How to Size a Junction Box

Electricity powers the devices that make the modern world run — from refrigerators to TVs to industrial machinery. But getting that electricity correctly set up and distributed within homes and businesses requires a lot of special equipment. That equipment includes conduits, raceways and the devices that we’ll discuss in this article: junction boxes.

When an electrician creates splits and branches in wiring connections that run through cable raceways and conduits, they enclose the connection points within a junction box. A junction box provides protection for these sensitive connections against bad weather, accidental contact, tampering and other hazards that can damage wiring.

Below, we’ll talk about how to size a junction box appropriately and determine which attributes your junction box needs to have. Before we dive in, we’ll discuss some of the basics everyone should know about what a junction box is and how one works.

What Is a Junction Box?

A junction box is an electrical enclosure that protects electrical conductors at the points where they are spliced, tapped and pulled. These enclosures allow electricians to easily access the conductors to perform work when needed while simultaneously keeping the conductors safe from damage and unauthorized access.

Some junction boxes protect the actual wired connection points and equipment. Others (known as pull boxes) don’t contain connectors, but play a key role in managing long cable runs and simplifying electrical installation.

Sometimes, you’ll hear the term “junction box” used to refer to an electrical fixture box. In the true technical definition of a junction box, wires should connect only to other wires, raceways and conduit bodies. However, in practice, the term is also frequently used to refer to many other electrical box types in which wires connect to a fixture such as a ceiling fan, light switch or wall socket.

For more information on the various applications and designs of junction boxes, make sure to see our complete guide to junction box types.

Considerations When Sizing a Junction Box

Sizing a junction box is critical for making sure the junction box functions correctly. In a junction box that’s too small, an electrician may have to bend conductors excessively to fit them fully inside the box, which risks damaging the insulation on the wires. Conductors and other components spaced too closely may also create interference or hazardous conditions.

Article 314 of the National Electrical Code contains most of the key regulations that you’ll need to follow when choosing, installing and sizing junction boxes. These regulations describe in detail the required junction box size, depending on the number and size of conductors and fittings enclosed within the box.

You’ll need to calculate both box volume and box fill when determining the appropriate size for a junction box. What do these concepts mean?

• Box volume is the total enclosed volume of the junction box, including any extended space provided by attachments such as domed covers or extension rings.
• Box fill is the total space occupied by the components that will be contained within the junction box.

The box volume should always be greater than or equal to the box fill. To calculate box volume and box fill, you’ll need the tables from NEC 314.16 and knowledge of how to use them. Next, we’ll look at how to use these important tools.

How to Size a Junction Box (18 AWG to 6 AWG)

If the conductors in your junction box are of size 18 AWG through 6 AWG, you’ll use the tables in NEC 314.16 to determine the minimum volume of your junction box. For your convenience, the tables are reproduced here.

Here’s table 314.16(A), which outlines the box volume for common trade sizes of metal junction boxes:

This table packs in a lot of information, so take a minute to examine all of its features. The columns on the left list common standard box dimensions and junction box types. The center columns list the minimum volume of a junction box of those dimensions in centimeters and inches, while the columns on the right list the maximum number of conductors in an enclosure, depending on conductor size.

Note, however, that you can only use this table as-is if your junction box doesn’t require any volume allowances for devices like switches and cable clamps. For most applications, that will rarely be the case, so you’ll typically also need to calculate box fill and compare it with this table.

When calculating the box fill of your junction box, you’ll need to reference table 314.16(B):

This table provides individual volumes for different AWG conductor sizes. To simplify calculation, the NEC also provides standards in 314.16(1) - 314.16(5) for calculating the box fill of various types of conductor hardware using the conductor volume values of the conductor sizes associated with the equipment.

Below, you’ll find simplified and abbreviated versions of the standards in NEC 314.16(B):

1. Conductor Fill: Conductors that run through the box or terminate inside it are counted as one conductor volume. Conductors at least twice the minimum free conductor length in NEC 300.14 are counted as two CVs. Conductors that both originate and terminate inside the box are not counted.
   • Example: 2x standard 14 AWG conductors = 2x 14 AWG CVs = minimum volume of 4 cubic inches
2. Clamp Fill: Any number of cable clamps inside the box are counted together as a single CV, based on the largest conductor present. Clamps outside the box aren’t counted.
   • Example: 2x standard 16 AWG cable clamps = 1x 16 AWG CV = minimum volume of 1.75 cubic inches
3. Support Fittings Fill: Each luminaire hickey or luminaire stud is counted as one CV, based on the largest conductor present.
   • Example: 3x 12 AWG luminaire studs = 3x 12 AWG CVs = minimum volume of 6.75 cubic inches
4. Device or Equipment Fill: Each single-device gang is counted as two CV, based on the largest conductor that terminates on the device. A multi-gang yoke wider than 50mm (2 inches) is counted as two CVs for each gang, based on the largest conductor that terminates on the device.
   • Example: 1x 14 AWG strap/yoke = 2x 14 AWG CVs = minimum volume of 4 cubic inches
5. Equipment Grounding Conductor Fill: Any number of grounding conductors or bonding jumpers are counted as a single CV. If your box has an additional set of conductors for an insulated grounding terminal (as outlined in NEC 250.146(D)), these conductors are counted as an additional single CV.
   • Example: 2x 16 AWG grounding conductors = 1x 16 AWG CV = minimum volume of 1.75 cubic inches

Once you have the relevant values, you’ll add them together to arrive at your box fill. Compare the box fill with the box volume from table 314.16(A) to find the minimum box volume you’ll need. If your conductors are all the same size, you can also get a quick estimate of whether your conductors will fit by looking at the right-hand column with the maximum number of conductors for each box size.

