Armoured Cable Size Calculator Uk

Estimate a practical SWA cable size for UK installations using design current, installation method, ambient temperature, grouping factor, cable run length, supply type, and voltage drop limits. This tool is designed for fast preliminary sizing of copper XLPE/PVC armoured cable in common low voltage applications.

Expert Guide to Using an Armoured Cable Size Calculator UK

Choosing the correct armoured cable size is one of the most important parts of electrical design. In the UK, steel wire armoured cable, often called SWA, is widely used for sub mains, outdoor distribution, detached buildings, workshops, plant rooms, EV infrastructure, pumps, and external equipment because it combines mechanical protection with solid electrical performance. A calculator helps speed up selection, but the best results come when you understand the engineering logic behind the number.

An armoured cable size calculator for UK projects usually works by checking two primary design limits. First, the cable must carry the design current safely after all correction factors are applied. Second, the circuit must stay within the permitted voltage drop for the type of load being supplied. In many real projects, the final cable size is driven by whichever condition is more onerous. For shorter high current runs, current carrying capacity often governs. For longer runs with moderate current, voltage drop can become the dominant factor.

This calculator uses a practical copper XLPE/PVC SWA approach suitable for preliminary sizing. It estimates the load current, applies ambient temperature and grouping corrections, compares the result to typical current carrying capacities for common installation methods, and then checks the expected voltage drop over the one way route length. That gives you a realistic starting point before a full design review against the latest edition of BS 7671, manufacturer data, and project specific conditions.

What affects armoured cable sizing in the UK?

Several variables change the minimum acceptable conductor size. If you ignore any of them, you can under size the cable and create overheating, nuisance tripping, poor equipment performance, or non compliance.

• Design current: The expected load in amps. This may come directly from equipment data or be derived from power demand.
• Supply type: Single phase and three phase circuits use different current calculations and voltage drop values.
• Installation method: A cable clipped direct or on tray can usually dissipate heat better than one enclosed in conduit or trunking.
• Ambient temperature: Higher temperatures reduce the current carrying capacity of cable insulation systems.
• Grouping: When circuits are run together, they heat each other and derating applies.
• Cable length: Longer runs create more voltage drop.
• Circuit type: Lighting circuits generally have a tighter voltage drop limit than many power circuits.
• Protective device coordination: The breaker or fuse rating must coordinate correctly with the cable and fault protection requirements.

How the calculator estimates current

If you know the current, the process is straightforward. If you only know power, the current can be estimated using standard formulas. For single phase loads:

Current = Power in watts / (Voltage x Power Factor)

For three phase loads:

Current = Power in watts / (1.732 x Voltage x Power Factor)

For example, a 20 kW three phase load at 400 V and 0.95 power factor produces a design current of about 30.4 A. This is why entering the right power factor matters. A lower power factor increases current for the same useful power.

Current carrying capacity and correction factors

In UK practice, cable tables are normally based on reference conditions. Real installations rarely match those ideal conditions exactly. Two of the most common corrections are temperature and grouping. If a cable table shows 82 A for a 10 mm² copper SWA cable under a chosen installation method, that does not automatically mean the cable is suitable for an 82 A load in every environment. If the ambient temperature is higher than the reference value or if multiple circuits are bunched together, the usable capacity falls.

A simple design approach is to divide the design current by the product of the applicable correction factors. That gives the minimum tabulated current carrying capacity that the selected cable size must have. For example, if the design current is 63 A, the ambient temperature factor is 0.94, and the grouping factor is 0.8, then the tabulated capacity required is:

63 / (0.94 x 0.8) = 83.8 A

In that example, a size with a nominal table value above 83.8 A would be required. This is why a cable that looks large enough at first glance may fail once derating is applied.

Copper SWA size	Typical clipped direct current capacity	Typical enclosed current capacity	Typical buried current capacity	Typical single phase voltage drop
2.5 mm²	36 A	30 A	42 A	18 mV/A/m
4 mm²	47 A	39 A	54 A	11 mV/A/m
6 mm²	60 A	49 A	69 A	7.3 mV/A/m
10 mm²	82 A	68 A	94 A	4.4 mV/A/m
16 mm²	110 A	91 A	125 A	2.8 mV/A/m
25 mm²	145 A	119 A	162 A	1.75 mV/A/m

The figures above are practical planning values often used for estimation. Exact values vary by cable construction, number of loaded conductors, installation details, ground thermal resistivity, and manufacturer data. That is why a calculator is a first pass tool, not the final sign off document.

