Junction Box Sizing Calculator
This junction box sizing calculator helps you determine the minimum required dimensions for electrical pull boxes and junction boxes in compliance with the National Electrical Code® (NEC® 2020) Article 314.28. Whether you're working on a straight pull, angle pull, or U-pull configuration, this tool ensures your boxes meet code requirements every time.
Finding the Right Electrical Junction Box Size
Until now, the easiest way to transport electricity from a power source to any equipment throughout a structure is through electrical wires. Connecting various devices like lighting fixtures and outlets to the power source means lots of cables to manage.
Since we don't want our cables lying around the floor or within the walls and ceiling, we use conduits like tubings and pipes to keep the wires together organized. We then connect these conduits using connectors and boxes where we can reorganize the wires to direct them to the locations they need to be.
We call these boxes our junction boxes or pull boxes (as we pull wires to them before inserting them back into other conduits). Just like when choosing the correct wire sizes or wire gauges suitable for our electrical needs, these boxes should also have the appropriate dimensions to accommodate the wires that enter them. We'll also have enough room for the necessary bending of cables so we don't end up folding the wires sharply, which could damage their insulation.
How to Size Junction Boxes (NEC® 314.28)
The NFPA® sets out guidelines in NEC® 2020 Article 314.28 for pull and junction box sizing. The rules depend on the pull type:
Straight Pulls
For a straight pull (conduit enters on one wall and exits on the opposite wall):
- Minimum box length = 8 × trade size of the largest conduit
Example: If the largest conduit is 2 inches, the box must be at least 16 inches long in the direction of pull.
Angle Pulls and U-Pulls
For an angle pull (conduit enters on one wall and exits on an adjacent wall) or a U-pull (conduit enters and exits on the same wall), the formula for each direction is:
- Minimum dimension = 6 × (largest conduit on that wall) + sum of all other conduit trade sizes on the same wall
This calculation must be performed for each wall/direction independently. The box dimensions must satisfy both calculations.
Measurement Systems
This calculator supports both measurement systems commonly used in electrical work:
- Imperial (US) — conduit trade sizes in inches (e.g., 1/2", 3/4", 1", 1-1/2", 2", 3", 4")
- Metric — conduit trade sizes in millimeters (e.g., 16 mm, 20 mm, 25 mm, 32 mm, 50 mm, 100 mm)
How to Use This Junction Box Sizing Calculator
- Select the measurement system — Imperial (inches) or Metric (mm)
- Choose the pull type — Straight Pull, Angle Pull, or U-Pull
- Enter conduit sizes for Wall 1 — enter the trade size of each conduit entering that wall. Use 0 for unused positions.
- For Angle/U-Pull, enter conduit sizes for Wall 2 as well
- Click Calculate to get the minimum required box dimensions per NEC® 314.28
FAQs
What is the difference between a junction box and a pull box?
Both terms refer to enclosures used in electrical systems. A pull box is used primarily to facilitate pulling wire through conduit, while a junction box is where conductors are spliced or connected. The NEC® Article 314.28 sizing rules apply to both.
What does "trade size" mean?
Trade size is the nominal designation for conduit size used in the United States. For example, a "2-inch" conduit has a trade size of 2, but its actual outside diameter is slightly larger. For this calculator, simply use the trade size as stated on the conduit or specifications.
Can I use this calculator for conduit bodies (LBs, Ts, Cs)?
No. Conduit body sizing follows NEC® Article 314.16 rules, not 314.28. This calculator applies only to pull boxes and junction boxes per Article 314.28.
What is the NEC® box fill (Article 314.16)?
Box fill refers to the volume requirements for outlet boxes containing conductors, devices, and fittings. This is different from pull/junction box sizing and is governed by NEC® Article 314.16. See our Box Fill Calculator for that calculation.
Are these minimum dimensions?
Yes. The values calculated here are the minimum dimensions required by NEC® 314.28. You may always use a larger box than the minimum.
Disclaimer
This calculator is provided for estimation purposes only and is not intended to substitute professional expertise. Always consult a licensed electrician and verify compliance with your local jurisdiction's adopted electrical code. The NEC® is adopted differently across states and municipalities — your Authority Having Jurisdiction (AHJ) has final say on code compliance.