Pull box sizing for 4 AWG and larger conductors

Size a pull box with 4 AWG and larger conductors by its raceway entries, not by a volume calculation. A straight pull needs a box length of at least 8 times the trade size of the largest raceway entering the box. An angle or U pull needs at least 6 times that trade size from the entry wall to the opposite wall, and raceways that carry the same conductor sit at least 2 times the smallest of them apart. The NEC volume-fill table covers small conductors, from 18 AWG up to 1 AWG; a pull box that splices 4 AWG and larger feeders sizes by geometry instead. A 42 in long box takes a straight pull up to 5 in trade size conduit (8 x 5 = 40 in), and a 24 in wall takes a 4 in angle pull (6 x 4 = 24 in). Here is how you work the numbers.

Why volume fill stops at these sizes

For conductors smaller than 4 AWG, you size a junction box by volume. The NEC publishes an allowance per conductor: 2.00 cu in for a 14 AWG conductor, 2.25 for 12 AWG, 5.00 for 6 AWG. You add the allowances, then extras for clamps and supports, and buy the box that holds the total. That arithmetic answers a small steel box with a handful of wires, which is how the 8 AWG three-phase splice sizing article works.

A pull box that splices 4 AWG and larger feeders follows the raceways instead. The code multiplies the largest conduit's trade size by 8 or by 6, because the limit is the bend the big conductor makes around the walls, not the air it occupies. Where the splices land on lugs rather than passing through, NEC 312.6 adds wire bending space at the terminals, with one table for conductors entering the terminal's own wall and one for entry through the opposite wall; the enclosure sizing guide carries both. The 350 kcmil pull box article shows the same geometry at its extreme.

The three entry rules

Three multipliers set the dimensions. Trade size means the nominal conduit size stamped on the fitting, not the outside diameter. Measure the box wall to wall, and raceway to raceway edge to edge.

Work each wall group on its own numbers. Three 3 in conduits in one angle pull need 6 x 3 = 18 in [455 mm] from their wall to the opposite wall. A pair of 2 in raceways that feeds and returns one feeder set sits with 2 x 2 = 4 in [100 mm] of metal between them. A straight 2½ in pull wants 8 x 2.5 = 20 in [508 mm] of box length. Stacked rows count one by one: check the 6 x dimension for every row on the wall. Leave about 2 in [50 mm] between unrelated hubs so the locknuts get wrench room.

Worked example: the largest raceway a 24 x 24 x 42 in box takes

Take a sheet steel pull box 24 in wide, 24 in high and 42 in long, splicing ten three-phase feeder circuits from 100 A to 225 A, all conductors 4 AWG and larger. The straight pull runs end to end in a 5 in trade size conduit, and the angle pulls enter groups of 4 in conduits on the long walls. Run the three checks:

• Straight pull: 8 x 5 in = 40 in against the 42 in length. It fits with 2 in to spare. The next trade size up fails: 8 x 6 in = 48 in, over the 42 in.
• Angle pull on a long wall: 6 x 4 in = 24 in, and the wall-to-wall dimension is 24 in. That is exactly the minimum, so it holds; a 4½ in group needs 6 x 4.5 in = 27 in and does not fit.
• Angle pull on an end wall instead: the pull dimension is the 42 in length, so a 6 in group holds: 6 x 6 in = 36 in against 42 in.
• Same-conductor pairs: two 4 in raceways carrying one feeder set keep 2 x 4 in = 8 in [200 mm] between their edges.
• Stacked rows of 2 in conduits on a long wall: each row needs 6 x 2 in = 12 in [300 mm] to the opposite wall, and the 24 in width covers it twice.

Decision: the box runs a straight pull up to 5 in trade size and a 4 in angle pull on its long walls; a 6 in straight pull calls for the next length up, 48 in. Where a 225 A phase splits into two parallel sets in matched raceways, size each raceway on its own trade size and keep the sets matched so the current divides evenly; matched-raceway parallel-conductor guide covers that side. Which wall the entries land on changes every number, which is why the parking-garage pull box article treats orientation alongside size and rating.

