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How to Calculate Underground Pull Box Sizing

Determining the size of underground pull and junction boxes to meet NEC 314.16 code standards is required to prevent damage to conductor insulation. Calculating the correct size of pull box needed can be complicated, so please feel free to contact us or call 800-767-1576 if you need assistance.

NEC 314.28 code requirements apply to installations with conductors of 4 AWG or larger that are required to be insulated, and for cables containing conductors of 4 AWG or larger.  These apply to splices, straight pulls, U pulls and angle pulls.

Calculating Pull Box Enclosure Sizes:

Minimum size requirements:  Minimum distance from where conductors enter to the opposite wall cannot be less than eight times the trade size of the largest raceway.

Straight Pulls

In this case, simply multiply your largest raceway size by 8, and that will give you the minimum length for your box.

calculate straight pull

Largest Raceway Size x 8 = Minimum Box Length

For example, if  your raceway is 3'':

3'' x 8 = 24''

Splices, Angle Pulls, U Pulls

These calculations also start with the largest raceway size, but then get a little more complicated. You need to multiply your largest raceway by 6, then add the sizes of any other raceways on the same wall and row to get minimum box size.

angle pull calculation

(Largest Raceway Size x 6) + Sizes of Raceways on the Same Row
= Minimum Box Length

For example, if you have a single Splice, Angle or U pull of 3'' raceway:

3'' x 6 = 18''

Multiple Raceways

If you have multiple rows of raceways, calculate each one separately and choose the largest distance.  

If you a have multiple rows of raceways, use the largest size calculated.

multiple raceways calculation

For example, if you have a row with 3-3'' raceways, and another row with 3-1'' raceways:

Calculate Row 1: (3'' x 6) + 3'' + 3'' = 24''

Calculate Row 2: (1'' x 6) + 1'' + 1'' = 8''

Choose 24'', the larger of the 2 sizes.

Distance Between Raceways

Additionally, the distance between raceways that enclose the same conductor cannot be less than six times the largest raceway, measured from the raceways’ nearest edge-to-nearest edge.

distance between raceways calculation

Largest Raceway Size x 6 = Minimum Distance Between Raceways

For example, if you have a 3'' raceway from left to bottom:

3'' x 6 = 18''

Pull Box Calculation Steps:    

You will want to calculate both horizontal (left to right and right to left) and vertical (top to bottom and bottom to top) dimensions, as well as calculating the distance between raceways. Because this can be confusing, you will want to draw out the configuration.

For example, if you have a pull box with 1-3'' raceway on the left, 1-2'' raceway on the left  1-2'' raceway on the right, and 1-3'' raceway on the bottom.  So you have a 2'' straight pull and a 3'' angle pull.

horizontal size calculation

Calculate Horizontal Dimension:

  1. Straight Pull:
    • Left to Right: 8'' x 2 = 16''
    • Right to Left: 8'' x 2 = 16''
  2. Angle Pull:
    • Left to Right: (3'' x 6) + 2'' = 20''
    • Right to Left:  None
vertical size calculation

Calculate Vertical Dimension:

  1. Straight Pull: None
  2. Angle Pull
    • Top to Bottom: None
    • Bottom to Top: 3'' x 6 = 18''
distance between raceways calculation

Calculate Distance Between Raceways:

  1. Distance between Raceways: 3'' x 6 = 18''


Your pull box must be at least 20'' wide by 18'' tall.

Need help choosing the right size pull boxes? Our knowledgeable staff would be happy to assist you. Contact us or call 800-767-1576.


Other Articles You May Enjoy:

Choosing the Right Underground Enclosures

Determining Proper ANSI Load Ratings for Underground Pull Boxes