If you’ve ever been caught off-guard by an electrical inspector citing NEC 314.28, you’re not alone.The difference between a pull box, junction box, and tap box isn’t just semantic—it directly impacts sizing requirements, installation costs, and code compliance. Whether you’re an electrical contractor, engineer, or facility manager, understanding these distinctions can save you thousands in materials and prevent costly project delays. In this comprehensive guide, we’ll break down the pull box vs junction box sizing rules, explain the critical NEC 314.28 calculations, and show you how proper nomenclature on your blueprints affects enclosure dimensions. Master these electrical code requirements and avoid the “coffin-sized box” surprise that catches so many professionals off-guard.
要点
- Pull boxes require 8x conduit diameter for straight pulls (NEC 314.28)
- Junction boxes use volume calculations based on conductor count, not conduit size
- Cutting and splicing wire can convert a pull box to a junction box, reducing size requirements
- Tap boxes need extra space for power distribution blocks and wire bending clearance
- Conductors 4 AWG and larger trigger strict NEC 314.28 geometric formulas
- Proper blueprint labeling (PB vs JB vs TB) determines which sizing rule applies
The Real-World Problem: When Box Labels Dictate Box Sizes
You’re reviewing an electrical floor plan. You see a box marked “PB” (Pull Box) located in a tight mechanical room. You assume it’s a standard 12×12 enclosure.
Then the electrical inspector arrives and fails the installation. He points to NEC Article 314.28 and says, “That box needs to be 32 inches long, not 12.”
You’re baffled. It’s just a metal box with wires passing through it. Why does it need to be the size of a coffin?
The answer lies in the name. In the eyes of the National Electrical Code (NEC), a Pull Box (PB), a Junction Box (JB), and a Tap Box (TB) aren’t just different labels—they represent fundamentally different electrical installations with distinct sizing requirements.
One name triggers a strict geometric formula based on conduit sizing (the “8x Rule”). The other name triggers a simple volume calculation. Using the wrong acronym on your blueprint can cost you thousands of dollars in steel and space.
Pull Box vs Junction Box vs Tap Box: Understanding the Definitions
To properly size any electrical enclosure, you must first identify the action happening inside the box. The wire’s behavior—whether it’s pulled through, cut and spliced, or distributed—determines which NEC sizing rule applies.
Quick Reference Comparison Table
| Box Type | 主要機能 | Wire Action | Sizing Rule | コードリファレンス |
|---|---|---|---|---|
| Pull Box (PB) | Wire passage | Continuous (uncut) | 8x conduit diameter | NEC 314.28 |
| Junction Box (JB) | Wire splicing | Cut and connected | Volume calculation | NEC 314.16 |
| Tap Box (TB) | 配電 | Split into branches | Hardware + bending space | NEC 312.6 |
プルボックス (PB): “The Gymnasium”
The Action: Wires enter, pass through, and exit without being cut. The conductor remains continuous from end to end.
The Physics Behind the Rule: Because the wire is continuous and stiff (often 4 AWG or larger), electricians need massive physical space to pull the loop out, create slack, and feed it back in without kinking the insulation or damaging the conductor.
The Sizing Rule: Strict Linear Geometry (NEC 314.28). Box dimensions are determined by the diameter of the conduit, not the size of the wire inside it.
一般的な用途： Long conduit runs, vertical risers, underground-to-overhead transitions.
ジャンクションボックス (JB): “The Surgery Room”
The Action: Wires enter, get cut, and are spliced together using wire nuts, compression lugs, or terminal blocks.
The Physics Behind the Rule: Once a wire is cut, it loses its tension and becomes limp and easy to manipulate. You don’t need room to pull continuous cable; you just need adequate space to store the splice safely.
The Sizing Rule: Volume (Fill) & Bending Radius. Box size is determined by the quantity and size of conductors, following NEC 314.16 volume calculations.
一般的な用途： Branch circuit connections, lighting control points, device installations.
Tap Box (TB): “The Distribution Hub”
The Action: A large feeder enters and splits into multiple smaller feeders or branch circuits.
The Physics Behind the Rule: This is essentially a heavy-duty junction box populated with Power Distribution Blocks (PDBs) that require substantial mounting space and wire bending clearance.
The Sizing Rule: Governed by wire bending space requirements (NEC 312.6) and the physical dimensions of the distribution hardware.
一般的な用途： Electrical rooms, service equipment, multi-tenant metering assemblies.
The NEC 314.28 Sizing Trap: Calculations for Conductors 4 AWG and Larger
Here’s where many electrical projects go wrong. When your installation involves conductors 4 AWG and larger, you must apply NEC 314.28 sizing rules. These aren’t simple volume calculations—they’re geometric formulas based on conduit diameter.
