Wire Pull Box Sizing

Pull box length and width from the conduit trade size and the pull type under NEC 314.28, the box volume with the 40 and 20 percent pull allowance, the conductor count, and the 270 degree rule.

A 2 in conduit in a straight pull wants an 8 x 4-1/2 in box (8 x 2 = 16 in long, 60 percent of that in width), and an angle pull wants 6 x 2 = 12 in long at 75 percent of the length. A 4 in hot-silver 360 degree body is 24 x 4 = 96 in, capped at a 24 in body with a 24 in wide pull.

—Required box length and width
—Box volume with the pull allowance
—Conductors that fit at your wire size
—The 8 in required-length cap
—Conduit body as a pull box
—Pull practice and the 270 degree rule
How the math works
  • Straight pull (NEC 314.28(A)(1)): box length not less than 8 times the trade size of the largest raceway, and width not less than 60 percent of the length (increased where conductors enter the side wall), with the required length capped at 8 in.
  • Angle pull (NEC 314.28(A)(2)): box length not less than 6 times the trade size of the largest raceway (angle walls), width not less than 75 percent of the length, with the same 8 in cap on the required length. A 2 in angle pull is a 12 x 9 box; a 4 in angle pull wants 24 in and the code caps the requirement at 8 in.
  • U-pull, 360 degree, hot silver (NEC 314.28(A)(3)): 24 times the trade size of the largest raceway, width not less than 75 percent of the length, with 6 in between raceways entering the same wall, and no 8 in cap. A 2 in hot silver is a 48 in pull; a 4 in is 96 in, which is a 24 in wide box with the 6 in raceway spacing.
  • Conduit bodies (NEC 314.16(E), 342.24(C)): a conduit body used as a pull box has to meet the pull box length for the largest raceway, and a 360 degree conduit body is a hot silver at 24 times the trade size. The pull-angle rule (270 degrees in one section, 360 degrees total between boxes) applies to every 360 degree section on the run.
  • Volume and the pull allowance (NEC 314.16(A) and Table 314.16(A)): conductor volume per AWG (14 AWG 2.00, 12 AWG 2.25, 10 AWG 3.10, 8 AWG 5.00, 6 AWG 5.00, 4 AWG 6.25, 3 AWG 8.25, 2 AWG 11.25, 1 AWG 9.25, 1/0 10.25, 2/0 13.50, 3/0 17.00, 4/0 21.50 cubic inches for the 60 and 75 degree insulation table; THHN is smaller), fittings at 2 of the largest conductor each, and the pull box spare allowance of 40 percent on boxes 70 cu in and under and 20 percent over 70 cu in.
Pull box sizing for common conduit in residential, commercial, and industrial runs. The conductor insulation type in Table 5A and Chapter 9 Table 1 changes the volume per conductor (THHN-2 and XHHW-2 are smaller than TW and THW, and a copperclad aluminum count is the same volume), a box with conduits entering the side wall needs the width increase, a 4 in square box on a 4 in conduit is a 16 in straight pull by the 8x rule (the 60 percent width is 9.6 in), a box in a hazardous location, a Class 1 wiring method, a cable tray, a wireway, a busway, a 120 degree cable bend in a 4 in box, a box used for a device, a 48 in box on a 4 in hot silver, and the box fill on a 4 wire delta with the equipment bonding conductor all change the answer. The box in the field is what the AHJ signs.

How big a pull box for 2 in conduit?

Straight pull: 8 times 2 = 16 in long, 9-1/2 in wide at 60 percent, 40 percent spare on a 16 x 9-1/2 x 3 box = 456 cu in. Angle pull: 6 times 2 = 12 in long, 9 in wide. A 12 in LB on 2 in conduit is exactly an angle pull box, and a 16 in LL is a straight pull box.

What is the 40 percent rule for pull boxes?

The pull box volume allowance: 40 percent of the box volume is reserved for the pull on a box 70 cu in and under, and 20 percent on a box over 70 cu in (NEC 314.16(A)). The conductors fill the rest, and the fill math runs off Table 314.16(A) by AWG with the fittings counted.

Does a conduit body count as a pull box?

Yes when it is used as a pull point, and NEC 314.16(E) wants the body as long as the box length the rule asks for. A 2 in LB used as a 90 degree pull is a 12 in LB; a 2 in LB that is 8 in long is a junction, not a pull box, and the 360 degree body is the 24x hot silver.

How many bends before a pull box?

NEC 342.24, 352.24, 358.24, and 362.24: no more than 270 degrees of bend between pull points, and 360 degrees total between boxes, so a run with three 90s and two 45s (270 degrees) needs a box at the far end. The 270 degrees is the pull tension answer, not the wire pulling answer.

Related tools

Conduit fill by countThe same conductors in the raceway Wire ampacity and derateThe circuit the box carries Voltage drop and wire sizeThe run length behind the box