The inspector opens your ジャンクションボックス. She counts wires. Checks the box stamp. Counts again. Then comes the red tag. “NEC 314.16 violation—box fill exceeded. This needs to be completely redone before I can sign off.”
That failed inspection just cost you a week of delays and several thousand dollars in rework—all because of a $4 junction box that was two cubic inches too small. Welcome to what electricians call “The Volume Trap.”
A junction box houses wiring connections, protecting them from damage and containing sparks to prevent fires. But sizing it correctly isn’t just best practice—it’s a critical safety requirement mandated by the National Electrical Code (NEC). An undersized box creates overcrowded wires, causing insulation damage, overheating, and creating serious fire hazards .
The NEC provides two distinct methods for sizing junction boxes, depending on wire size:
- NEC 314.16 (Box Fill): For smaller conductors (6 AWG and smaller), sizing is based on total volume required. Think of it as “The Fill Factor”—every component inside that box gets a vote, and you need to count them all.
- NEC 314.28 (Pull Box): For larger conductors (4 AWG and larger), sizing is based on physical dimensions required to safely pull and bend thick cables. This is where “The 8x Rule” comes into play.
This guide walks you through both calculation methods to ensure your installations are safe and code-compliant.
Part 1: Sizing for Small Conductors (6 AWG and Smaller) – NEC 314.16 Box Fill Calculation
When working with common household wiring sizes like 14 AWG, 12 AWG, or 10 AWG, you must use the “box fill” calculation. This method determines the minimum required cubic inch (cu in) volume of the box. Miss this calculation, and you’ll meet The Volume Trap firsthand—usually during an inspection you thought was routine.
How to Calculate Box Fill
Follow these steps to determine total volume needed. Each component in the box is assigned a “conductor equivalent” value based on the largest wire size connected to it. Think of it as democracy for wires—every element gets a vote, but some count for more than others.
- 導体： Count each wire that originates outside the box and terminates or is spliced inside it as one conductor.
- Devices: Count each device (e.g., a switch or receptacle) as 二つの conductors. Devices are double-voters in this election.
- Clamps: Count all internal cable clamps together as one conductor—they vote as a bloc.
- Ground Wires: Count all bare copper or green ground wires together as one conductor. Ground wires travel in packs; they all count as one.
- Fixture Studs/Hickeys: Count each fixture stud or hickey as one conductor .
Once you have the total conductor equivalent, multiply that number by the volume allowance for that wire size, found in the table below.
Table 1: Wire Volume Allowance (per NEC 314.16(B))
| ワイヤーサイズ | Volume per Conductor (cu in) |
|---|---|
| 14AWG | 2.00 cu in |
| 12AWG | 2.25 cu in |
| 10AWG | 2.50 cu in |
| 8AWG | 3.00 cu in |
| 6AWG | 5.00 cu in |
Table 2: Standard Box Volumes (per NEC 314.16(A))
| Box Type | Minimum Volume |
|---|---|
| Single-gang | 18 cu in |
| 2-gang | 36 cu in |
| 4″ Square | 30.3 cu in |
| 4-11/16″ Square | 42 cu in |
Box Fill Example
シナリオ You’re installing a single receptacle in a box. Two 12/2 non-metallic sheathed cables (each with hot, neutral, and ground) enter the box, which has internal clamps. Here’s how the votes stack up:
- 導体： 2 hot + 2 neutral = 4
- Ground Wires: The two ground wires count as 1 (traveling as a pack)
- Clamps: The internal clamps count as 1 (voting as a bloc)
- Device: The receptacle counts as 2 (double-voter)
Total Conductor Equivalents: 4 + 1 + 1 + 2 = 8
Volume Calculation: The wire size is 12 AWG, which requires 2.25 cu in per conductor.
8 conductors × 2.25 cu in/conductor = 18.0 cu in
You must select a box with at least 18.0 cu in of volume. A standard single-gang box (18 cu in) meets this minimum requirement exactly—which means you’re at 100% capacity. In electrical work, “exactly at capacity” is another way of saying “go up one size if you value your inspection approval.”