The 8x Rule: Straight Pull Sizing Requirements
If you label a box as a “Pull Box” and the conduit enters one side and exits the opposite side (straight-through configuration):
NEC 314.28(A)(1) Rule: The length of the box must be at least 8 times the trade diameter of the largest raceway.
Real-World Example:
You have a 4-inch conduit carrying 500 MCM cable in a straight pull configuration.
- 計算： 4 inches × 8 = 32 inches
- Required Box Length: 32 inches minimum
- Common Mistake: Ordering a standard 12×12 or 18×18 box (both fail inspection)
ト: Always identify your largest conduit diameter first, then multiply by 8 before specifying box dimensions on your submittal drawings.
The 6x Rule: Angle Pull and U-Pull Sizing Requirements
When conduit enters and exits at different angles (90° turn) or exits the same wall (U-pull configuration), the calculation becomes more complex:
NEC 314.28(A)(2) Rule: The distance to the opposite wall must be 6 times the trade diameter of the largest raceway, PLUS the sum of the diameters of all other raceways on the same wall.
Real-World Example:
You have a 4-inch conduit and a 2-inch conduit entering the bottom wall, with exits on the side wall.
- 計算： (4 inches × 6) + 2 inches = 26 inches
- Required Dimension: The vertical dimension must be at least 26 inches
Why This Matters: Angle pulls require extra space because the wire must bend around corners. The 6x multiplier ensures adequate bending radius to prevent insulation damage during installation.
Common Sizing Mistakes and How to Avoid Them
Mistake #1: Using wire size instead of conduit diameter for calculations.
- ❌ Wrong: “I have 500 MCM wire, so I’ll use that measurement”
- ✅ Correct: “My 500 MCM wire is in a 4-inch conduit, so I multiply 4 × 8”
Mistake #2: Forgetting to add all raceway diameters in angle pull calculations.
- ❌ Wrong: Only multiplying the largest conduit by 6
- ✅ Correct: Multiplying largest by 6, then adding all others
Mistake #3: Applying pull box rules to junction boxes.
- ❌ Wrong: Using 8x rule when wires will be spliced
- ✅ Correct: Identifying the wire action first, then applying the appropriate rule
The “Cut-Wire Loophole”: Converting a Pull Box to a Junction Box
Here’s a field-tested strategy known by veteran electrical contractors and project managers—though it comes with important trade-offs.
The Situation
You ordered a 24-inch box for what you thought was a simple pass-through involving a 4-inch conduit. During inspection, you realize NEC 314.28 requires 4 × 8 = 32 inches. Your box is 8 inches too short, and the inspector is writing a violation.
ソリューション
The electrician cuts the continuous wire and re-connects it with a properly-rated splice kit, compression lug, or mechanical connector inside the box.
By introducing a splice, you’ve transformed the “Pull Box” into a “Junction Box”:
- Pull Box Requirement: 8x conduit diameter = 32 inches
- Junction Box Requirement: Adequate volume for conductor fill and bending radius (often 24 inches is sufficient)
Suddenly, your 24-inch box is code-compliant.
The Trade-Off Analysis
利点がある：
- Saves replacement box costs ($200-$500 depending on size)
- Avoids delays when space is physically constrained
- Legitimate code-compliant solution when done properly
デメリット
- Every splice is a potential failure point
- Increases long-term maintenance requirements
- May void certain warranty provisions
- Requires proper splice hardware (adding material costs)
When to Use This Strategy:
This approach makes sense in renovation projects where a larger box physically won’t fit between existing structural elements. However, for new construction with adequate planning, always size boxes correctly from the start.
Best Practice: If you must introduce a splice, use high-quality mechanical connectors or compression lugs rated for the full conductor ampacity, and document the splice location on as-built drawings for future maintenance.
Sizing Tap Boxes: It’s All About the Hardware
While “Tap Box” isn’t a strict NEC-defined term like “Pull Box,” it represents a specific installation type that requires careful sizing consideration beyond simple pull-through calculations.
What Makes a Tap Box Different
A tap box contains Power Distribution Blocks (PDBs)—specialized hardware that allows one large feeder to split into multiple smaller feeders or branch circuits. Think of it as an electrical junction point for power distribution.
Sizing Constraints for Tap Boxes
Constraint #1: Distribution Block Dimensions
The box must be deep enough to mount the PDB hardware, which can be substantial for high-amperage applications.