There are some important caveats you should be aware of when using these tables, including:

• Standard-sized junction boxes don’t have their volume printed on them, so you’ll need to use these tables to find their volume. Non-standard-sized boxes and non-metallic junction boxes should have their volume printed on the electrical enclosure itself.
• Certain types of components aren’t counted when calculating box fill. These include pigtails, cable and raceway fittings such as bushings and locknuts, wire connectors and some other types of small components.
• All of the requirements we’ve discussed here are minimum requirements. When in doubt, it’s always best to use an electrical enclosure that’s larger than necessary. The only downside is the potentially higher cost of a larger-sized enclosure and space expenditure, but even these drawbacks can be mitigated by carefully planning conduit runs to use a smaller number of larger-sized junction boxes.

Finally, remember that there’s no substitute for following the complete NEC code when sizing a junction box. This guide provides a quick and easy starting point, but be sure that you’re familiar with the relevant sections of NEC 314 in their entirety before choosing a junction box.

How to Size a Junction Box (4 AWG and Above)

If your conductors are of size 4 AWG or larger, you’ll need to follow a different set of standards for sizing your junction box. These larger connectors can have their insulation easily damaged if they’re forced into a junction box that’s too small, so the minimum size must be calculated based on the arrangement of connectors and the size of the largest conduit or raceway present.

You’ll find the standards for these larger-sized conductors in NEC 314.28. Here are the basic procedures for how to size a junction box with 4 AWG conductors or above, depending on what kind of connector arrangement you’re using inside the box:

• Straight Pulls: The distance from the conductor entry to the opposite wall of the enclosure must be at least eight times the trade size of the largest raceway entering the enclosure.
  • Example: If you’re installing a straight pull of a 3-inch raceway through your junction box, the box would need to be at least 24 inches on one side to accommodate the pull. (8 x 3 = 24)
• Angle Pulls: The distance from the conductor entry to the opposite wall of the enclosure must be at least six times the trade size of the largest raceway entering the enclosure, plus the sum of the trade size of any other raceways on the same row and wall.
  • Example: For an angle pull of a 3-inch raceway with another 2-inch raceway entering on the same wall, the box would need to be at least 20 inches on one side. ((6 x 3) + 2 = 20)
• U-Pulls: The distance from the conductor entry to the opposite wall of the enclosure must be at least six times the trade size of the largest raceway entering the enclosure, plus the sum of the trade size of other raceways on the same wall.
  • Example: For a U-pull of a 3-inch raceway that exits the box through another 3-inch raceway on the same wall, the box would need to be at least 21 inches on one side. ((6 x 3) + 3 = 21)
• Splices: The distance from the conductor entry to the opposite wall of the enclosure must be at least six times the trade size of the largest raceway entering the enclosure, plus the sum of the trade size of other raceways on the same wall.
  • Example: For a spliced conductor with three 3-inch raceways entering on the same wall, the box would need to be at least 24 inches on one side. ((6 x 3) + 3 +3 = 24)
• Distance Between Raceways: The minimum distance between raceways that contain the same connector must be at least six times the trade size of the largest raceway, measured from near edge to near edge of the raceway entries.
  • Example: For an angle pull of a 3-inch raceway, the near sides of the raceway’s entry and exit would need to be at least 18 inches apart. (6 x 3 = 18)

If you have more than one type of connection in the same box, use the largest measurement. For example, let’s take a pull box sizing scenario where you have both a straight pull and an angle pull in your junction box. The straight pull’s calculation demands a 24-inch box, while the angle pull’s calculation demands only a 21-inch box. In this case, you’d need to use the larger measurement and purchase a 24-inch enclosure.

Which Junction Box Sizes Are Available?

Polycase offers outdoor electrical junction boxes in a wide range of standard NEMA enclosure sizes. We have a full selection of both metallic and non-metallic junction boxes that are built for the highest standards of NEMA and IP ingress protection, including popular ratings like NEMA 4X enclosures and IP68 enclosures.

Our junction boxes are also easy to modify with custom-machined enclosure cutouts. We can perform most types of custom cutouts with minimal scrap and ultra-precise tolerances of just +/- 0.005”. If you need a quick set of standard raceway cutouts, our Polycase SK Series comes equipped with easy-to-use knockouts that are perfect for many common junction box configurations.

At Polycase, we make it easy to find the right junction box for your application through our friendly, knowledgeable customer service and our industry-leading selection of electrical enclosures. Just call us at 1-800-248-1233 for in-depth guidance on how to size a junction box – or contact us online with your questions.