SWA for man cave

[G and J]

At the bottom of my garden there is a man cave and playing with the online voltage drop calculators it seems I would be ok with 6mm but I may go for 10mm to be sure.  
 

What I can’t work out is if I need a two or three core cable.  I’m finding it confusing as they don’t seem to have insulation colours that I would expect.  
 

What do I need?

[andyscotland]

Historically, in most cases people used 2-core SWA for single phase circuits, using the outer steel armour as the CPC (earth). This is often ok in principle although it does need to be checked/calculated as the resistance of the steel isn't always appropriate for the circuit.

 

Many people (including me) these days prefer to use a copper core for the CPC even if we could theoretically use the armour. The resistance is better from day 1, less to check, and armour and its terminations often corrodes & degrades over time. The sheath still needs to be earthed at at least one end to protect the cable, but is not officially part of the circuit.

 

3-core SWA was traditionally used for balanced 3-phase loads e.g. motors, as these do not have a neutral. This is why the cores are coloured for the three live phases (brown, black, grey).

 

You can use that cable for a single phase circuit if you sleeve the cores at both ends. However there are manufacturers now making it in the correct colours (brown,blue,green&yellow) and it would be better practice to use this - it's fairly easy to source.

 

This all assumes that it is ok to export the supply earth to your outbuilding. That depends on a number of factors including distance, type of construction of the building, whether any other services are present etc. It may not be safe to use the house earth - if not then you would use a 2-core cable with the armour earthed at one end and then fit an earth spike.

 

Bear in mind for cable sizing,  voltage drop needs to be considered to the furthest point of your circuits (e.g. including the wiring within the man cave) and you also need to consider the Earth Fault Loop Impedance for the circuits to suit the type(s) of protective devices.

 

I'd almost always go up a size for something like this: the extra cost is marginal and it buys you a lot of future-proofing. You only want the hassle of running the cable once.

 

To be honest as you can maybe see there are a few site-specific things that need to be checked/calculated to do this safely.

 

Terminating SWA properly also takes a bit of skill/experience.

 

This is therefore quite an involved job to attempt without an electrician to advise - it's beyond the normal realm of DIY (and I think notifiable in England & Wales? Not sure as I'm in Scotland and our rules are different).

 

If you are keen to do it yourself then you'll probably want to do more research on some of the things mentioned above, and you'll definitely want to buy extra cable to have a practice before you attempt to terminate the real thing.