raceway on the next wall. The length is 6 × 3 + 3 = 21 in and the width is 6 × 3 = 18 in. The Pull Box Sizing Calculator shows 21 in × 18 in Minimum Pull Box.

Each wall sets one dimension, which is why the Second Wall section has its own size and count. Smaller raceways in the row go in as a sum under Mixed Sizes. Two 2 in and one 1 in raceway add 5 in. Where a wall has more than one row, each row is figured alone and the largest answer is used.

The first card gives one more number. Raceways that hold the same conductors must be six times the larger trade size apart, which is 18 in here. Mike Holt notes that this is measured from the nearest edge of one raceway to the nearest edge of the other, not center to center.

U-Pulls, Splices and Combined Pulls

A U-pull enters and leaves through the same wall. It uses the same six-times rule, measured to the opposite wall. Two 3 in raceways in one row need 21 in, and the result reads 21 in Minimum to the Opposite Wall. The 18 in entry spacing applies along the wall between the two raceways.

That spacing runs from the nearest edge of one raceway to the nearest edge of the other, so the wall also has to hold both openings. The calculator allows eight times the raceway size for this, which is 24 in for a 3 in U-pull. The box check uses it, and the next standard box becomes 24 in × 24 in. Real fittings need a little more.

A straight-through splice also uses six times, not eight. A single 3 in raceway with spliced conductors needs an 18 in box, against 24 in for the same raceway pulled straight through. Other raceways in the row add their trade sizes as before.

Many boxes hold a straight pull and an angle pull together. Each direction is worked out both ways and the larger figure wins. Mike Holt's first example has two 3 in raceways on the left, one on the right and one at the bottom. The straight pull needs 24 in and the angle pull 21 in, so the box is 24 in by 18 in.

In the Pull Box Sizing Calculator, Straight and Angle Together returns the same 24 in × 18 in for those entries. His second example mixes a 2 in straight pull with a 3 in angle pull that shares its row with the 2 in raceway. The answer is 20 in by 18 in, and the calculator matches it with 6 × 3 + 2 on the length.

NEC Pull Box Sizing Chart

This chart gives the minimum inside dimension for common trade sizes. The angle columns also apply to U-pulls and splices. For a pull box on 4 in conduit, the answer is 32 in for a straight pull. One angle pull needs 24 in, and two 4 in raceways in the row need 28 in.

The Pull Box Sizing Chart by Raceway Size in the Pull Box Sizing Calculator lists all twelve trade sizes from 1/2 in to 6 in. Its last column follows the number of raceways entered, so a row of three or four shows its own figures.

Standard Pull Box Sizes

The code minimum is rarely a size on the shelf. The second card in the Pull Box Sizing Calculator rounds up to the next standard box, using the footprints in the Hubbell-Wiegmann SC series catalog of screw cover pull boxes. The 21 in × 18 in minimum becomes a 24 in × 18 in box.

Standard dimensions for an electrical pull box in that series run from 4 in × 4 in to 36 in × 36 in. Depths run from 4 in to 12 in. Square boxes come in 4, 6, 8, 10, 12, 15, 16, 18, 24, 30 and 36 in. The common rectangular sizes are below.

The same card gives the junction box size in mm. The default minimum is 533 mm × 457 mm and the standard box is 610 mm × 457 mm, which is 61.0 cm by 45.7 cm. A pull that needs more than 36 in reads Larger Than the Catalog Sizes, and a custom box is the answer.

Depth is not set by the eight-times and six-times rules. It has to fit the largest raceway with its locknuts and bushings. Where conductors enter opposite a removable cover, Mike Holt points to the bending distance in NEC Table 312.6(A) for the space between the entry and the cover.

Box Check in the Pull Box Sizing Calculator

Enter the inside length and width of a box under Proposed Box and Conductors. The fourth card compares it with the minimum and tries the box both ways round. A 30 in × 30 in box on the default pull reads Your 30 × 30 in Box Passes. It has 9 in of spare length and 12 in of spare width.

An 18 in × 18 in box fails the same pull. The card turns red and shows Short on Length By 3 in. The alert then names 24 in × 18 in as the next standard size that fits. A 24 in × 24 in box passes with 3 in and 6 in to spare.

The third card works the rule backward. For the 30 in box it reads Up to 4 in Raceway in Your Box, keeping the same pull type and row counts. It also shows 3 1/2 in as the largest straight pull and room for 5 raceways of the entered size in one row.

Junction Boxes, 100 Foot Runs and Grounding

Is a pull box the same as a junction box?

The enclosure is the same, and the name follows the use. In a pull box the conductors pass through unbroken. In a junction box they are spliced or tapped. A pull box can be used as a junction box, and NEC 314.28 sizes both.

To calculate junction box size for 4 AWG and larger in the Pull Box Sizing Calculator, pick Straight-Through Splice, or the angle and U-pull types if the conductors turn. Each uses six times the largest raceway plus the others in its row.

Do you need a pull box every 100 feet?

The NEC sets no distance limit for power conductors. It limits bends instead, to 360° between pull points. A pull box restarts that count, so runs with many offsets and 90° bends need one. The Offset Bend Calculator shows how many degrees each offset uses.

The 100 ft figure comes from the telecom standard TIA-569. As Cabling Installation & Maintenance summarizes it, no section of conduit is to be longer than 100 ft or hold more than two 90° bends between pull points. That applies to data cabling, and many job specifications adopt it.

Do pull boxes need to be grounded?

Metal ones do. NEC 314.4 requires metal boxes to be grounded and bonded in line with Article 250. A box joined by metal conduit is normally bonded through the raceway. With PVC conduit, the equipment grounding conductor has to be connected to the metal box.

Nonmetallic pull boxes have no metal to bond. The grounding conductor still runs through them with the circuit conductors, and the Pull Box Sizing Calculator dimensions apply to them in the same way.