Understanding voltage drop limits

Voltage drop is the reduction in voltage between the origin of the circuit and the load. In the UK, common design limits are 3% for lighting circuits and 5% for many other final circuits, subject to the details of the installation and any project specification. Excessive voltage drop can cause dim lighting, weak motor starting, poor heating performance, and erratic electronic behaviour.

The calculator estimates voltage drop using the common relationship:

Voltage drop = mV/A/m x design current x route length / 1000

For example, if a single phase load of 50 A is supplied over 60 m using a cable with a voltage drop of 7.3 mV/A/m, the estimated drop is:

7.3 x 50 x 60 / 1000 = 21.9 V

On a 230 V system, that is roughly 9.5%, which is too high for most final circuits. The result clearly shows that long runs often need a larger cable than thermal sizing alone would suggest.

Design criterion	Typical UK planning value	Why it matters	Common consequence if exceeded
Lighting circuit voltage drop	3% of nominal voltage	Maintains lamp output and stable driver performance	Visible dimming, driver instability
General power circuit voltage drop	5% of nominal voltage	Supports reliable operation of socket loads and fixed equipment	Under voltage, nuisance faults
Reference low voltage single phase supply	230 V	Used when converting design drop into percentage	Incorrect percentage calculations if assumed wrongly
Reference low voltage three phase supply	400 V	Needed for accurate three phase drop assessment	Oversized or undersized cable decisions

Why armoured cable is often chosen

SWA is popular because it is robust and versatile. The steel wire armour provides mechanical protection, helps the cable resist impact and crush damage, and suits outdoor, underground, or industrial environments much better than a non armoured alternative. It is commonly used where the route passes outside a building, across a yard, underground to a detached garage, or through service zones where accidental damage is possible.

That said, armour does not remove the need for proper installation. You still need to consider burial depth, warning tape, glands, earthing arrangements, support method, fault protection, and whether the armour is being used as a circuit protective conductor. A calculator does not determine all of those points. It only helps with conductor sizing and voltage drop checks.

How to use the calculator properly

1. Choose whether you are entering current directly or calculating from power.
2. Select the supply type, either single phase 230 V or three phase 400 V.
3. Enter the route length as a one way distance in metres.
4. Select the installation method closest to the actual site condition.
5. Enter ambient temperature and the number of grouped circuits.
6. Choose whether the circuit is lighting or another type of power circuit.
7. Click calculate and review both the recommended cable size and the voltage drop check.
8. If the result is close to a limit, consider moving up one cable size for design margin.

Common design mistakes

• Using connected load instead of realistic design load without diversity where appropriate.
• Ignoring power factor on larger motors, compressors, or mixed inductive loads.
• Forgetting grouping where multiple armoured cables are installed together.
• Assuming buried cable always runs cooler without checking soil conditions and route details.
• Checking current carrying capacity only and forgetting voltage drop.
• Not coordinating the selected cable with the protective device and disconnection requirements.
• Using a nominal site voltage that does not match the actual supply arrangement.

When the calculator result should be reviewed by a qualified designer

Every installation should be reviewed if there are unusual conditions. Examples include motor starting currents, harmonic rich loads, EV charging arrays, agricultural premises, fire alarm circuits, explosive atmospheres, medical locations, high ambient temperatures, thermal insulation contact, or long underground runs in poor soil. You should also review the design if the fault level is high, if the armour is intended to carry fault current, or if the cable route passes through multiple installation methods.

For formal compliance and safety, consult the latest edition of BS 7671, manufacturer data sheets, and project specifications. The calculator is best viewed as an efficient early stage design tool that helps narrow down the likely conductor size before you complete a detailed verification.

Useful UK authority references

For broader regulatory and safety context, the following official sources are useful:

• Health and Safety Executive: Electricity safety guidance
• Legislation.gov.uk: Building Regulations 2010
• UK Government: Approved Document P, electrical safety in dwellings

Final practical advice

As a rule, if your route is long or if future load growth is likely, selecting the next cable size up can be a smart commercial decision. The additional cable cost is often small compared with the cost of excavation, installation labour, downtime, and replacement later. Upsizing also helps reduce voltage drop, improves efficiency, and can improve motor starting performance. In many UK projects, this makes the slightly larger cable a better whole life decision.

Use this calculator to obtain a technically informed estimate, then confirm the design against the latest standards, real installation conditions, protective device data, and cable manufacturer information. That combined approach is the best way to achieve a safe, durable, and regulation aligned armoured cable installation in the UK.

This page provides a preliminary engineering estimate for copper armoured cable sizing in common UK low voltage scenarios. Final cable selection should be verified by a competent person against BS 7671, equipment characteristics, fault protection requirements, and manufacturer data.