Method of selecting wire-conduit (raceway) diameters: the 40% cross-sectional fill rule, the diameter formula, and companion provisions on circuit grouping / wire-count limits / long runs with bends / pulling cables

Topic Positioning

This card covers the selection and checking of wire-conduit (raceway) diameters in electrical conduit work: how to determine the diameter from the wire count and cross-sections by the 40% fill rule, plus the companion bases on circuit grouping, wire-count limits, correction for long runs with many bends, and cable pulling. Applicable to the design, construction and acceptance of building-electrical lighting/power conduit runs.

Core Conclusions

1. Basic rule - 40% cross-sectional fill

• DL/T 5390-2014 Clause 8.7.3: "For lighting circuits run in conduits, the sum of the wire cross-sectional areas including insulation shall not exceed 40% of the conduit's internal cross-section, or the conduit's internal diameter shall not be less than 1.4 to 1.5 times the diameter of the wire bundle. The conduit-compatibility table for plastic-insulated wires may be selected per Appendix F of this provision."
• GB 51348-2019 Clause 8.3.3: "For insulated wires in metal conduits (except two wires), their total cross-sectional area (including outer sheath) shall not exceed 40% of the conduit's internal cross-section." The commentary notes the ratio is determined not only by the heat-dissipation requirement of energized wires but also by the requirement that wires and their insulation not be damaged during installation or future replacement.
• GB 50575-2010 Clause 5.2.4: "The total cross-sectional area of wires in a conduit (including outer sheath) shall not exceed 40% of the conduit's internal cross-section, and the total number of wires should not exceed 8."
• GB 50327-2001 Clause 16.3.4: "Wires of the same circuit shall be run in the same conduit, but the total number of wires per conduit shall not exceed 8, and the total wire cross-sectional area (including insulation) shall not exceed 40% of the conduit's internal cross-section."
• GB 50054-2011 Clause 7.2.22: "For wiring in flexible metal conduits, the total cross-sectional area of wires should not exceed 40% of the conduit's internal cross-section."

2. Diameter formula (derived from the 40% rule) For n wires of outside diameter d: n x d2 <= 0.4 x D2 (D = conduit internal diameter), i.e. D >= d x sqrt(2.5n) = about 1.58 x d x sqrt(n) (in mm; d = actual insulated outside diameter of the selected product; the two-wire exception aside). Round up to a standard diameter. Example: wire outside diameter 3.9 mm, 3 wires -> D >= 1.58 x 3.9 x sqrt(3) = about 10.7 mm, so the actual conduit must have a larger internal diameter (verify with product outside diameters). Without product outside-diameter data, use the fallback of DL/T 5390-2014 Clause 8.7.3: conduit internal diameter not less than 1.4 to 1.5 times the wire-bundle diameter.

3. Circuit grouping and wire-count limits

• GB 50303-2015 Clause 14.1.2: "Unless required by the design, insulated wires of different circuits, different voltage levels, and AC and DC lines shall not be run in the same conduit." (The provision guards against short-circuit faults and interference. Inspection: 20% of the circuits per inspection batch, not fewer than 1 circuit; method: observation.)
• Wires of the same circuit shall be run in the same conduit; the total per conduit should not exceed 8 (for residential decoration: shall not exceed 8).

4. Correction for long runs and many bends (DL/T 5390-2014 Appendix F, Table F) Table F is the "conduit compatibility table for 500 V plastic-insulated wires": look up the conduit diameter directly by conductor cross-section and wire count; table notes:

• Note 1: the table applies to BV single-core wires; Note 4: diameters in mm.
• Note 2: "Where the run length equals or exceeds 50 m with one bend, 40 m with two bends, or 20 m with three bends (bend angles all 90 deg or 105 deg), fit a junction box or select the next larger diameter."
• Note 3: "Each two 120 deg, 135 deg or 150 deg bends count as one 90 deg or 105 deg bend."

5. Different basis for pulling cables (not the 40% rule)

• GB 50217-2018 Clause 5.4.4: "Selection of protective-pipe diameter and cable count shall comply with: 1 preferably one cable per conduit. Except at power plants, substations and other critical sites, all circuits of one motor or all low-voltage motor circuits of one equipment may share up to 3 power cables or multiple control cables per conduit; 2 the internal diameter should be not less than 1.5 times the cable outside diameter or the envelope diameter of multiple cables; the bore diameter of duct banks should be not less than 75 mm."
• GB 50054-2011 Clause 7.6.12: "For cables run buried in conduits indoors, or through walls and floors in conduits, the conduit internal diameter shall not be less than 1.5 times the cable outside diameter."

Applicable Boundaries

• The 40% rule addresses insulated wires (including insulation/outer sheath) in conduits, balancing heat dissipation in service against undamaged installation and replacement - not merely the mechanical fit.
• Metal conduits carrying only 2 insulated wires are exempt from the 40% limit (GB 51348-2019 Clause 8.3.3), though operability and heat dissipation must still be ensured.
• Power/control cables are checked not by 40% but by "internal diameter >= 1.5 x cable (or envelope) outside diameter", preferably one cable per conduit, duct-bank bores not less than 75 mm.
• Different circuits, different voltage levels, and AC vs DC lines shall not share one conduit; one circuit one conduit; total wires per conduit preferably not more than 8.
• For long runs or many bends, add junction boxes or go one size up.
• Recompute with the actual outside diameters of the selected wires and follow the code editions executed by the project and local plan-review requirements; this card is a general-rule compilation and does not replace project design.

Further Reading

• DL/T 5390-2014 "Technical Rules for Lighting Design of Power Plants and Substations", Clause 8.7.3 and Appendix F, "Table F conduit compatibility table for 500 V plastic-insulated wires" (look up conduit diameter by conductor cross-section and wire count; mind the notes on run length/bend count and junction boxes or one size up).
• GB 51348-2019 "Standard for Electrical Design of Civil Buildings", Clause 8.3.3; GB 50575-2010 "Code for Construction and Acceptance of Wiring Works up to 1 kV", Clause 5.2.4; GB 50303-2015 "Code for Acceptance of Construction Quality of Building Electrical Works", Clause 14.1.2; GB 50327-2001 "Code for Construction of Residential Decoration Works", Clause 16.3.4; GB 50054-2011 "Code for Design of Low-Voltage Electrical Installations", Clauses 7.2.22 and 7.6.12; GB 50217-2018 "Standard for Design of Electric Power Engineering Cables", Clause 5.4.4.