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SWA to outbuilding
Question
Hi all, quick question; I am running a 100m of 10mm 2 core SWA to a workshop, there will be a consumer unit and earth spike in the workshop. My question is; can I take a split from the DNO disconnect and feed the SWA from there? I really don't want to come out of the house Consumer unit, so thought I would come from the disconnect and put in a 40a c curve breaker before the SWA. and earth the armour at both ends. 
Or should it be a 40a RCBO before the SWA?
The house is on a TN-C-S, will the earth spike at the workshop cause a problem with this?
Many thanks
Kris
 
13 Replies
OlympusMons 11001209648
117 Posts
Have you bought the cable yet (thinking VD)?
What is the construction of the workshop (wooden/metal)?
Any water pipes etc that might need bonding?
Dbat 11001212107
33 Posts
Going to need bigger than 10mm for 40A over that difference, even without looking in the book.
10mm should be okay according to calcs, the workshop only needs 3kw max, most of my stuff is cordless, I only thought 40a breaker at the head because that is what the cable is rated to, I could put in a 20a and it would be plenty. Workshop will be wooden construction, no pipes to bond. 



 
UKPN 1100233915
103 Posts
Others will advise voltage drop, but if you intend using PVCSWA for this project the minimum size for 2 core for PME is 95mmsq. Earths spikes have no effect and can in fact interfere with the DNO network.

Regards, UKPN⚡
95mm2 for a 20A supply!! I don't have 95mm coming to my house. I've been working in events for over 25 years and we use 95mm powerlok for 200A supplies, I think I'll stick with the calcs in BS 7671.
UKPN 1100233915
103 Posts
You wont find the calculations for PME requirements for SWA equivalent to copper cable in non legal guide books, I will set out the calculation for you on here if you like, but deep in your heart, as the days go by and my colleagues confirm my post, you will run a separate 10mmsq with the SWA or disconnect the armour at the workshop end, fit a stake and rcd.

Regards, UKPN,⚡
AJJewsbury 77361768
1608 Posts
I think UKPN is referring to the requirement for PME bonding conductors in other metals to have the equivalent conductance of the stated sizes in copper - i.e. the usual 10mm² requirement becomes about 90mm² in steel (but note that would be the c.s.a. of the armour, rather than the copper cores).

Although if the outbuilding has no extraneous-conductive-parts to bond, the SWA wouldn't need to function as a bonding conductor, so just the usual sizing for c.p.c.s will do.

    - Andy.
AJJewsbury 77361768
1608 Posts
earth spike in the workshop. .... and earth the armour at both ends.
What's the spike meant to achieve?

If the intention is to TT the workshop, then it needs to be kept completely isolated from the PME earth (so as the armour would be connected to the PME earth at the house end it can't be connected to the TT earth at the remote end).

If it's meant to be an additional PME electrode, then in BS 7671 terms the rod would be an extraneous-conductive-part and the connection to it would strictly speaking be classed as a main bond - so the protective conductor between the house and the workshop would have to meet main bonding requirements (typically 10mm² copper .... or 9x that in steel).

    - Andy.
AJJewsbury 77361768
1608 Posts
My question is; can I take a split from the DNO disconnect and feed the SWA from there?
Is the meter in an external meter box? If the most DNOs & suppliers prohibit customer equipment other then meter tails in the box (even if there's lots of spare space, it's reserved for being able to add extra meters or even a temporary cut-out).

If your additional can be internal, then as long as you have a single main switch that isolates the complete installation (the supplier's isolator will do as long as it's DP and accessible) - then yes you can Henley block and take a separate feed to a switchfuse or whatever you need to protect the SWA.

Although if it's designed only for 20A there might be a temptation for putting a 20A MCB at the house end and dispense with the consumer unit in the workshop (just have a 5A FCU for lighting).

   - Andy.
Thanks Andy, that’s all a lot clearer now. 
lyledunn 83218531
292 Posts
Have you checked cost for your cable? I managed to get 16mm2 3-core cheaper than 16mm2 2core.
By the way, my post is coming up with an irritating intermittent Message that it has been “Auto Saved”. Anyone else getting this or is it something on my IPad that requires attention?
AJJewsbury 77361768
1608 Posts
lyledunn:

By the way, my post is coming up with an irritating intermittent Message that it has been “Auto Saved”. Anyone else getting this or is it something on my IPad that requires attention?

Yup - me too. Usually right on top of what I'm typing. Looks like we've had another "upgrade" - I notice they've 'fixed' the post ordering - in that it now defaults to oldest first like it used to. A pity it still jumps back to the first page after you've submitted a post (now the oldest) - so you new contribution isn't shown on longer threads.

   -  Andy.

Dbat 11001212107
33 Posts
Through The Haze:
10mm should be okay according to calcs, the workshop only needs 3kw max, most of my stuff is cordless, I only thought 40a breaker at the head because that is what the cable is rated to, I could put in a 20a and it would be plenty. Workshop will be wooden construction, no pipes to bond. 



 

If your putting a 40A MCB upfront to protect the cable it doesn't matter that it will only has a 3kw load occasionally when designing the circuit though. Regardless of what the cable is rated to

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