Large conductors need room to bend without damage, so the NEC sizes pull and junction boxes from the trade size of the conduits rather than from conductor volume. For straight pulls the box must be eight times the largest conduit long; for angle and U pulls, six times the largest conduit plus the diameters of the other conduits entering the same wall.
How to use the Junction Box Sizing Calculator
- Pull type
- Largest conduit (trade size)
- Sum of other conduits in the same row
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the type of pull, the largest conduit trade size and the sum of the trade sizes of the other conduits on the same wall.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Pull type | Choose one: Straight pull, Angle or U pull |
| Largest conduit (trade size) | Choose one: ½ in, ¾ in, 1 in, 1¼ in … |
| Sum of other conduits in the same row | Units: in, cm, mm, ft, m |
Example values
How the main result changes when Sum of other conduits in the same row changes and everything else stays at the example values above:
| Sum of other conduits in the same row (in) | Minimum box dimension (in) |
|---|---|
| 1 | 16 |
| 1.5 | 16 |
| 2 | 16 |
| 2.5 | 16 |
| 3 | 16 |
| 4 | 16 |
Formula
Straight pull: length ≥ 8 × largest raceway trade sizeAngle or U pull: distance ≥ 6 × largest raceway + Σ other raceways in the same rowRaceways enclosing the same conductor: ≥ 6 × the larger raceway apartWorked example
Inputs — Pull type: Straight pull; Largest conduit (trade size): 2 in; Sum of other conduits in the same row: 2 in.
Results — Minimum box dimension = 16 in; Min. distance between entries of the same conductor = 12 in.
Frequently asked questions
How big should a pull box be for 2 in conduit?
16 in long for a straight pull, or 12 in plus the other conduits on that wall for an angle pull.
Does NEC 314.28 apply to small wires?
It applies when conductors are 4 AWG or larger; smaller conductors use the box-fill rules of 314.16.
What is a U pull?
The conductors enter and leave through the same wall of the box.
What formula does the Junction Box Sizing Calculator use?
Straight pull: length ≥ 8 × largest raceway trade size.