Mastering SWA Cable Sizing
The Complete Guide to BS 7671 Correction Factors and SWA sizing.
Sizing Steel Wire Armoured (SWA) cable is one of the most technical tasks governed by BS 7671. The current-carrying capacity Iz is not just a table look-up, it’s the result of applying the relevant different Correction Factors to account for real-world installation conditions
SWA sizing requires multiplying various Correction factors (Ca, Cg, Ci, Cd, Cs…} based on installation method (e.g., buried depth, grouping, soil thermal resistivity). This complexity makes manual work time-consuming and error-prone.
Remember, you are calculating the effective current-carrying capacity (Iz), which is derived from the cable’s tabulated current (It) after applying the correction factors
In addition to calculating the maximum current carrying capacity for an SWA cable, you also need to take into account of voltage drop, length of run and the max Zs of the protective device.
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The SWA Correction Factor Checklist
To demonstrate compliance and ensure safety, you must apply the appropriate correction factors. If your SWA run is laid in the ground, grouped with other cables, or installed in an area of high ambient temperature, you must apply a correction factor for each:
- Ca: Ambient Air Temperature (Table 4B1)
- Ca: Ambient Ground Temperature (if buried – Table 4B2)
- Cg: Grouping (running with other circuits – Tables 4C1 to 4C6)
- Ci: Thermal Insulation (if passing through walls/ceilings etc. – Table 52.2)
- Cd: Depth of Burial (for direct buried cables – Table 4B4)
- Cs: Soil Thermal Resistivity (for ground installations – Table 4B3)
A common mistake is forgetting a single correction factor, which results in a dangerously undersized cable, or over-applying factors, which results in an unnecessarily large, expensive drum of SWA cable.
Calculating your final SWA cable size requires multiplying all of these factors against the tabulated current (It). This is where human error, time pressure, and liability risk are highest.
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