NEC 314.28 Pull Box Sizing Rules: A Practical Guide

> **Quick Answer:** NEC 314.28 requires pull boxes containing conductors 4 AWG or larger to be sized using specific formulas based on pull type. For straight pulls, the box length must be at least 8× the trade size of the largest conduit. For angle and U-pulls, the distance from the conduit entry to the opposite wall must be at least 6× the largest conduit, plus the sum of all other conduits entering on the same wall.

What Is NEC Article 314.28?

NEC Article 314 covers outlet, device, pull, and junction boxes — and 314.28 is the section that specifically governs pull boxes, junction boxes, and conduit bodies containing conductors 4 AWG and larger. If you've got smaller conductors only, 314.28 doesn't apply; you'd fall back to the conductor fill rules in 314.16.

The reason 314.28 exists is simple: large conductors have minimum bending radii. You can't just cram a run of 500 kcmil into a 12×12 box and expect the conductors to survive the installation — or the inspection. The code gives you a floor, not a ceiling.

This section applies to any enclosure used for pulling, splicing, or directing conductors 4 AWG and larger. That includes pull boxes on feeder runs, service entrance equipment, motor circuits, and anywhere else you're routing large wire through a change in direction.

The Straight Pull Formula — 314.28(A)(1)

A straight pull is the simplest case: conductors enter one side of the box and exit through the opposite side in a straight line. No turns.

**Formula:** Box length = 8 × (trade size of the largest conduit)

**Example:** You've got a 3-inch conduit on each end, straight through.

- 8 × 3 = 24 inches

Your box needs to be at least 24 inches in the direction of the pull. Width and depth are governed by other considerations (conduit spacing, cover bolt access), but the minimum length along the pull direction is 24 inches.

That's it for straight pulls — one formula, one conduit to worry about. Where it gets more interesting is angle pulls.

The Angle Pull Formula — 314.28(A)(2)

An angle pull means the conductors change direction inside the box — entering one wall and exiting through a different wall (typically a 90-degree turn). This is where most field errors happen.

**Formula for each wall:** Distance from the conduit entry to the opposite wall = (6 × largest conduit on that wall) + (sum of remaining conduits on the same wall)

You apply this formula independently for each wall that has conduits entering it, then the box dimensions have to satisfy all of them simultaneously.

**Worked Example:**

You have:

- One 2-inch conduit entering the left wall, exiting the bottom wall

- Two 1.5-inch conduits also entering the left wall, exiting the bottom wall

For the **left wall** (determining the box width — distance from left wall to right wall):

- Largest conduit on left wall: 2 inches

- Remaining conduits on left wall: 1.5 + 1.5 = 3 inches

- Required distance: (6 × 2) + 3 = 12 + 3 = **15 inches**

For the **bottom wall** (determining the box height — distance from bottom wall to top wall):

- Largest conduit on bottom wall: 2 inches (same conductors, same conduits)

- Remaining conduits on bottom wall: 1.5 + 1.5 = 3 inches

- Required distance: (6 × 2) + 3 = **15 inches**

So you need at least a 15×15 box. If you only had the single 2-inch conduit making the turn, you'd need 6 × 2 = 12 inches. The additional 1.5-inch conduits add 3 inches to each dimension.

Don't forget: you also need to check that conduit entries are spaced far enough apart on each wall. The NEC requires that where multiple conduits enter a wall, the distance between them (measured center-to-center) must not be less than 6× the largest conduit. This prevents conductors from crossing each other in a way that makes bending radii impossible to maintain.

Use the [pull box sizing calculator](/pull-box-sizing-calculator) to run these numbers automatically — it handles all three pull types and flags the controlling dimension for you.

The U-Pull Formula

A U-pull (sometimes called a parallel pull) happens when conductors enter and exit through the same wall. They come in one conduit, make a U-turn inside the box, and leave through another conduit on the same wall.