- 400A PDB: Typically 8-12 inches deep
- 600A PDB: Often 12-18 inches deep
- 1200A+ PDB: May require 20+ inches depth
Constraint #2: Wire Bending Space (NEC 312.6)
The box must provide adequate “Wire Bending Space” between the lug and the enclosure wall. This isn’t about pulling tension—it’s about the physical space needed to bend large conductors into terminal connections.
NEC 312.6(B) Wire Bending Space Requirements:
| ワイヤーサイズ | Minimum Bending Space |
|---|---|
| 250-350 kcmil | 4 inches |
| 400-500 kcmil | 5 inches |
| 600-700 kcmil | 6 inches |
| 750-900 kcmil | 8 inches |
Real-World Example:
For a tap box with 500 kcmil feeders connecting to a distribution block:
- Distribution block depth: 12 inches
- Wire bending space: 5 inches (per NEC 312.6)
- Wall thickness and hardware: 2 inches
- Minimum box depth: 19 inches
Professional Sizing Process for Tap Boxes
- Select your distribution hardware first (don’t guess the box size)
- メーカーの仕様を確認する for mounting dimensions
- Calculate NEC 312.6 wire bending space for your largest conductors
- Add clearances for heat dissipation (especially for high-amperage installations)
- Specify the enclosure based on total calculated dimensions
ト: When working with high-quality distribution equipment, always consult the manufacturer’s installation instructions. They often provide recommended enclosure dimensions that exceed minimum code requirements for optimal performance and heat management.
Conduit Sizing and Box Selection: Making the Right Choice
Understanding the relationship between conduit sizing, wire fill, and box requirements is essential for code-compliant electrical installations.
Start with Wire Fill Calculations
Before you can size a pull box, you need to know your conduit diameter, which depends on conductor fill:
NEC Chapter 9, Table 1 – Maximum Fill Percentages:
- One conductor: 53% fill
- Two conductors: 31% fill
- Three or more conductors: 40% fill
計算例:
Installing (3) 500 kcmil THHN conductors:
- Individual conductor area: 0.7073 sq. inches (NEC Chapter 9, Table 5)
- Total conductor area: 2.12 sq. inches
- Required conduit at 40% fill: 3.5-inch or 4-inch trade size
Once you know the conduit size (4-inch), you can apply the pull box formula: 4 × 8 = 32 inches.
材料選択の考慮点
Steel vs. Aluminum vs. Non-Metallic:
| 素材 | 利点 | 一般的なアプリケーション |
|---|---|---|
| Galvanized Steel | High strength, cost-effective | General-purpose, indoor installations |
| ステンレス鋼 | 耐食性 | Coastal, chemical, washdown areas |
| アルミニウム | Lightweight, corrosion-resistant | Outdoor, rooftop, weight-sensitive |
| PVC/Fiberglass | Non-conductive, corrosion-proof | Hazardous locations, underground |
Box Rating Considerations:
- NEMA 1: Indoor, general purpose
- NEMA 3R: Outdoor, rain-tight
- NEMA 4/4X: Washdown, corrosive environments
- NEMA 12: Indoor, dust and drip protection
Blueprint Best Practices: Proper Labeling Saves Money
The nomenclature you use on electrical drawings has real-world cost implications. Here’s how to specify boxes correctly and avoid expensive change orders.
Standard Blueprint Notation
Clear Labeling Format:
PB-32x12x8 (NEMA 3R)
↓ ↓
Size Rating
Include These Details:
- Box type designation (PB, JB, or TB)
- Dimensions (Length × Width × Depth)
- NEMA rating or environmental requirements
- Mounting method (surface, flush, ceiling)
- Material specification if non-standard
Coordination with Other Trades
Mechanical Coordination:
- Verify adequate clearance around HVAC equipment
- Check for conflicts with ductwork and piping
- Confirm accessibility for maintenance
Structural Coordination:
- Ensure adequate support for heavy enclosures (TB boxes can exceed 200 lbs loaded)
- Verify mounting surface can support weight
- Check for conflicts with structural framing
Architectural Coordination:
- Confirm finished ceiling heights allow for box depth
- Verify door swing clearances in tight spaces
- Coordinate with finish schedules for trim requirements
Electrical Code Compliance Checklist
Use this checklist before your inspection to ensure NEC 314.28 compliance:
For Pull Boxes (4 AWG and Larger):
- ☐ Measured largest conduit trade diameter correctly
- ☐ Applied 8x rule for straight pulls (multiply largest conduit × 8)
- ☐ Applied 6x rule for angle pulls (multiply largest × 6, then add all others)
- ☐ Verified box length meets or exceeds calculation
- ☐ Confirmed all conduit entries are properly secured
- ☐ Ensured adequate bonding/grounding per NEC 314.4
For Junction Boxes (Any Size):
- ☐ Calculated total conductor volume per NEC 314.16
- ☐ Accounted for clamps, fittings, and devices in fill calculation
- ☐ Ensured adequate wire bending radius inside box
- ☐ Verified splice methods are appropriate for conductor type and ampacity
- ☐ Confirmed box is properly supported per NEC 314.23
For Tap Boxes:
- ☐ Selected distribution block hardware first
- ☐ Calculated wire bending space per NEC 312.6(B)
- ☐ Added hardware mounting depth to calculation
- ☐ Verified adequate heat dissipation for ampacity
- ☐ Confirmed accessibility for future maintenance
よくある質問（FAQ）
What is the difference between a pull box and a junction box?
