1. What Is a Junction Box? (Definition and Classification)
A junction box is asealed enclosure for branching, connecting, or transitioning conductors, classified by purpose:
| Type | Application | Typical applications |
|---|---|---|
| Branch box | Branching a sub-circuit off a main line | Cable trench / cable tray branching |
| Transition box | Transitioning between different cable sizes | Cable cross-section changes |
| Pull box | Facilitating pulling at conduit bends | Long conduit runs |
| Junction box | Equipment incoming/outgoing connections | Lighting fixtures / switches / receptacles |
2. Why Use a Junction Box? (3 Key Functions)
- Branching: When a main cable needs to branch off at a point, the connection must be made inside a box — never by stripping insulation and twisting wires directly.
- Transition: When cable cross-section changes (e.g., 6mm² to 2.5mm²) or material changes (copper-to-aluminum transition), it must be done inside a box.
- Protection: Splices are sealed inside the box, keeping out dust, water and rodents. The IP rating is selected to suit the environment.
3. What Happens Without a Junction Box? (3 Risks)
① Exposed splices→ moisture ingress → rising contact resistance → overheating → fire; ② No mechanical protection→ pulling force → loosened splice → open circuit or short circuit; ③ Hard to service→ splices buried in walls or conduit → fault cannot be located → wall must be opened up.
4. Junction Box Selection: Material and Protection (Comparison of 4 Materials)
| Material | IP rating | Where it is used | Price |
|---|---|---|---|
| ABS plastic | IP20–IP44 | Indoor, dry | Low |
| Flame-retardant PC | IP44–IP54 | Indoor, damp | Medium |
| Stainless steel 304 | IP54–IP66 | Outdoor / corrosive | High |
| Stainless steel 316 | IP66 | Coastal / chemical | Highest |
5. Junction Box Installation: 3 Requirements (GB 50303)
GB 50303 Clause 12.2: ① Junction boxes must be positioned for easy service access, with the box opening flush with the wall surface; ② Splices inside the box must be crimped or bolted — twisting is not permitted; ③ Box covers must be sealed; use a gasket in damp locations. Zhejiang Golden Unicorn builds to drawing —Stainless steel / powder-coated steel / PVC · MOQ 1 unit · 15–25 days.
FAQ FAQ
What is the difference between a junction box and a splitter box?
A splitter box is a type of junction box used specifically forbranching a main line; junction box is the broader term and also covers pull boxes, transition boxes, equipment junction boxes and more.
Does a junction box have to be metal?
Not necessarily. An ABS plastic box is acceptable in a dry indoor area; use a stainless steel box in damp or corrosive environments; use a cast aluminum / cast iron explosion-proof box in hazardous areas.
How do I choose the ingress protection (IP) rating for a junction box?
IP20 is sufficient indoors in dry areas; IP44 for damp indoor areas; IP54 is typical outdoors; IP65 for outdoor rain exposure; IP68 for submersion.
Can a junction box be recessed?
Yes, it can be recessed, but the box opening must be flush with the wall surface and the cover plate must be removable. Recessed boxes in damp locations require a gasket.
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