Cable Ampacity Software - British Standard

The British Standard BS 7671* applies to low voltage cables rated under 1 kV. Therefore, this standard is mostly for building cables. The derated cable ampacity calculation module uses the following parameters.

*Additional Module - Not included in ETAP Base Package

Cable Raceway Installations

ETAP is a powerful ampacity analysis software tool for aboveground (A/G) and underground (U/G in duct and buried) installations as followed:

• A/G Conduit (no thermal insulation)
• A/G Trays (perforated)
• Air Drop (suspended)
• Brackets
• Building Voids (with or without cable thermal insulation)
• Cleats
• Embedded Direct in Building Material
• Ladder
• Open & Clipped Direct
• Trenches (open/ventilated or closed)
• Trunking (wall/suspended or flush floor)

Cable Layout

ETAP’s cable ampacity analysis software gives the option to select from the cable layouts listed here below.

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

Grouping Factors

The cable ampacity, as part of the ampacity analysis tools, allows to select from the following grouping factors. The ampacity is adjusted accordingly based on the user’s selection.

• Number of Circuits as limited based on the standard
• Simultaneously Overloading option (used for cable sizing)

Temperature Correction

The cable ampacity assessment application calculates the derated ampacity based on the ambient and conductor temperatures. In this ampacity analysis application for buried, aboveground, and underground cables, the user can select the base temperature provided by the manufacturer or the operating temperature of the cable calculated by this module.

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

Cable Fire Protection

Users of this cable ampacity analysis program can select from a variety of fire protection layers available in the ETAP Library. ETAP’s ampacity analysis program offers the following cable fire protection adjustments:

• Cable Thermal Insulation
• Cable Insulation Thickness of able thermal insulation

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. The alerts in ETAP can be based on the options listed below.

• Based on the calculated Derated Ampacity
• User-Defined value
• Based on thermal analysis results of underground raceway systems module

This cable ampacity method is based on BS 7671 - 2001, Requirements for Electrical Installation. This cable ampacity calculation tool applies to a number of types of installation, including above ground and underground configurations. This cable ampacity analysis method can be used for cables at nominal voltages up to and including 1000V a.c. and 1500V d.c.

The displayed cable Base ambient temperature (Ta) is fixed at 30 Co per BS 7671. The cable Base maximum conductor temperature (Tc) is determined based on cable conductor type and insulation type corresponding to Tables 4D1A and onwards of BS 7671.

Cable Base Ampacity is from Tables 4DA1 and onwards of BS 7671. Under certain conditions, the base ampacity may not be available from these tables and the “Base” Ampacity field will show zero. In this case, the user may refer to ETAP log pane for more information.

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 523-04 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.

If a derating factor cannot be determined, the “Derated” Ampacity field on the Ampacity page will show zero. In this case you may refer to ETAP log pane for more information.

Sheath and Jacket Layer

According to BS 7671, sheath layer is an important factor in ampacity calculation and the sheath layer can be either metallic or non-metallic. In the current version of ETAP, a sheath layer is only metallic: lead or aluminum. A Jacket in ETAP is treated as non-metallic sheath for BS 7671 based calculation. Therefore, for cable ampacity calculation 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. It is required to enter a nonzero thickness value if a Sheath/Armor or Jacket Type is selected. Note that Paper type Jacket is not treated as a sheath for BS 7671 based calculation.