Armoured Cable Voltage Drop Calculator
Calculate voltage drop for Steel Wire Armoured (SWA) cables under BS 7671 UK Wiring Regulations. Verify compliance for single & three-phase circuits.
Armoured Cable Voltage Drop Calculator
How to Use Armoured Cable Voltage Drop Calculator
Verifying that SWA cable runs comply with voltage drop guidelines is critical under the IET Wiring Regulations. Follow these step-by-step instructions to assess compliance:
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1Select supply type. Choose between Single Phase (230V) or Three Phase (400V) depending on your UK supply configuration.
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2Select SWA cable size. Choose the cross-sectional area of your conductor from the dropdown menu (ranging from 1.5 mm² to 300 mm²).
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3Enter load current. Input the design current (Ib) of your connected load in Amps.
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4Enter cable length. Input the physical routing path length of the SWA run, measured in meters.
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5Choose circuit type. Select whether the feed represents a Final Circuit (5% maximum drop limit) or a Distribution Circuit (3% limit).
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6Click calculate. Click the calculate button to compute the absolute voltage drop, percentage, remaining voltage, and compliance.
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7Review compliance result. Inspect the BS 7671 status card to see if your SWA cable sizing passes the compliance check or if a larger conductor is needed.
How to Calculate Armoured Cable Voltage Drop
Under BS 7671 (IET Wiring Regulations), electrical designs must restrict voltage drop to guarantee safe operation. Voltage drop calculations for SWA cables use standard millivolts per Ampere per meter (mV/A/m) constants derived from Appendix 4.
Formula 1 — Calculate Voltage Drop (V)
The total voltage drop in volts along the SWA cable routing is calculated by multiplying the mV/A/m constant, design current, and physical cable run length, then dividing by 1,000 to convert to volts:
Formula 2 — Calculate Voltage Drop (%)
The percentage drop evaluates how much voltage is lost relative to the standard nominal supply (230 V for single phase, 400 V for three phase):
Real Example
Let's run a complete step-by-step compliance calculation using standard UK parameters:
- Supply Phase: Single Phase 230V
- Cable Size: 25 mm² SWA (Steel Wire Armoured)
- Load Current: 80 A
- Cable Length: 75 m
- Voltage Drop Constant: 1.75 mV/A/m
- Circuit Type: Final Circuit (5% maximum limit, equivalent to 11.5 V)
Step 1 — Calculate Voltage Drop in Volts
Using the primary formula:
Step 2 — Calculate Percentage Voltage Drop
Compare this drop with the single-phase supply voltage:
Step 3 — Compliance Analysis
The calculated drop of 4.57% (10.50 V) is less than the 5% BS 7671 limit for final circuits (11.5 V). Therefore, the compliance result is a PASS. However, if this cable were used as a distribution circuit with a 3% limit (6.9 V), it would result in a FAIL, and a larger SWA cable (e.g. 35 mm²) would be required.
Armoured Cable Voltage Drop Chart
This reference chart lists standard mV/A/m voltage drop constants for 90°C XLPE-insulated copper SWA cables (multicore BS 5467/BS 6724) under BS 7671 (Table 4E4B) along with typical UK applications.
| Cable Size (mm²) | Single-Phase Constant (mV/A/m) | Three-Phase Constant (mV/A/m) | Typical Application |
|---|---|---|---|
| 1.5 mm² | 29 | 25 | Domestic lighting circuits, low-power control links |
| 2.5 mm² | 18 | 15 | Ring final circuits, radial power circuits, garage feeds |
| 4 mm² | 11 | 9.5 | Domestic cookers, small heat pumps, radial power sockets |
| 6 mm² | 7.3 | 6.3 | Electric showers, residential EV chargepoints, outbuilding feeds |
| 10 mm² | 4.4 | 3.8 | High-power showers, domestic sub-mains, commercial EV chargers |
| 16 mm² | 2.8 | 2.4 | Residential service feeds, sub-mains to distribution boards |
| 25 mm² | 1.75 | 1.5 | Industrial machinery feeds, three-phase sub-mains |
| 35 mm² | 1.25 | 1.1 | Commercial building sub-mains, heavy industrial loads |
| 50 mm² | 0.90 | 0.80 | Main distribution boards, high-power plant machinery |
| 70 mm² | 0.63 | 0.55 | Large industrial distribution feeds, sub-station links |
| 95 mm² | 0.46 | 0.40 | Heavy factory plant, primary distribution switchboards |
| 120 mm² | 0.36 | 0.32 | Infrastructure distribution lines, transformer feeds |
| 150 mm² | 0.29 | 0.25 | Large commercial site mains supply cables |
| 185 mm² | 0.23 | 0.20 | Industrial switchgear connections, high-capacity sub-mains |
| 240 mm² | 0.18 | 0.155 | Heavy power plant distribution networks |
| 300 mm² | 0.14 | 0.12 | Grid-scale distribution substation connections |
Note: Values are representative design figures. Refer to current BS 7671 and manufacturer data for final design.
Armoured Cable Voltage Drop Frequently Asked Questions
Under BS 7671 regulations, the recommended voltage drop limit for installations supplied from the public low voltage network is 3% for lighting circuits and 5% for other circuits (such as power, heating, and socket radial/ring circuits). These percentages are calculated from the origin of the installation to the furthest terminal of the load.
The millivolts per Ampere per meter (mV/A/m) value is found in BS 7671 Appendix 4. It depends on conductor material (copper or aluminium), insulation type (PVC or XLPE), core layout (2-core for single-phase, 3 or 4-core for three-phase), and conductor operating temperature (typically 70°C or 90°C).
For three-phase SWA circuits, the voltage drop represents the line-to-line drop, and the mV/A/m values in BS 7671 Table 4E4B already incorporate the square root of three multiplier. Thus, you use the same formula: (mV/A/m * I * L) / 1000, but compare the result against the nominal 400V supply rather than 230V.
Long cable runs naturally increase the circuit resistance, leading to a higher voltage drop. To keep the drop within BS 7671 limits, electrical designers must select a larger cable size (cross-sectional area) with a lower mV/A/m value, even if the smaller cable is thermally capable of carrying the design current.
The steel wire armour (SWA) provides mechanical protection and serves as a protective earth conductor (CPC), but it does not carry active load current under normal operating conditions. Therefore, the armouring does not affect the active mV/A/m voltage drop values, which are determined by the copper or aluminium cores.
XLPE insulated SWA cables can operate at a higher conductor temperature of 90°C compared to 70°C for PVC cables. Because copper resistance increases with temperature, XLPE cables operating at full load have slightly higher resistance and voltage drop constants (mV/A/m) than PVC equivalents under BS 7671.
In an existing installation, if the voltage drop is too high, you can reduce the load current (e.g. by using more energy-efficient equipment), balance the loads more evenly across phases in three-phase systems, or shorten the cable path. Otherwise, replacing the cable with a larger size is required.
Temporary voltage drops due to high inrush currents, such as when starting large electric motors, are generally excluded from the standard 3% and 5% limits in BS 7671, provided that the transient voltage drop does not cause malfunctions or safety issues for other connected equipment.