Shed electrics are in

[phil_ballard]

Thanks to all who helped along the way (here and elsewhere), my shed electrics had the big switch-on last night and all seems OK. One final question, if I may; the cable to the shed goes to a 'garage CU' with one 16A and one 6A MCB. The cable from kitchen CU to shed is 6mm T+E (inside) and 4x2.5mm SWA with cores doubled-up to 2x5mm (outside).
I used an unused slot in the Wylex kitchen CU and replaced the wired fuseholder with a Wylex 32A plug-in MCB. This now leaves the kitchen CU with 1x5A, 1x15A and 1x30A wired fuses, 1x32A MCB and the cooker fuse (45A? can't remember, will look tonight). The CU has a 'max 80A' sticker on it. Could someone please show me how to do the diversity calc to check all is still OK? Electric fan oven, gas hob.

Yes, I know I should have had that worked out before I started ...

 

[ban-all-sheds]

The cable from kitchen CU to shed is 6mm T+E (inside) and 4x2.5mm SWA with cores doubled-up to 2x5mm (outside).

Why didn't you use the right size SWA in the first place?

Would anybody with the data tables care to check that the CSA of the armour on 4-core 2.5mm² is the right size for it to be the cpc on that circuit?

 

[phil_ballard]

I already had this piece - it's easy to change if necessary, the trench is (deliberately) not filled in yet. Would would the 'right' SWA have been?

 

[ban-all-sheds]

6mm². 3-core L/N/E, with the armour also earthed.

There is a school of thought which says that as you only want to dig that trench and lay cable the once, you should use 10mm², on the grounds that "you never know".

 

[phil_ballard]

I can understand that, but the trench is only about four feet long and in easy soil so not a big issue. Of course I'm not questioning your concern - re the current capacity of the SWA armour - but it's CSA appears way larger than the earth conductor of the 6mm T&E used in the same cct, which is why I never questioned it. Ah well, live and learn.

Thanks Ban.

 

[ban-all-sheds]

Don't forget that the armour is steel, not copper, so for a given csa it has higher resistance.

See here:

http://www.screwfix.com/talk/thread.jspa?forumID=23&threadID=6084&messageID=60083

And this is the table of armour values (with many thanks to Lectrician for sending it to me).

(but it was I that turned it into a JPG and put it on the interweb)

So doing the sums for 5mm² conductor size (your 2 x 2.5mm²) your armour would need a csa of (5 x 143)/46 = 15.54mm². The armour csa for 4-core 2.5mm is 20mm², so you'd be OK, but I hope you can see why, when going from memory as I was yeserday, that I thought it should be checked.

Also, I'll bet that those ROT calculations assume worst-case 5s disconnect on a semi-enclosed fuse......

 

[phil_ballard]

Excellent help, thanks again Ban.

Returning to the question I posted to ask, re 80A CU and diversity calc - could someone pls show me how that is calc'd and whether I'm in the clear?

MTIA

 

[plugwash]

first it must be borne in mind that the diversity calculations in the wiring regs are only guidelined

general assumption in domestic installations is that your fine if the CU is rated as high or higher than the service fuse

 

[phil_ballard]

This is one of 2 similar CUs ... I don't know the rating of the service fuse ...

 

[Lectrician]

I recognise that chart BAS

 

[ban-all-sheds]

And with good reason...