Please confirm SWA size.

[Zebsport]

Hi Guys,

Could some one please confirm the following just in case i made some stupid error before I spend £200 on cable!

Cable required for workshop at end of garden. I worked out the max power use in the garage would be 8kw (air compressor 2.2KW, heater, power tools, lights etc) I can't see this ever happening all at once, it would be more likely 3-4kw on a regular basis.

SWA 10mm
Load 8kw
Length 50metres
Voltage drop 6.6v
% drop 2.9%
Current Load 34.78 amps
Max cable load 44.5 amp

No point in using 16mm as 10mm covers everything and more?
The mcb at the house end should be 40amp just below max load of cable?

Thanks

 

[riveralt]

6mm might be enough for you:


http://www.tlc-direct.co.uk/Products/SW6slash3.html

 

[riveralt]

Hold that, I just ran your figures through the TLC cable calcualtor and according to it you will need the 10mm cable.

http://www.tlc-direct.co.uk/Technic...230&length=50&submit=Calculate+Min+Cable+Size

 

[Zebsport]

Thanks i'll take a look. Recent threads about my brothers very similar workshop set up has encouraged me to dig up the old supply and do it properly. So want to get it just right.

 

[ban-all-sheds]

SWA 10mm
Load 8kw
Length 50metres
Voltage drop 6.6v
% drop 2.9%
Current Load 34.78 amps
Max cable load 44.5 amp

I see what you've done.

You've used the 3-phase voltage drop figure of 3.8mV/A/m, not the single-phase figure of 4.4.

34.78A over 50m = 7.7V drop, = 3.3%


You need to be careful using the TLC calculator:

1) If you specify 3-core SWA it will do 3-phase calculations, so select 2-core even if you are using 3-core.

2) If you select XLPE it will assume that you can run it up to 90°, which you probably can't, so select PVC even if you are using XLPE.

 

[Zebsport]

I presume I select a MCB just below the cables maximum capable load. So in this case I would select a 40 amp mcb at the supply end?

 

[ban-all-sheds]

//www.diynot.com/wiki/electrics:mcbtocable

 

[riveralt]

Surely the value of 1.45 is not need in this calculation - doesn't it cancel itself out?

 

[ban-all-sheds]

Only for BS EN 60898 MCBs, i.e. Types B, C & D.

For other types of device I2 is a different multiple of In.

For example with BS 3036 fuses it's 2.0In.

And 1.45 ÷ 2.0 = 0.725, the de-rating factor for cables protected by that type of device.

 

[spalding1sparks2]

what was the start current on the compressor
and are you doing a TT system

 

[Zebsport]

Hi.

The compressor is rated at 2.2 kw but have know idea what it will be on start up. Obviously more so I rounded it up to 3kw (pretty scientific way!!).

Well i was going to connect everything back to the house and have an earth rod at the workshop as well, thinking more was best. Some recent knowledge gained from the forum has told me this will be wrong so i've gone for just the earth rod. The 3rd core and armour in the swa will be not be connected at the workshop end. Something to do with exporting the earth and the DNO not being happy.

 

[flameport]

The 3rd core and armour in the swa will be not be connected at the workshop end.

In that case, buy 2 core cable, which will be significantly cheaper.

 

[ban-all-sheds]

The compressor is rated at 2.2 kw but have know idea what it will be on start up. Obviously more so I rounded it up to 3kw (pretty scientific way!!).

Not a sound engineering calculation. You need to find out what the makers say about the protective device - it's possible you'll need a Type C. Hopefully not a D.

Well i was going to connect everything back to the house and have an earth rod at the workshop as well, thinking more was best. Some recent knowledge gained from the forum has told me this will be wrong so i've gone for just the earth rod. The 3rd core and armour in the swa will be not be connected at the workshop end. Something to do with exporting the earth and the DNO not being happy.

Why throw away a perfectly good DNO earth unless you really really have to?

... i was going to ..... thinking ... Some recent knowledge ... Something to do with ...

Are you starting to think that there's more to all of this than you initially thought, and that maybe you aren't really ready yet to tackle a job like this?