Part 2: Sizing for Large Conductors (4 AWG and Larger) – NEC 314.28 Pull Box Calculation
For large conductors (4 AWG and larger), the focus shifts from volume to physical dimensions. The wires are thicker. The rules are simpler. The box must be large enough to allow conductors to be pulled through and bent without damaging their insulation. The rules depend on pull type.
Straight Pulls
In a straight pull, conductors enter one side of a box and exit through the opposite wall. This is where The 8x Rule earns its reputation for elegant simplicity.
- Rule: The length of the box must be at least eight times (8x) the trade size of the largest raceway (conduit) .
A 2-inch conduit entering and exiting opposite walls? Your box needs to be at least 16 inches long (2″ × 8 = 16″). That’s it.
Angle Pulls, U-Pulls, or Splices
This applies when conductors enter one wall and exit through an adjacent wall (angle pull), the same wall (U-pull), or are spliced in the box.
- Rule: The distance from the raceway entry to the opposite wall must be at least six times (6x) the trade size of the largest raceway, plus the sum of the diameters of all other raceways on the same wall and in the same row . Call this “The Six-Times Safety Net.”
Distance Between Raceways
- Rule: The distance between raceways enclosing the same conductor run must be at least six times (6x) the trade size of the largest raceway.
Pull Box Example (Combination Pulls)
When a box contains both straight and angle pulls, you must calculate for both and use the largest resulting dimension for each direction (horizontal and vertical).
シナリオ A pull box has the following raceways:
- Left Wall: One 3″ and one 2″ raceway.
- Right Wall: One 2″ raceway.
- Top Wall: One 3″ raceway.
- The 2″ raceways are for a straight pull (left to right).
- The 3″ raceways are for an angle pull (left to top).
1. Calculate Horizontal Dimension (Length):
- Straight Pull (Left to Right): The largest raceway in this pull is 2″.
- 8 × 2″ = 16″
- Angle Pull (Originating on Left Wall): The largest raceway is 3″. The other raceway on that wall is 2″.
- (6 × 3″) + 2″ = 18″ + 2″ = 20″
- Result: Choose the largest dimension: 20 inches.
2. Calculate Vertical Dimension (Height):
- Angle Pull (Originating on Top Wall): The largest (and only) raceway is 3″.
- 6 × 3″ = 18″
- Result: The vertical dimension is 18 inches.
Final Box Size: The box must be at least 20″ long by 18″ high.
安全への配慮
Before starting any electrical work, always prioritize safety.
- Turn Off Power: De-energize the circuit at the メインブレーカー panel.
- Verify Power is Off: Use a voltmeter or non-contact circuit tester to confirm there is no voltage in the wires you will be handling .
- Use Insulated Tools: Work with tools designed for electrical work.
- Wear PPE: Safety glasses and insulated gloves are recommended.
- Work in a Clear, Lit Area: Ensure your workspace is well-lit and free of obstacles.
共通の間違いを避ける
- Overcrowding the Box: Never exceed the calculated box fill. This is the most common cause of overheating and electrical fires—and the #1 reason for failed inspections. The Volume Trap claims another victim.
- Choosing the Wrong Box Type: Use weatherproof boxes for outdoor or damp locations and metal boxes for metal-clad (MC) cable or conduit.
- Incorrect Placement: Install boxes in accessible locations. Do not bury them inside a finished wall where they cannot be serviced. Inspectors have long memories for boxes they can’t find.
- Poor Grounding: Ensure all ground wires are securely connected to each other and to metal boxes to prevent shock hazards.
- Ignoring Local Codes: The NEC is a national standard, but your local jurisdiction may have additional requirements. Always check local regulations . What passes in Phoenix might fail in Chicago.
結論
Properly sizing a junction box is a fundamental skill for safe and reliable electrical work. By understanding and correctly applying The Fill Factor (NEC 314.16 for small conductors) and The 8x Rule (NEC 314.28 for large conductors), you can ensure your installations are compliant, functional, and—most importantly—safe.
And when that inspector opens your junction box? You’ll get the green tag, not the red one.
免責事項: This guide is for informational purposes only. Electrical work can be dangerous. Always consult the latest version of the National Electrical Code, check local amendments, and consider hiring a qualified electrician for any work you are not comfortable performing yourself.