A pull box contains continuous conductors that pass through without being cut, requiring sizing based on conduit diameter (8x rule per NEC 314.28). A junction box contains spliced conductors and is sized based on volume calculations. The key difference is whether wires are cut and connected inside the box.
When does NEC 314.28 apply to electrical boxes?
NEC 314.28 applies specifically to boxes containing conductors 4 AWG and larger. For smaller conductors (6 AWG and below), use NEC 314.16 volume calculations instead. The 8x and 6x sizing rules only apply to larger conductor installations.
Can I use a junction box instead of a pull box to save space?
Yes, but only if you’re willing to introduce splices. By cutting the continuous conductors and properly splicing them inside the box, you convert a pull box installation to a junction box installation, allowing smaller dimensions. However, every splice is a potential failure point, so this should only be done when space constraints are unavoidable.
How do I calculate pull box size for multiple conduits?
For straight pulls, use 8x the largest conduit diameter. For angle pulls or U-pulls, use 6x the largest conduit diameter, then add the diameters of all other conduits entering/exiting the same wall. Always use conduit trade size (diameter), not wire size, for these calculations.
What is a tap box and when is it required?
A tap box is an enclosure containing power distribution blocks (PDBs) that split one large feeder into multiple smaller feeders. While not a specific NEC term, tap boxes require sizing based on the distribution block dimensions plus NEC 312.6 wire bending space requirements. They’re commonly used in electrical rooms and multi-tenant service installations.
Do I need a pull box for underground to overhead transitions?
Yes, when conductors 4 AWG and larger transition from underground conduit to overhead conduit, a pull box sized per NEC 314.28 is typically required. This provides adequate space for the pulling operation and prevents damage to conductor insulation during installation. Consult NEC 300.5 for additional underground installation requirements.
Conclusion: Mastering Box Sizing for Code-Compliant Installations
Understanding the distinctions between pull boxes, junction boxes, and tap boxes—and knowing when to apply NEC 314.28 sizing rules—is fundamental to successful electrical installations. The “8x rule” for straight pulls and “6x rule” for angle pulls aren’t arbitrary requirements; they’re engineered to ensure safe installation practices and protect conductor integrity.
Key Principles to Remember:
- Action dictates sizing: Identify what’s happening inside the box (pulling, splicing, or distributing) before calculating dimensions
- Conduit diameter, not wire size: NEC 314.28 calculations use conduit trade diameter
- 4 AWG threshold: Larger conductors trigger strict geometric formulas
- Blueprint accuracy matters: Proper labeling (PB vs JB vs TB) prevents costly field changes
- When in doubt, size up: Larger boxes are easier to work in and future-proof your installation
Whether you’re an electrical contractor bidding projects, an engineer designing systems, or an inspector ensuring code compliance, mastering these sizing principles will save time, money, and frustration on every job.
Need Expert Guidance on Your Next Project?
Selecting the right enclosure and ensuring code compliance doesn’t have to be complicated. For high-quality electrical enclosures, power distribution blocks, and expert technical support, consult with experienced manufacturers who understand both NEC requirements and real-world installation challenges.
Ready to specify your next pull box, junction box, or tap box installation? Review your conductor sizes, calculate your box dimensions using the formulas in this guide, and always verify with the latest edition of the NEC before finalizing your design.
Have questions about specific installations or need help interpreting NEC 314.28 for your project? Bookmark this guide and refer back whenever you’re sizing electrical enclosures for conductors 4 AWG and larger.
Disclaimer: This guide provides general information about NEC 314.28 and electrical box sizing requirements. Always consult the current edition of the National Electrical Code and local amendments, and seek guidance from licensed electrical professionals for specific installations. Code requirements may vary by jurisdiction.
