SWA Cable Calculator
Choosing an SWA cable is not simply a matter of matching the load to the nearest cable size.
The cable also has to cope with the length of the run, operating voltage, current, installation conditions and allowable voltage drop. A cable that works perfectly for a short run may produce excessive voltage drop when the same load is supplied over a much longer distance.
This SWA Cable Calculator helps you estimate a suitable steel wire armoured cable size from your electrical load and cable-run information.
Enter the load, supply details and cable length to get an estimated cable requirement and voltage-drop information.
SWA Cable Calculator
Estimate the required steel wire armoured cable size from your electrical load, supply voltage, cable length and voltage-drop limit.
Calculation summary
What does SWA mean?
SWA stands for Steel Wire Armoured.
SWA cable is a type of armoured electrical cable with a layer of steel wires surrounding the insulated conductors. The armour provides additional mechanical protection, which makes SWA cable particularly useful for installations where a cable may be exposed to physical damage.
Typical applications include:
- Underground electrical supplies
- Garden buildings and outbuildings
- Garages and workshops
- External power supplies
- Commercial installations
- Sub-main distribution
- Long outdoor cable runs
- Industrial electrical installations
The exact cable construction, conductor material, insulation and armour arrangement depend on the cable specification being used.
How the SWA Cable Calculator Works
A cable-size calculation starts with the electrical demand.
If the load is given in watts or kilowatts, the calculator can determine the approximate current required from the supply voltage. If the current is already known, it can be entered directly.
For a simple single-phase resistive load:
Current = Power ÷ Voltage
For example, a 4,600 W load on a 230 V supply would require approximately:
4,600 ÷ 230 = 20 A
That 20 A is only the starting point.
The cable length then becomes important because a longer conductor produces a greater voltage drop.
For BS 7671-style calculations, voltage drop can be expressed using the manufacturer’s or cable-table value in mV/A/m:
Voltage drop (V) = mV/A/m × current × length ÷ 1,000
This approach is commonly used for SWA cable voltage-drop calculations.
Why Cable Length Matters
Imagine two installations carrying the same 20 A load.
One cable is only 10 metres long.
The other is 100 metres long.
They have the same load current, but the longer cable has much more conductor resistance over the complete run. Consequently, the voltage drop can become the factor that determines the required cable size.
This is why simply saying “20 amps needs X mm² cable” can be misleading.
For a long SWA run, moving to a larger conductor can reduce voltage drop even when the smaller cable has sufficient current-carrying capacity.
SWA Cable Size Is More Than Current Capacity
When selecting an armoured cable, there are several things to consider rather than looking at ampacity alone.
1. Design current
The cable must be capable of carrying the expected load current under the actual installation conditions.
2. Voltage drop
The voltage available at the far end of the cable should remain within the applicable design limit.
3. Installation method
A cable installed in free air does not necessarily have the same current-carrying capacity as the same cable installed under different conditions.
For example, BS 7671-based SWA sizing commonly distinguishes between reference methods such as clipped direct and buried installations.
4. Cable construction
Two-core, three-core and four-core SWA cables can have different electrical characteristics and applications.
5. Environmental conditions
Temperature, grouping with other cables, thermal insulation and installation conditions can require derating.
6. Fault and protective requirements
Final cable selection may also require checks for protective-conductor requirements, short-circuit withstand and the characteristics of the protective device.
Single-Phase vs Three-Phase SWA Cable
The supply configuration can significantly affect the calculation.
Single-phase
A typical UK-style single-phase supply is 230 V.
Single-phase SWA cable is commonly encountered in supplies to garages, workshops, garden buildings, lighting installations and other smaller loads.
Three-phase
A typical three-phase supply is 400 V between phases.
Three-phase SWA is frequently used for larger loads, motors, distribution circuits and commercial or industrial applications.
The current required for a given amount of power can therefore be substantially different between single-phase and three-phase systems.
For three-phase power:
I ≈ P ÷ (√3 × V × power factor)
For real installations, the applicable load characteristics and power factor should be considered.
What SWA Cable Size Do I Need?
There is no single SWA cable size that is correct for every load.
A suitable size depends on the combination of:
Load + voltage + current + cable length + installation method + cable construction + voltage-drop limit
For example, a relatively small load might use a small cable on a short run but require a larger conductor on a long run because voltage drop becomes significant.
Common SWA conductor sizes include:
1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm², 16 mm², 25 mm², 35 mm², 50 mm², 70 mm², 95 mm² and larger sizes.
These are examples of commonly encountered sizes, not a universal recommendation for a particular load. Actual current-carrying capacities depend on the cable construction and installation conditions.
Understanding Voltage Drop in SWA Cable
Voltage drop is one of the most important reasons a cable may need to be larger than expected.
Suppose the equipment requires 230 V at the source. Because the cable has electrical resistance, the voltage measured at the far end will be lower while current is flowing.
The greater the:
- Cable current
- Cable length
- Cable resistance
the greater the voltage drop.
For SWA cable, manufacturers and cable tables commonly provide voltage-drop figures in mV/A/m.
A simplified calculation is:
Voltage Drop = (mV/A/m × Amps × Length) ÷ 1,000
The result can then be compared with the permitted voltage-drop value applicable to the installation.
SWA Cable Calculator Example
Consider a hypothetical installation with:
- Supply: 230 V
- Load: 4,600 W
- Cable length: 50 m
- Supply: Single phase
The approximate load current is:
4,600 ÷ 230 = 20 A
The next question is not simply whether a particular cable can carry 20 A.
The voltage drop over the 50 m run must also be checked.
If the selected cable has a voltage-drop value of mV/A/m, the calculation becomes:
Voltage drop = mV/A/m × 20 × 50 ÷ 1,000
A larger cable generally has a lower voltage-drop value, so increasing the conductor size can improve the voltage available at the load.
This illustrates why cable size and cable length need to be considered together.
SWA Cable Calculator Limitations
This calculator should be treated as a planning and estimation tool, not as a substitute for a complete electrical design.
A real installation may require additional checks, including:
- Design current
- Protective-device characteristics
- Current-carrying capacity
- Correction factors
- Voltage drop
- Earth-fault loop impedance
- Protective conductor requirements
- Short-circuit withstand
- Earthing arrangement
- Installation method
- Ambient temperature
- Cable grouping
- Thermal insulation
- Starting current for motors
- Local electrical regulations
For a final installation, use the applicable wiring regulations, manufacturer’s cable data and the requirements of the relevant electrical authority or qualified electrical professional.