BS 7671 Amendment 4: Buried Cable Duct Voltage Drop Data

Amendment 4 expands Appendix 4 with new voltage drop data specifically for cables installed in buried ducts. Previously, designers had to interpolate between the direct-buried values and the in-air values, or use manufacturer-specific data for ducted installations. The new appendix tables provide mV/A/m values for common cable types in standard duct sizes, eliminating this uncertainty.

The new data covers single-core and multicore cables from 1.5 mm² to 400 mm² in duct sizes from 50 mm to 160 mm diameter. The voltage drop values for ducted cables are generally higher than direct-buried values because the duct creates a thermal barrier — heat generated by the cable must transfer through the air gap between cable and duct wall, then through the duct wall, before dissipating into the surrounding soil. This thermal resistance increases cable operating temperature, which increases conductor resistance and therefore voltage drop.

Duct fill ratio correction factors are provided for the first time. When multiple cables share a single duct, the mutual heating and reduced air circulation increase the cable temperature further. The correction factors apply to the mV/A/m values based on the ratio of total cable cross-sectional area to duct internal cross-sectional area. For a fill ratio above 40% (the maximum recommended by BS 7671 for ease of pulling), the voltage drop correction can increase the mV/A/m value by up to 15%.

Soil thermal resistivity is factored into the ducted cable data. The standard tables assume a soil thermal resistivity of 1.2 K·m/W (typical for damp UK clay soils). Where actual soil conditions differ — sandy soils can reach 2.5 K·m/W when dry — correction factors are provided. This is particularly relevant for buried duct runs in areas with seasonal soil moisture variation, where summer drying can significantly increase soil thermal resistivity and cable operating temperature.

The practical impact for designers is that voltage drop calculations for buried ducted cables are now more accurate but may show higher voltage drop values than previously assumed. This is especially significant for long EV charging feeder cables in car parks, BESS feeder cables from substation to battery enclosure, and solar PV DC cables in ground-mounted arrays.

Duct arrangement corrections are also included. The thermal interaction between ducts in a multi-duct bank (trefoil, flat-spaced, or stacked arrangements) affects each cable's operating temperature. Amendment 4 provides correction factors for standard duct bank configurations up to 3 × 3 arrangements.

ECalPro's Voltage Drop Calculator incorporates the new Appendix 4 ducted cable data, duct fill corrections, soil thermal resistivity adjustments, and multi-duct bank configurations.