Junction Box Sizes

What Exactly Is a Junction Box?

Any box mainly used for safely connecting, organizing and protecting multiple electrical wires is called a junction box.

Choosing the right specifications and sizes of junction boxes is crucial to the installation, wiring capacity, safety and neatness of your project. An oversized box wastes space and incurs additional costs, while an undersized one will be incompatible with the project. In this guide, I will elaborate on the standard sizes, specifications, sealed openings and wire capacity of junction boxes, and show you how to select the appropriate junction box according to your project.

Common Size Categories of Junction Boxes

Example: By external dimensions (Length × Width × Height)

• Small size: Commonly used for low-voltage systems, lighting, household wiring, LED applications, etc.
• Medium size: Commonly used for indoor electrical wiring and branch circuits in distribution boxes.
• Large size: Commonly used for industrial applications, multi-circuit wiring, and outdoor centralized wiring.

Small enclosures: Ideal for installations with operating pressure ≤24V, low current, and limited installation space.

Medium enclosures: Can accommodate slightly more wires and terminals, supporting internal wire splitting and transfer. Flexible installation, moderate structural strength. Neither as cramped as small boxes nor as bulky as large industrial boxes, offering high cost performance.

Large enclosures: Must feature high IP rating, corrosion resistance, weatherability, etc. Large internal space, strong load-bearing capacity, and capable of handling complex wiring layouts.

Specifications vary slightly by manufacturer with no mandatory global unified standards, but common industry practices apply.

By Inlet / Opening Specifications

Common types include:

• G9 / PG11 / PG13.5 / PG16 / PG21
• M12 / M16 / M20 / M25
• 1/2", 3/4" NPT (American standard)

Cable glands must be matched to the corresponding dimensions to ensure a tight connection at the joint and edge, achieving high-strength waterproof performance.

By Internal Wiring Capacity

Small size

• Applicable wire cross-section: 0.5–2.5 mm²
• Number of wires accommodated: 2–6 pcs
• Suitable circuits: 1–2 circuits

Small internal space, only withstands local heat generated by low current. Mostly matched for low-current, low-voltage scenarios.

Medium size

• Applicable wire cross-section: 1.5–4.0 mm²
• Number of wires accommodated: 4–12 pcs
• Suitable circuits: 2–4 circuits

Moderate internal space, capable of holding terminal blocks and wiring.

Large size

• Applicable wire cross-section: 2.5–6.0 / 10 mm²
• Number of wires accommodated: 10–25 pcs and above
• Suitable circuits: 4–12 circuits and above

Large internal space, suitable for multi-circuit layout with large cross-section wires.

Size Differences Between Plastic & Metal Junction Boxes

Plastic: All components including bolts are plastic, providing 360° non-conductivity. Significant differences in volume and weight compared to metal junction boxes, as well as certain differences in electrical performance. More standardized dimensions, lighter and thinner housing, lower cost. Commonly used for indoor or outdoor waterproof applications.

Metal housing: Used in explosion-proof, high-temperature and industrial environments. Larger volume, higher cost. For general applications without special requirements, plastic housing is an ideal choice.

How to Choose the Right Size for Your Scenario

• LED / Indoor lighting: Choose small sizes (80/100/120 series)
• Outdoor waterproof, garden lights, surveillance: Medium size + IP65/IP66/IP67
• Industrial equipment, control cabinets, multiple wires: Large size + multiple openings
• Solar / PV wiring: Special sizes + high protection level
• Ships, harsh environments: Thickened and enlarged models

Unsure about the size? We can provide drawings / custom sizes.---CNPNJI

If you need a complete junction box size chart or custom dimensions for your project, feel free to contact us for a free quote and catalog.