NEC Pull Box Sizing Calculator (314.28)

What is NEC 314.28: Conductor Bending Radius Limits?

Pull boxes are junction points installed in long electrical pipe runs. They allow electricians to tie off ropes and physically pull heavy wire without exceeding 360-degrees of total bends. Because large internal wires have strict physical 'Bending Radius' limits, you cannot bend a 600kcmil wire around a tight 90-degree corner without cracking its insulation. NEC Article 314.28 mandates strict mathematical models for how large the physical metal box must be to safely accommodate the wire bend.

Mathematical Foundation

Straight Pull (NEC 314.28(A)(1))

L_{min} = D_{largest} \times 8

L_{min}

= Minimum pull box length required.

D_{largest}

= Trade size of the single largest conduit entering the box.

Angle / U-Pull (NEC 314.28(A)(2))

L_{min} = (D_{largest} \times 6) + \sum D_{additional}

L_{min}

= Minimum pull box dimension required in the direction of the pull.

D_{largest}

= Trade size of the largest conduit entering the wall.

\sum D_{additional}

= The mathematical sum of all other conduits entering the same wall.

Laws & Principles

Straight Pull (314.28(A)(1)): The simplest form. The wire enters one wall and exits the opposite wall in a perfectly straight line without bending inside the box. The box length between those walls must be 8-times the diameter of the largest pipe. Any other smaller pipes entering the same wall DO NOT add to the box size math.
Angle Pull (314.28(A)(2)): The wire enters one wall and turns 90-degrees to exit an adjacent wall. Because the wire must bend inside the box, the wall distance must be perfectly calculated. Multiply the largest pipe by 6, then ADD the raw diameter of every other pipe sharing that same wall.
U-Pull (314.28(A)(2)): The wire enters the box, loops, and exits out the EXACT SAME wall it entered. The math is identical to the Angle Pull (Largest × 6 + Sum of Others).
Distance Between Raceways: When making an Angle or U-Pull, the physical distance between the two pipes enclosing the same conductor must be at least 6-times the trade size of the largest pipe.

Step-by-Step Example Walkthrough

" An electrician is piping into a metallic junction box. On the bottom wall, they have one 3-inch pipe and two 2-inch pipes entering. The wires will turn 90 degrees and exit the side wall. "

1. Identify Pull Type: The wires are turning 90 degrees, making this an Angle Pull governed by 314.28(A)(2).
2. Identify Largest Conduit on the wall: The largest is the 3-inch pipe.
3. Apply Primary Multiplier: 3 inches × 6 = 18 inches.
4. Identify Additional Conduits: There are two extra 2-inch pipes on the same wall.
5. Apply Summation: 18 inches + 2 inches + 2 inches = 22 inches total.

Final Result: The distance from the bottom wall (where the pipes enter) to the opposite top wall must be an absolute minimum of 22 inches to legally pass the inspection. The electrician will likely order a standard 24x24 metal enclosure.

Quick Answer: How do you size an electrical pull box?

You size an electrical pull box based on the trade diameters of the conduit entering the enclosure. For a straight pull, the length must equal the largest conduit multiplied by 8. For an angle pull, multiply the largest conduit by 6, then strictly add the diameters of all other conduits on that same wall. Use this NEC Pull Box Sizing Calculator to instantly calculate the legal minimum enclosure dimensions required to pass inspection.

Common NEC Multipliers & Distance Limits

Pull Configuration	Largest Conduit Multiplier	Addition Rules
Straight Pull	8×	Ignore all additional conduits
Angle Pull (90°)	6×	Add all other conduits on same wall
U-Pull (180°)	6×	Add all other conduits on same wall
Conduit Separation	6×	Physical distance between same-wire pipes

Code Violations & Diagnostics

The Multiplier Mistake

An apprentice is building a straight pull box with one 4-inch pipe and three 2-inch pipes entering the same wall. They use the Angle Pull framework by mistake, calculating: (4" × 6) + 2" + 2" + 2" = 30 inches. This is structurally incorrect. Because the wire does not bend, the straight pull rule dictates: 4" × 8 = 32 inches. The apprentice's 30-inch box failed the inspection and had to be ripped out of the wall.

The Removable Cover Loophole

An electrician calculates that a massive 4-inch conduit requires a 24-inch angle pull box (4" × 6 = 24"). They don't have room for a square 24x24 box in the cramped ceiling. The NEC explicitly allows the conduit to enter the wall directly opposite a removable cover. If the pipe enters facing the lid, the box depth only needs to satisfy standard bending radius charts, allowing a much thinner, rectangular enclosure profile.

Field Design Best Practices

Do This

✓Use nominal Trade Sizes. A 2-inch PVC pipe has an actual Outer Diameter of 2.375 inches. The NEC does not care about the outer diameter. Always use the stated 'Trade Size' (the number printed on the pipe) when executing 314.28 multiplier math.
✓Check all four walls. In a complex junction box, conduits might enter the top, bottom, and left sides. You must run the (Largest × 6) + Additional formula independently on EVERY wall, and size the final box to the largest restriction found.

Avoid This

✗Do not group unrelated rows. If a box is extremely deep and you have two completely distinct, horizontal rows of conduit, you do not sum them together. Calculate the rows individually; the box size is dictated by whichever single row generates the largest mathematical requirement.

Frequently Asked Questions

What is the 6 times rule for pull boxes?

The 6x rule applies to Angle and U-Pulls. The NEC requires the distance to the opposite wall to be at least six times the diameter of the largest entering conduit. If multiple conduits exist, you multiply the largest by 6, then add the diameters of the remaining conduits on that specific wall.

What is the 8 times rule for pull boxes?

The 8x rule exclusively governs Straight Pulls. When a thick wire enters a box and leaves through the exact opposite wall without bending, the length of the box must simply be eight times the thickness of the largest conduit pipe. Additional smaller pipes entering the wall are entirely ignored in this math.

Do these rules apply to small #12 wire?

No. NEC Section 314.28 mathematically triggers only when you are pulling conductors sized #4 AWG or larger. Small household wiring (Sizes 14, 12, 10, 8, 6) is incredibly flexible and is governed instead by 'Box Fill Volume' tables rather than rigid bend-radius lengths.

How do you calculate conduit separation distance?

If you are bending a wire inside a box (Angle or U-Pull), the physical distance between the entry pipe and the exit pipe must be mathematically separated. Measure center-to-center between the two conduits. That distance must be at least 6-times the trade dimension of the largest pipe.

NEC 314.28 Box Sizing

NEC Pull Box Sizing (314.28)

Conduit Entry Layout

Enclosure Restrictions