Pull box size calculator

How the pull box dimensions are calculated

This page applies the sizing rules of NEC 314.28(A) to pull boxes and junction boxes that hold conductors of 4 AWG and larger at 1000 volts or less. A straight pull is sized from the largest raceway alone, an angle or U pull adds the other raceways of the same wall and row, and the entries of one set of conductors are kept a set distance apart. The Code text was not read for this page, so the rules are taken from EC&M, NY Engineers and VoltageLab pages that agree on every figure used, and each of their worked layouts is reproduced below. Box depth, bending space and a catalog size are not calculated.

Straight pull

length >= 8 x trade size of the largest raceway in the pullConductors that enter one wall and leave by the opposite wall need 8 times the trade size of the largest raceway. A trade size 2 straight pull needs 16 inches, and a trade size 3 needs 24 inches.

Angle or U pull

distance to the opposite wall >= 6 x largest raceway + sum of the other raceways in the rowConductors that enter one wall and leave by an adjacent wall, or come back out of the same wall, need 6 times the trade size of the largest raceway plus the trade sizes of the other raceways entering that wall in the same row. A trade size 3 raceway beside a trade size 2 one gives 6 x 3 + 2 = 20 inches.

Rows

each row of a wall is calculated alone, and the largest result governsRaceways in different rows do not add to each other. A wall with a trade size 2 row and a row of two trade size 3 raceways gives 12 inches for the first row and 21 inches for the second, so 21 governs. Adding the first row's raceway to the second row would wrongly give 23.

Straight and angle pulls together

use the greater of the two results for the directionWhen a wall has both kinds of pull, both are calculated and the greater is used. In EC&M's code quiz a trade size 4 straight pull gives 32 inches and the trade size 2 angle pull with the trade size 4 beside it gives 16 inches, so the box is 32 inches in that direction.

Entries of the same conductors

distance >= 6 x trade size of the larger raceway, edge to edgeThe raceway that brings conductors into the box and the one that takes them out are kept 6 times the larger trade size apart, measured between the nearest edges and leaving out locknuts and bushings. Two trade size 3 raceways are 18 inches apart. This page reports the distance and does not check your layout against it.

Which wall sets which direction

left and right walls set the horizontal dimension, top and bottom walls the verticalThe distance is taken from the wall the raceway enters to the opposite wall. Raceways on the left wall or the right wall set the horizontal dimension, and raceways on the top wall or the bottom wall set the vertical one. In the default case the left wall gives 20 inches and the top wall gives 18.

Large boxes

any dimension over 6 ft needs cabled or racked conductorsEC&M describes 314.28(B) as calling for the conductors to be cabled or racked in an approved manner when a dimension passes 6 ft, or 72 inches. A trade size 6 raceway with 40 inches of others in its row gives 76, which trips the note.

Worked examples from published pull box sizing

The default case is EC&M's second example. Its left wall holds a trade size 2 raceway in a straight pull and a trade size 3 raceway in an angle pull, the top wall holds a trade size 3 raceway, and the right wall holds a trade size 2 raceway. The straight pull gives 8 x 2 = 16 inches and the angle pull gives 6 x 3 + 2 = 20 inches, so the horizontal dimension is 20. Its top wall gives 6 x 3 = 18 inches. The two trade size 3 entries of the same conductors are 18 inches apart.

The table below runs every layout printed in the sources through the same function as the form. Sources print the horizontal and vertical figures, and some also print the distance between entries of the same conductors, which the tests check. Every row agrees with its source. Two of the sources work a straight pull and an angle pull on one wall, and both take the greater of the two, 32 against 16 inches in the code quiz.

Eight cases below run through the same functions the form uses. Six come straight from the sources. The seventh is a layout made up to show the rule that each row stands alone, with a trade size 2 row at 12 inches and a row of two trade size 3 raceways at 21. The eighth is also made up and uses fractional trade sizes.

Standards and data sources

• EC&M (Mike Holt), Taking the Mystery Out of Sizing Pull Boxes and Junction Boxes - The straight pull rule of 8 times the trade size of the largest raceway, the angle and U pull rule of 6 times the largest plus the other raceways of the same wall and row, the separate calculation of each row, and the 6 times distance between entries of the same conductors. Its two worked boxes are reproduced here, 24 by 18 inches and 20 by 18 inches.
• EC&M, Code Calculations (March 2004) - A box with two trade size 4 raceways on the left and two trade size 2 on the top, 28 by 14 inches, with 24 inches between the trade size 4 entries of the same conductors. It also gives a depth example, two trade size 2 conduits with 4/0 AWG conductors entering a 24 by 24 inch box, which needs 4 inches of depth by Table 312.6(A). Depth is not calculated on this page.
• EC&M Code Quiz (Steven Owen, September 8, 2004, 2002 NEC) - A trade size 4 straight pull and a trade size 2 angle pull on one wall, solved as 32 by 12 inches. It shows that the greater of the straight and angle results governs a direction, and that the other raceway of the wall is added to the angle pull.
• NY Engineers, article on pull box sizing - A straight pull, an angle pull of trade sizes 4, 2 and 2 at 28 inches, a U pull of two trade size 3 raceways at 21 inches with 18 from entry to exit, and a combination box of 18 by 20 inches. The page does not name an NEC edition.
• VoltageLab, pull box sizing - The scope of 314.28 as conductors of 4 AWG and larger, with 6 AWG and smaller going to the 314.16 box fill rules, and two more worked layouts, trade sizes 3, 2 and 2 on one wall at 22 inches and trade sizes 4 and 2 on the left with a 3 on the bottom at 26 by 18 inches.
• EC&M, NEC Rules for Pull Boxes, Junction Boxes and Conduit Bodies - The cabled or racked conductor requirement of 314.28(B) for a box with any dimension over 6 ft, and the separation of pull and junction boxes from the outlet box rules of 314.27.

