BS 7671 - British Standard

BS 7671

BS 7671 - British Standard

The British Standard BS 7671 applies to low voltage cables with nominal voltages up to and including 1 kV AC and 1.5 kV DC.

Cable Raceway Installations

ETAP’s cable capacity calculation handles both above ground (A/G) and underground (U/G in duct and buried) installations:

  • A/G Conduit (thermally insulated wall, attached to wall, masonry, unventilated or ventilated channel)
  • A/G Trays (perforated, unperforated)
  • Air Drop (suspended, on insulators)
  • Architrave 
  • Window Frame
  • Building Voids (direct, conduit, ducting, ceiling, suspended floor)
  • Brackets
  • Chanel (open, ventilated)
  • Embedded Direct (thermally insulated wall, mouldings, masonry)
  • Ladder
  • Wire Mesh
  • Open & Clipped Direct (on wall, under ceiling, spaced from ceiling)
  • Trunking (on wall, suspended, flush floor, skirting)
  • U/G Duct
  • U/G Buried (direct, conduit)
  • Masonry (cable ducting)
BS 7671 Cable Raceway Installation Types

Cable Layout

ETAP’s cable current carrying capability software gives the option to select from the following layouts:

  • Horizontal layout - Touching
  • Horizontal layout - Spaced
  • Vertical layout – Touching
  • Vertical layout - Spaced
  • Trefoil layout
  • Flat Layout

Temperature Correction

The cable ampacity assessment application calculates the derated capacity based on the ambient and conductor temperatures.

  • Base Ambient Temperature
  • Base Conductor Temperature
  • Operating Ambient Temperature
  • Operating Conductor Temperature

Alert for Allowable Ampacity

ETAP’s ampacity analysis program provides alerts to the user when a critical situation occurs such as overloading. A critical situation may deteriorate the cable and tremendously reduce its lifetime.

This cable current capability calculation method is based on BS 7671 - 2018, Requirements for Electrical Installation. 

Cable Derated Ampacity is displayed in the "Derated" Ampacity field. The following derating factors are considered for cable derating calculation:

  • Ca – Correction factor for ambient temperature. The factor is from Table 4C1 or Table 4C2 of BS 7671 Appendix 4.
  • Cg – Correction factor for grouping. The factor is from Tables 4B1, 4B2 or 4B3 of BS 7671 Appendix 4.
  • Ci – Correction factor for conductors embedded in thermal insulation. Refer to Section 523.9 of BS 7671 for detailed information.
  • Cf – Correction factor for BS 3036 fuse. A factor of 0.725 is applied for cables protected by a fuse to BS 3036. Refer to Section 5 of BS 7671 Appendix 4 for detailed information. A BS 3036 Fuse is specified on the Protection page by selecting User-Defined Overload Protection device.

Sheath Layer

According to BS 7671, sheath layer is an important factor in the current carrying capability calculation. The sheath layer can be either metallic or non-metallic. In ETAP, sheath layer is metallic: lead, aluminum, or copper. 

Jacket Layer

A Jacket in ETAP is treated as non-metallic sheath for BS 7671 based calculation. Therefore, for cable capacity calculations based on BS 7671, it is considered that the cable has a sheath layer if the cable has either a sheath layer or a jacket layer.




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