Armoured Cable Size Calculator
Calculate the minimum SWA armoured cable size required for UK installations under BS 7671. Determine current capacity, voltage drop, and compliance status for single and three-phase circuits instantly.
Armoured Cable Size Calculator
How to Use Armoured Cable Size Calculator
Verifying both thermal current capacity and voltage drop compliance is standard practice for UK electrical designers. Follow this workflow to determine the proper SWA cable size:
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1Choose Phase & Voltage. Select Single Phase (230V) for typical residential feeds or Three Phase (400V) for commercial and industrial connections.
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2Enter Design Current (Ib). Input the design load current in Amps. This value represents the maximum continuous current flow expected.
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3Input Cable Length. Enter the physical length of the planned SWA routing run in meters. Longer runs increase resistance and voltage drop.
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4Select Conductor Material & Method. Choose Copper or Aluminium SWA, then select the installation environment (e.g. clipped direct, buried, or in ducts).
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5Define Ambient Temp & Max Drop. Specify the local operating temperature to apply BS 7671 thermal derating. Set your maximum allowed voltage drop (standard is 3% for lighting and 5% for power).
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6Click Calculate. Review the recommended SWA size, corrected carrying capacity, drop percentage, and the detailed engineering design statement.
How to Calculate Armoured Cable Size
Under BS 7671 (IET Wiring Regulations), selecting an SWA cable involves checking both thermal capacity and active voltage drop.
1. Design Current (Ib) and Cable Capacity (Iz)
The corrected current capacity (Iz) of the SWA cable must be greater than or equal to the circuit design current (Ib), which in turn must not exceed the protective device rating (In):
Where Itab is the tabulated base current capacity from BS 7671, Ca is the ambient temperature correction factor, and Cg is the grouping correction factor.
2. Voltage Drop Sizing Check
Voltage drop calculations use standard millivolts per Ampere per meter (mV/A/m) constants. These represent the millivolts lost per Amp of load current for every meter of circuit run.
Single Phase Voltage Drop Formula
Three Phase Voltage Drop Formula
For balanced three-phase systems under BS 7671, the line-to-line drop incorporates the √3 factor. The three-phase mV/A/m constants in BS 7671 Tables (such as Table 4E4B) already have this factor built-in. Therefore, the calculation simplifies to:
If starting from per-phase active and reactive parameters or using a raw line-to-neutral equivalent, the general expression is:
Voltage Drop Percentage Formula
Conductor Cable Selection Rule
The sizing algorithm selects the smallest standard cable cross-sectional area that satisfies both:
- Thermal carrying capacity: Iz ≥ Ib
- Voltage drop restriction: Vd (%) ≤ Max allowed drop %
Worked Sizing Example
Let's size a three-phase copper SWA cable for the following parameters:
- Configuration: Three-Phase balanced load, 400 V nominal supply
- Design Current (Ib): 80 A
- Length (L): 50 m
- Installation Method: Clipped Direct (Ref Method C)
- Conductor Type: Copper SWA XLPE (BS 5467)
- Ambient Temp / Max Drop: 30°C / 5% limit (equivalent to 20 V)
Step 1: Check Current Capacity
Using Table 4E4A for copper SWA XLPE cable (Method C, 3/4-core) with ambient factor Ca = 1.0:
- 10 mm² capacity = 73 A. Since 73 A < 80 A, it fails.
- 16 mm² capacity = 94 A. Since 94 A ≥ 80 A, it passes thermal capacity.
Step 2: Check Voltage Drop for 16 mm²
The three-phase mV/A/m drop constant for 16 mm² is 2.4 mV/A/m (from Table 4E4B):
Percentage drop calculation:
Since 2.40% is less than the 5.0% limit (equivalent to 20 V), the 16 mm² Copper SWA cable satisfies all BS 7671 design requirements and is the recommended size.
