1. Installation Location Requirements (GB 50303 Clause 12.2)
GB 50303 Clause 12.2 requirements for junction box location:
- Box opening flush with the wall surface, deviation ≤ 2mm, no protrusion or recess.
- Box position must allow access for maintenance; must not be buried in non-removable finish layers.
- In damp locations (bathrooms/kitchens), the box must have a sealing gasket, protection ≥ IP54.
- Outdoor box bottom must be ≥ 200mm above ground to prevent water ingress.
2. Mounting Methods (4 mounting types)
| Installation | Mounting method | Application | Notes |
|---|---|---|---|
| Surface mount | 4-point expansion bolt mounting | Indoor/outdoor wall surface | Box protrudes from wall |
| Flush mount | Cement mortar embedment | Pre-embedded during finishing stage | Box opening flush with wall |
| Suspension | Bracket + bolt mounting | Cable tray / cable trench | Fall-arrest measures required |
| Floor-standing | Base + expansion bolts | Large junction boxes | Base ≥ 100mm |
3. Sealing Requirements (Dust and Water Protection)
① A silicone sealing gasket must be fitted between the cover and the box bodysilicone sealing gasket; no gaps after tightening. ② Cable entry and exit holes must usewaterproof cable glands(PG/M thread); the cable must not pull out after tightening. ③ Unused entry and exit holes must beplugged with sealing plugs— never left open.
4. Grounding Requirements (PE bar must be connected)
Metal junction boxes must be grounded:a PE terminal bar inside the box, with the PE bar reliably connected to the metal enclosure of the box; grounding resistance ≤ 4Ω. Plastic junction boxes do not require grounding, but the PE conductor must still pass through and be connected inside the box.
5. Acceptance Checklist (5 items mandatory)
- Location: box opening flush and level for easy maintenance
- Fixed: bolts tightened, no looseness
- Sealing: gasket intact, cable glands tightened
- Grounding: PE bar connected to box body, grounding resistance ≤ 4Ω
- Marking: circuit numbers and destination labels complete
FAQ FAQ
Is there a height requirement for junction box installation?
Surface-mounted: bottom edge 1.3–1.5 m above floor. Flush-mounted: level with the switch (about 1.3 m). Outdoor junction boxes: bottom at least 200 mm above ground to prevent water accumulation.
Must a junction box be grounded?
Metal junction boxes must be grounded(PE bar connected to the box enclosure). Plastic junction boxes do not require grounding, but the PE conductor must pass through.
What if the sealing gasket ages?
Inspect regularly. Replace immediately if the gasket is aged, deformed or cracked — otherwise the ingress protection rating drops.
What does junction box acceptance mainly check?
Five items: position, mounting, sealing, grounding and labeling. If any one is missing, acceptance fails.
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