Question 1

What size SWA cable do I need for a garage supply?

Accepted Answer

A typical garage with lighting, a couple of sockets, and perhaps a small heater might total 5-8kW. With 0.6 diversity, the design current is around 13-21A at 230V. A 6mm² 2-core SWA (rated 51A buried, 46A clipped) is usually adequate. Always verify voltage drop for runs over 25m.

Question 2

Can I use T&E cable to run power to an outbuilding?

Accepted Answer

No. T&E (Twin and Earth) cable is not rated for burial, direct outdoor exposure, or most external routes. SWA (Steel Wire Armoured) cable must be used for underground runs to outbuildings. The steel armouring provides mechanical protection and serves as the CPC (circuit protective conductor).

Question 3

How deep must SWA cable be buried?

Accepted Answer

BS 7671 Regulation 522.8.10 requires buried cables to be at a depth sufficient to avoid damage. The accepted minimum is 500mm (0.5m) for most routes, increasing to 600mm under driveways or areas subject to vehicle traffic. Warning tape or cable tiles should be placed above the cable.

Question 4

Do I need an earth rod for an outbuilding on a PME supply?

Accepted Answer

Yes. Under Regulation 411.4.2, if the main supply is TN-C-S (PME), the PEN conductor must not be used as the sole means of earthing for an outbuilding. A local earth electrode is required at the outbuilding, and the SWA armour provides the CPC back to the main installation.

Question 5

What diversity factor should I use for a sub-main?

Accepted Answer

Typical domestic diversity factors range from 0.4 (light use, e.g. shed with lighting) to 0.8 (heavy use, e.g. workshop with power tools and heating). A garden office with sockets, lighting, and occasional heating typically uses 0.5-0.6. The IET On-Site Guide provides detailed diversity allowances by load category.

Question 6

What is the maximum cable length for a 10mm² SWA sub-main?

Accepted Answer

The maximum length depends on the design current and the voltage drop limit. For example, at 30A design current and 3.8 mV/A/m for 10mm², the maximum length is (11.5 x 1000) / (30 x 3.8) = 100.9m. At lower currents, longer runs are possible. Always calculate for your specific load.

Question 7

Can I run a three-phase supply to an outbuilding?

Accepted Answer

Yes, provided your property has a three-phase supply from the DNO. Use 4-core SWA cable and size it using the three-phase formula (I = P / V x 1.732). Three-phase is advantageous for high loads (workshops, large workshops) as it distributes the load across three phases, reducing cable size requirements.

Question 8

What MCB do I need at the main consumer unit for the sub-main?

Accepted Answer

The MCB rating must satisfy the coordination rule: design current (Ib) ≤ MCB rating (In) ≤ cable derated capacity (Iz). For a typical 10mm² SWA sub-main at 30A diversified load, a 32A or 40A Type B MCB is appropriate. The sub-board should have its own main switch and 30mA RCD protection.

Cable (mm²)	Rating (A)	mV/A/m	Typical Use
4	40	9.5	Small shed / lighting
6	51	6.4	Garage with sockets
10	69	3.8	Workshop / garden office
16	91	2.4	Annexe / extension
25	116	1.5	Large loads
35	143	1.1	High demand

Sub Main Cable Size Calculator

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Quick Reference -- SWA Cable Ratings

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