What you need

• The raceway schedule: every entry with its trade size, wall and position.
• The splice plan: which feeders pass straight, which turn, which entry-and-exit pairs carry the same conductors.
• One hub or gland per entry, matched to the trade size; gasketed hubs on wet and outdoor walls.
• Bonding jumper stock and grounding bushings for the metal raceway entries.
• The multipliers on the drawing: 8, 6 and 2.
• The box itself: 304 stainless for indoor washdown and most outdoor sites, 316L for salt air, brine and chemical areas, powder-coated carbon steel for dry indoor walls. We cut custom bodies from one piece in your dimensions through the custom enclosures builder.

Sizing and layout steps

1. List every raceway with its trade size, wall and position. The largest trade size on the box sets the straight-pull number.
2. Mark each pull straight, angle or U, and group the raceways by the wall they enter.
3. Size the straight pulls: largest trade size x 8, against the box length in the pull direction. For a 3 in largest raceway that is 8 x 3 = 24 in.
4. Size each angle and U group: largest trade size in the group x 6, against the distance from that wall to the opposite wall. A 2½ in group needs 6 x 2.5 = 15 in.
5. Space the pairs: 2 x the smallest trade size, edge to edge, between raceways that carry the same conductor. Two 2½ in raceways need 2 x 2.5 = 5 in.
6. Set the depth: panel practice is the largest trade size plus 2 in [50 mm], so a 5 in entry wants about 7 in of depth for the hub and the first bend.
7. Add NEC 312.6 bending space where conductors land on lugs, taking its same-wall table when the conductor enters the terminal's wall and its opposite-wall table when it does not.
8. Put every entry on the approval drawing with trade size, wall and coordinates; the cutout drawing checklist on the stainless cutouts page keeps that drawing complete.

Checks when the box is laid out

• Straight-pull length: at least 8 x the largest trade size; 5 in needs 40 in of box.
• Every angle group: wall to wall at least 6 x the group's largest trade size; 3 in needs 18 in.
• Every pair: at least 2 x the smallest trade size, edge to edge; 2½ in needs 5 in.
• Working space before you mount: NEC 110.26(A) wants at least 3 ft [900 mm] clear in front of equipment up to 150 V to ground in Condition 1, with the door opening at least 90 degrees.
• Wet or outdoor walls: a gasketed hub on every entry, plus drainage openings of 3 mm to 6 mm (1/8 in to 1/4 in) where NEC 314.15 permits them in damp and wet locations.
• Bonding: every metal raceway entry bonded for fault current, with paint and coatings cleared from the contact surfaces as NEC 250.12 calls for, and the bonding conductor sized from the overcurrent device ahead per NEC Table 250.122.

Mistakes that cost the size or the rating

• Sizing a 4 AWG-plus splice box by volume. That returns a box that fails the entry checks; run the 8, 6 and 2 multipliers instead.
• Measuring the 6 x dimension to the near wall. The dimension runs from the entry wall to the wall opposite it; on the example box that is 24 in on the long walls and 42 in end to end.
• Skipping the pair spacing in U pulls. Under 2 x metal between entry and exit, the conductor's bend pinches at the corner.
• Cutting field holes oversize or leaving them bare. Cut to the hub's stated diameter, deburr the edge and close it with a gasketed hub, the way the field-drilled conduit entry article lays out for a Type 4X box.
• Reusing a cutout for a smaller raceway. Fit a listed reducing hub or a safe transition, as the conduit entry mismatch article shows.
• Blocking the working space at installation. Keep the 3 ft [900 mm] clear in front of the box.

What to put on the order

Name the enclosure type number on the drawing: NEC 110.28 says an IP code cannot stand in for it in US installations. Our stainless pull boxes come rated NEMA 4X / IP66 by the manufacturer; not UL listed. Every cutout rides on the approval drawing with its trade size, wall and coordinates, so the rating survives the entries. From your approval of that drawing, delivery runs 10 business days, anywhere in the world. For the rating side of the order, see whether a pull box counts as an enclosure under a NEMA 4X spec; for the outdoor hardware list, the outdoor service conductor pull box article carries gaskets, hubs and drainage together.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions