Reserve rules for cables passing manholes and reserve (surplus) lengths for power / railway signal / urban rail signal / communication cables

Topic Positioning

Answers "must cables reserve length when passing cable manholes, and how much", consolidating reserve (additional, surplus) length rules and values for power, railway signal, urban rail signal, communication, and road lighting cables — for length calculation, BOQ compilation, and construction allowances.

Core Conclusions

1. Master principle: reserve at manholes; reserves count as quantity, not loss

• GB 50217-2018 Standard for Design of Electric Power Cable Works, Clause 5.1.17: cable calculated length includes the actual route plus additional length, accounting for: 1 terrain elevation changes along the route, expansion joints, or contingency margins; 2 the snaking increment of cables above 35 kV; 3 reserve sections for stripping at terminations/joints and leads to equipment. For 35 kV and below, additional lengths per Appendix G.
• Measurement: cable laying is measured in meters per single-run design length; reserves and additional lengths are components of the laying quantity and enter it; where the design specifies, follow it, otherwise the standard reserve tables (SJG 181.7-2024 Clause 5.2.3; BOQ per GB/T 50856-2024 Table D.9.1, code 030409).
• Direct bases for surplus at manholes: reserve surplus at stations, manholes, duct portals, terminations, and joints (CJJ/T 72-2015 Clause 5.10.22); reserve some length at building entries, joints, cable heads, trenches, and tunnels (GB 50054-2011 Clause 7.6.7); reserve standby length near terminations and joints (GB 50168-2018 Clause 6.1.5).

2. Manhole reserve values (by discipline; not interchangeable)

• Communication duct cables in manholes: 1-2 m per end, max 3 m (GB 51158-2015 Clause 7.3.6).
• Urban rail signal (DB11/T 2382-2024 Table 10.4.1): 2 m in cable manholes; 5 m entering buildings (indoor reserve); 2 m ring reserve per indoor end (1 m below 20 m); 2 m per end for duct exit/entry and termination; 3 m per terminal; 5 m to distribution cabinets; 0.5 m vertical-to-horizontal; cabinets/boxes by half-perimeter; bending factor 2% of total length.
• Railway signal (TB 10007-2017 Clause 17.0.11): cable length includes 1 actual duct length between equipment; 2 per-end duct exit and termination at 2 m; 3 per splice 2 m; 4 per-end surplus (1 m if run < 20 m, else 2 m); 5 track crossing at 5.5 m per track plus inter-track spacing beyond 5 m; 6 indoor entry surplus >= 5 m; 7 bending factor >= 2% of the sum; 8 other special-section surplus.
• Urban rail signal acceptance (GB/T 50578-2018 Clause 4.3.9): indoor surplus >= 5 m; outdoor equipment ends >= 2 m (>= 1 m for runs < 20 m); bridge ends >= 2 m; splice ends >= 2 m; through civil-defense/flood gates per design; across expansion joints >= maximum movement.
• Power cables: no unified national "N m per plain manhole transit" item — terminations/joints in manholes per GB 50217-2018 Appendix G (0.5 m each for termination/joint making, etc.) and design; plain transits per the maintenance-surplus clauses with designer confirmation.
• Road lighting (CJJ 89-2012 Clause 6.1.4): laying length preferably 110% of route length; at in-pole splices, >= 2 m per side per pole; at riser T-connections, >= 1.5 m per pole.

3. Manhole setting and reserves GB 50217-2018 Clause 5.4.7: working manholes are required at: 1 spacing limited by pulling tension (max duct length per Appendix H); 2 cable branches and joints; 3 major direction changes or duct-to-direct-burial transitions; 4 steep slopes needing anti-slip anchoring. Manholes exist mostly for pulling, branching, and jointing: joint/splice reserves apply where joints are made (2 m per end, 3 m per terminal, etc.); plain transits take the maintenance-surplus caliber.

Applicable Boundaries

• Order of values: design first; absent design, the discipline's reserve table.
• Values belong to different discipline standards — railway signal (TB 10007-2017), urban rail signal (GB/T 50578-2018, DB11/T 2382-2024), communication ducts (GB 51158-2015), power (GB 50217-2018), road lighting (CJJ 89-2012) — not directly transferable.
• DB11/T 2382-2024 is a local consumption standard for local pricing only; other regions follow their own.
• Reserves, already counted as quantity, must not be re-counted as construction loss; mind the base for percentage factors (sag, bends, crossings add to the sum of drawn length plus reserves).
• Values here are fallbacks absent design; the specific project follows the drawings and disclosure.

Further Reading

• GB 50217-2018 Clauses 5.1.17, 5.4.7, Appendix G (additional lengths <= 35 kV; note: building entries, direct burial per terrain, trench/tunnel/tray risers, terminations and joints may take 5% of drawn route length).
• GB/T 50856-2024 Appendix D, Table D.9.1 (030409).
• GB 51158-2015 Code for Design of Communication Line Engineering, Clause 7.3.6.
• TB 10007-2017 Code for Design of Railway Signaling, Clause 17.0.11.
• GB/T 50578-2018 Standard for Constructional Quality Acceptance of Urban Rail Transit Signal Works, Clause 4.3.9.
• DB11/T 2382-2024 Tables 10.4.1 (signal) and 10.3.1 (communication: 3 m at splices, 5 m at equipment, +3 m per 200 m on fiber brackets, 5 m at repeaterstation entries, main-frame terminations by pair count, etc.).
• CJJ 89-2012 Code for Construction and Acceptance of Urban Road Lighting Works, Clause 6.1.4; CJJ/T 72-2015 Clause 5.10.22; GB 50054-2011 Clause 7.6.7; GB 50168-2018 Clause 6.1.5; SJG 181.7-2024 Clause 5.2.3.