Armoured Cable Size Chart
This reference table details standard design current-carrying capacities and voltage drop constants for 90°C XLPE-insulated multicore copper SWA cables (BS 5467) under BS 7671 reference conditions.
| Cable Size (mm²) | Current Capacity (A) — Method C | Typical Drop mV/A/m (3-Ph) | Typical Applications |
|---|---|---|---|
| 1.5 mm² | 23 A | 25 | Lighting circuits, secondary control wiring |
| 2.5 mm² | 31 A | 15 | Ring final mains, outhouse sockets, garden lighting |
| 4 mm² | 42 A | 9.5 | Radial socket feeds, light industrial machinery |
| 6 mm² | 53 A | 6.3 | Domestic showers, EV charge points, solar array feeds |
| 10 mm² | 73 A | 3.8 | Sub-mains to outbuildings, high-power cooking loads |
| 16 mm² | 94 A | 2.4 | Main distribution boards, residential service feeds |
| 25 mm² | 124 A | 1.5 | Heavy factory machines, three-phase sub-mains |
| 35 mm² | 154 A | 1.1 | Commercial mains distribution, heavy manufacturing plant |
| 50 mm² | 187 A | 0.80 | Sub-station links, large industrial distribution panels |
| 70 mm² | 238 A | 0.55 | Primary site intakes, heavy manufacturing lines |
| 95 mm² | 289 A | 0.40 | Grid-level substation connections, industrial main feeds |
Note: Capacities shown assume Method C (clipped direct) at 30°C ambient temperature. Values will vary under different installation methods and BS 7671 correction factors.
Armoured Cable Size Calculator Frequently Asked Questions
Under BS 7671 (IET Wiring Regulations), SWA cable sizing must simultaneously satisfy two critical criteria: the thermal current-carrying capacity (Iz) must be greater than or equal to the circuit design current (Ib), and the voltage drop under full load must not exceed the recommended limits (typically 3% for lighting and 5% for other circuits).
The installation method alters the cable's thermal dissipation. Cables installed in free air (Method E) or on trays have higher capacities due to cooling. Cables direct buried in soil (Method D) have moderate capacity, while cables in buried ducts (Method D in duct) have the lowest capacity due to heat retention, often requiring a larger conductor size for the same design current.
For low voltage installations supplied directly from a public network, BS 7671 Table 4Ab specifies a maximum voltage drop of 3% for lighting circuits (6.9 V for single-phase, 12 V for three-phase) and 5% for other circuits like power and socket outlets (11.5 V for single-phase, 20 V for three-phase), measured from the origin to the load.
XLPE (cross-linked polyethylene) insulation can safely operate at a conductor temperature of 90°C, compared to 70°C for thermoplastic PVC. This higher thermal threshold allows XLPE SWA cables to carry higher current loads for the same cross-sectional area, reducing the required copper or aluminium size, though voltage drop limits still apply.
Tabs in BS 7671 assume 30°C in air and 20°C in ground. If actual temperatures are higher, correction factors (Ca) from Table 4B1 or 4B2 must be applied. This derates the cable's thermal capacity. For example, at 40°C in air, an XLPE cable is derated to 91% of its base rating, meaning a larger cable size may be necessary.
Generally, aluminium SWA cables are not used for small domestic installations because they are typically manufactured only in sizes of 16 mm² and above. Copper SWA is used for smaller domestic feeds (such as garage sub-mains or EV chargers) due to its higher conductivity, flexibility, and availability in small sizes like 2.5 mm² or 6 mm².
The power factor represents the ratio of active power to apparent power. A lower power factor increases the total design current required for a given active kW load, which subsequently increases the current flow and the voltage drop across the cable run. In larger cables, reactive impedance also influences the total mV/A/m drop.
Direct burial (Ref Method D) places the armoured cable in direct contact with soil, which provides relatively good heat dissipation. Installing the SWA cable in a plastic duct buried underground restricts airflow and acts as a thermal barrier. Under BS 7671, this reduces the current capacity by about 15% to 20%, requiring larger SWA sizing.
For balanced three-phase systems under BS 7671, the line-to-line voltage drop is calculated as: (mV/A/m * Ib * L) / 1000. The tabulated three-phase mV/A/m values in Table 4E4B already incorporate the sqrt(3) factor, so no manual multiplication is needed, and the result is compared directly against the 400 V nominal supply.