Limitations of this calculator

• The text of the NEC was not read for this page. NFPA sells the Code, and no open reprint of 314.28 was found. The rules come from the EC&M, NY Engineers and VoltageLab pages listed, which agree on every figure used, and the page reproduces each layout they print.
• Those pages were written on the 2002 and 2005 NEC or do not name an edition. The 2023 wording of 314.28(A) was not available, and neither was the text of the provision headed Smaller Dimensions in 314.28(A)(3), so no smaller box allowed by that provision is modeled. Check the edition the authority having jurisdiction has adopted.
• The sources scope 314.28 to conductors of 4 AWG and larger at 1000 volts or less. A box with only 6 AWG and smaller conductors goes by box fill under 314.16, which is a separate calculator, and a box with both kinds is sized by 314.28 for its overall dimensions according to VoltageLab. Over 1000 volts is outside the sources and is not sized here.
• When a straight pull and an angle pull share a wall, the page takes the largest raceway from the angle or U pull itself and adds every other raceway of the row, the straight pull included. EC&M works its examples and its code quiz this way. If the authority having jurisdiction reads the largest raceway as the largest in the whole row, enter that size.
• You decide which raceways are straight pulls, which are angle or U pulls and which row each sits in. The page cannot see your layout, so a raceway entered in the wrong place changes the result. A wall with several rows needs each row entered on its own.
• Box depth, the cover and the bending space at the terminals are not calculated. For a wall that faces a removable cover EC&M describes the distance as the bending space of Table 312.6(A) for one conductor per terminal, and its example needs 4 inches of depth for 4/0 AWG in trade size 2 conduit.
• Distance between entries, 6 times the larger trade size, is reported and not checked. The page does not know where your knockouts or hubs sit, whether the raceways fit along the wall, or the width of locknuts and bushings, which EC&M leaves out of the measurement.
• Sizes are conduit trade sizes from 1/2 to 6 inches, and a row sum up to 60 inches is accepted. Cable tray, wireways, conduit bodies, handholes and cable assemblies are outside the page, and so are dimensions over 72 inches, which carry the cabled or racked requirement and are flagged, not designed.
• The result is a smallest dimension, not a catalog size. Round up to a size the maker sells and confirm its depth and listing. The sources give no table of standard box sizes, so none is applied.
• This is a planning estimate, not a design drawing. A licensed electrician, the manufacturer's data and the authority having jurisdiction decide the box, its installation and its listing.

Common pull box sizing mistakes

1. Using the straight pull rule for an angle pull. A straight pull takes 8 times the trade size and an angle pull takes 6 times plus the other raceways. For a trade size 3 box the straight pull is 24 inches and the angle pull with one more trade size 3 beside it is 21. Running the wrong rule on a turn oversizes the box, and running the angle rule on a straight pull undersizes it.
2. Leaving the other raceways of the wall out. The angle rule adds every other raceway entering the same wall in the same row. In EC&M's second example the trade size 3 angle pull on the left wall has a trade size 2 raceway beside it. 6 x 3 alone gives 18, and with the 2 added it is 20.
3. Adding raceways from another row. Each row of a wall is calculated by itself. A trade size 2 row and a row of two trade size 3 raceways give 12 and 21 inches. Pooling them as one row would put 23 inches on the box and size it larger than the rule asks.
4. Reading the wrong direction. Raceways on the left and right walls set the horizontal dimension, and raceways on the top and bottom walls set the vertical one. In the default case the top wall's trade size 3 raceway gives 18 inches vertically and does not touch the 20 inch horizontal figure.
5. Sizing a 6 AWG box here. Conductors of 6 AWG and smaller are counted by box fill under 314.16, not by 314.28. The sources scope the pull box rules to 4 AWG and larger, and the form sends a smaller conductor to the box fill calculator.
6. Ordering the exact minimum. The figure is the smallest dimension the pulls allow. A box also needs depth, and EC&M's example of 4/0 AWG in trade size 2 conduit needs 4 inches by Table 312.6(A). Choose a catalog size at or above both dimensions and check its depth against the data sheet.

Common questions about pull box size

Related calculators

• Box fill calculator Box volume under 314.16 for boxes with 6 AWG and smaller conductors.
• Conduit fill calculator The trade size of the raceways that enter the box.
• Conduit offset bend calculator Marks and spacing for the bends in a run between boxes.
• Equipment grounding conductor size calculator The grounding conductor that runs through the box with the feeder.
• All tools Every public BuildSolver calculator in one list.

Used on the same job

• Wire size calculator The conductor size that decides whether the box falls under 314.28.
• Voltage drop calculator Voltage drop on the long run between pull points.
• Wire ampacity derating calculator Ampacity of the conductors that share the raceways entering the box.