**Formula:** Distance from the conduit entry wall to the opposite wall = 6 × (largest conduit entering that wall) + (sum of remaining conduits on same wall)

This is mathematically identical to the angle pull formula — because the physics of the bend is the same. The difference is that you only have one wall to worry about for the turn direction; the opposing wall sets the depth of the U.

**Example:** Two 2.5-inch conduits on the same wall, making a U-turn.

- (6 × 2.5) + 2.5 = 15 + 2.5 = **17.5 inches**

The box needs to be at least 17.5 inches from the entry wall to the opposite wall.

The Depth Requirement

NEC 314.28 doesn't set a specific depth requirement beyond what's needed to accommodate the conduit hubs and maintain wiring space. But in practice, most engineers specify a minimum of 6 inches of depth on any pull box to allow for conduit connectors and to give conductors room to lay without being crushed against the cover.

Check with your AHJ — some jurisdictions have specific depth requirements, and for boxes larger than 24×24, you'll often want at least 8–12 inches of depth to make the conductor management workable.

Applying 314.28 in the Field

Here's how to approach it on a real job:

1. **Identify all conduits entering each wall.** List trade sizes, not actual outside diameters.

2. **Determine the pull type** for each set of conductors. Are they going straight through? Turning? U-turning?

3. **Run the formula for each wall** separately. The box dimension in each direction must satisfy the formula for that direction.

4. **Check conduit spacing** — the 6× rule for center-to-center spacing still applies within each wall.

5. **Order up.** Always round up to the next available standard box size, never down.

The most common field mistake is applying the straight pull formula when you actually have an angle pull configuration. If the conduit routing drawings show any turn inside the box, you're in angle pull territory, and the 8× formula is wrong. See [common pull box sizing mistakes](/pull-box-sizing-mistakes) for a full breakdown of where electricians go wrong.

Conduit Bodies as an Alternative

For single conduits making a 90-degree turn, a conduit body (LB, LR, LL, T, or C fitting) can substitute for a pull box — but only if all conductors are 6 AWG or smaller, per 314.16(C). For 4 AWG and larger, conduit bodies don't cut it unless specifically listed for the application. You need a real box.

Real-World Scenarios

**Commercial feeder run:** You're pulling a 400A feeder with 3/0 AWG conductors in 2.5-inch conduit from the service room to a panel 200 feet away, with a 90-degree turn at a chase wall. This is an angle pull. Using the formula: 6 × 2.5 = 15 inches minimum in both directions. A standard 16×16×6 pull box works for a single-conduit turn.

**Service entrance:** A 2-inch service entrance conduit from underground makes a 90-degree turn to enter the meter socket. Single conduit, angle pull: 6 × 2 = 12 inches. A 12×12 box satisfies the minimum — though many inspectors want a 16×16 here for workability.

**Motor branch circuit:** A 100 HP motor circuit running 350 kcmil in 3.5-inch conduit hits a pull box before transitioning to a 90-degree run into the MCC. Straight pull through the box: 8 × 3.5 = 28 inches minimum box length. You'd order a 30×24×8 or similar.

For any of these, run your dimensions through the [NEC 314.28 pull box calculator](/pull-box-sizing-calculator) before you order material — especially when multiple conduits are involved on the same wall.

Working with Your AHJ

NEC 314.28 is a floor, not a ceiling. Some jurisdictions adopt local amendments that go beyond the NEC minimum. It's worth a quick call to the local AHJ on larger projects before you order boxes, especially for service entrances and utility interconnection work where inspectors often have strong opinions about box sizing.

Also keep in mind that 314.28 doesn't address conductor fill for splices. If you're making connections inside the box — not just pulling through — you'll need additional space per 314.16(B) fill rules, and possibly per 314.28(B) for boxes containing splices. Plan your box size accordingly and don't assume the minimum pull size is enough if you're also splicing.

For more on where pull boxes are required by code (and where conduit bodies work instead), read [when do you need a pull box](/when-do-you-need-a-pull-box) for the full rundown.

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