Free SWA Armoured Cable Calculator | Cable Size & Voltage Drop

How to use this calculator

We've designed this tool to be as simple as possible while still adhering to standard UK electrical principles (BS 7671). To get started, simply select your supply phase, enter your load in Amps or kW, and specify the cable run length. For common scenarios like a garage supply, shed, or EV charger, you can use the quick preset buttons to automatically fill in typical values.

If you need more control, open the Advanced Options to adjust the ambient temperature, grouping factors, or add a 20% future capacity headroom to ensure your cable is future-proof.

What affects SWA cable size?

Several critical factors determine the correct steel wire armoured (SWA) cable size for your project. Ignoring any of these can result in a cable that fails to meet safety regulations.

Load and cable run length

The load current (in Amps) dictates the absolute minimum thickness of the copper conductors needed to carry the power without melting. However, the longer the cable run, the more resistance the electricity faces. A long run often requires you to step up the cable size, even if the load is relatively small.

Voltage drop

As electricity travels through a cable, some voltage is lost as heat. The IEE Wiring Regulations (BS 7671) state that the maximum allowable voltage drop is 3% for lighting circuits and 5% for power circuits. Our calculator automatically checks the volt drop and will recommend a larger cable if your run is too long for the minimum size.

Installation method

How you install the cable affects how well it can dissipate heat. A cable clipped direct to a wall cools down much faster than one buried direct in the ground or enclosed in a duct. The calculator adjusts the current carrying capacity based on your chosen installation method.

Correction factors and site conditions

If your cable runs through a hot environment (ambient temperature) or is bunched together with other live cables (grouping factor), its ability to carry current safely is reduced. You must apply correction factors to compensate for these conditions, which may force you to choose a thicker cable.

Common mistakes people make

• Ignoring voltage drop: Many people only check the current carrying capacity and forget that a 50-metre run to a garden office will suffer significant voltage drop, requiring a thicker cable.
• Forgetting future loads: Sizing a sub-main exactly for today's load means you might have to dig up the garden again when you want to add a hot tub or an EV charger later. Always consider future-proofing.
• Using the wrong protective device: The cable must be protected by an appropriately sized MCB or RCBO. The cable's capacity must always be greater than or equal to the rating of the protective device.

Frequently Asked Questions

Important Disclaimer