Prefabricated branch cables: definition, applicable scenarios, fire-service selection, and shaft-riser installation essentials (GB 51348-2019 / GB 50054-2011 / T/ASC 23-2021 / DB11/T 1832.17-2021)

Topic Positioning

Prefabricated branch cables (trade name "pre-branch cables"; "pre-branched power cables" in the Beijing work rule) are finished power cables with machine-prefabricated branch joints. This card compiles the definition and varieties, applicable scenarios and length limits, mandatory selection for fire-service feeders, and shaft-riser installation essentials (hanging supports, fixing, bend radius, fire sealing) — for electrical design selection, workmanship checks, and budgeting.

Core Conclusions

1. Definition (GB 51348-2019, 8.8.2, verbatim) Prefabricated branch cables are power cables — PVC-insulated or XLPE-insulated PVC-sheathed, non-FR/FR/fire-resistant, PVC-sheathed or steel-tape-armored, single- or multi-core — on which the manufacturer machine-fabricates branch joints along the whole length per the design section and branch spacing, with superior supply reliability.

Points: trunk and branch cables supplied integrally; joints factory-made; no field joints (unlike field tee-offs or insulation-piercing connectors); varieties cover single/multi-core, non-FR/FR/fire-resistant, optionally armored.

2. Scope and length limits (GB 51348-2019, 8.8.1, verbatim) Prefabricated branch cable wiring suits indoor low-voltage tree-trunk distribution in high-rise, multi-story, and large public buildings. Single-phase branch cables preferably <= 120m; three-phase <= 300m. High ampacity, corrosion and water resistance, easy installation — widely used as low-voltage distribution mains in high-rise/multi-story and large public buildings.

3. General installation requirements (verbatim)

• GB 51348-2019, 8.8.2: install exposed on supports or ladders indoors and in electrical shafts; for vertical runs, reserve slab penetrations per the maximum trunk diameter and a hanging hook in the top slab above the trunk.
• GB 50054-2011, 7.6.58: bind branch cables tightly to the trunk during laying and release the bindings after the trunk is fixed; do not over-stretch branch cables.

4. Mandatory selection for fire-service feeders (T/ASC 23-2021, 5.3.5, verbatim) Fire-service feeder trunks should have no splices. Feeders of fire equipment with current <= 400A on tree-trunk supply shall use fire-resistant (pre-)branch cables. Where radial feeders truly need a splice, no more than one, located in a fire-protected equipment room or shaft.

5. Key shaft-riser workmanship (DB11/T 1832.17-2021, 11.4.6) (1) Hanging supports: shaft-top support via cable clamping devices (wire-rope slings) or suspended insulating devices; with suspended devices, the civil crew embeds hooks in the cast slab; with wire-rope slings, channel-hook cross members on the shaft wall; single/dual-circuit multicore prefabricated branch cables also need support clamps at each floor slab. (2) Fixing and load: fix the trunk on wall supports immediately after hoisting — the hanging ring carries no further total weight; fix the trunk 300mm above and below each branch joint edge; branch start supports 300mm from the trunk center; end supports 300mm from the bend midpoint; add intermediate supports for long branches, spacing preferably <= 400mm, evenly. (3) Single-core eddy currents: with single-core trunks, prevent eddy currents — ferromagnetic clamps and pipe clips forbidden. (4) Bend radius: single-core >= 20x OD; multicore >= 15x OD. (5) On ladders/trays/boxes: fix with portal or flat-steel brackets; besides the ends, fix at 400mm from the end brackets and 400mm above/below slabs; intermediate bracket spacing <= 1m; trunk tie spacing <= 1m; tie branches at every ladder rung; use equal/variable-width horizontal tees at branches and 90-degree down-bends at branch corners. (6) Fire sealing: after electrical handover tests pass, seal per design — single cables through slab reserves with SDF-II sealant plus mineral wool or glass fiber; multiple cables with SDF-II flush with the slab, capped with SDF-III above and below; ladders/trays/boxes through slabs also sealed around the trunk. (7) Tags: cable number, spec, type, voltage, and start/end; tags at ends, bends, and crossings for conduit runs, added at intervals on straights.

Applicable Boundaries

• Scope: indoor low-voltage tree-trunk distribution mains in high-rise, multi-story, and large public buildings; 120m/300m are "preferably" limits, not mandatory caps — meet them or justify otherwise.
• Mandatory: fire-service feeders <= 400A on tree-trunk supply shall use fire-resistant (pre-)branch cables ("shall").
• Quantified workmanship (300mm, 400mm, 1m, 20x/15x bend radius, SDF-II/III) is from DB11/T 1832.17-2021, a Beijing local standard; other regions follow local work rules, design documents, and manufacturer instructions.
• This card covers definition, selection, and installation only — not quantity takeoff or pricing; price the finished cable as main material and check local norm items.

Further Reading

• GB 51348-2019 Section 8.8: 8.8.1 (scope and lengths), 8.8.2 (definition and installation).
• GB 50054-2011, 7.6.58.
• T/ASC 23-2021, 5.3.5 (fire-service trunk selection and splices).
• DB11/T 1832.17-2021, 11.4.6 with Figs. 11.4.6-1~4 (slings, clamps, riser details, hoisting).