CU quiery

Discussion in 'Electricians' Talk' started by pete4690, Jan 30, 2015.

  1. pete4690

    pete4690 New Member

    on my CU I have a spare 40a mcb I am planning on powering my shed from this into a smaller CU in my shed which has a 40a 32ma RCD, a 32a and a 6a MCB which will be used for lighting and sockets, the feed will be via SWA whick would be better 4mm or 6mm? the shed is 23 metres away from the house CU and can I run the cable from the 40a mcb in the house to the 40a rcd in the shed? or do I need different size RCD/MCB?

    thanks.
     
  2. JP.

    JP. Screwfix Select

    Whip some 10mm in m8
     
  3. pete4690

    pete4690 New Member

    i looked at 10mm but thought that might be a bit overkill lol, do you know if running the cable from a 40a mcb into 40a 32ma rcd up at the shed would be ok?
     
  4. JP.

    JP. Screwfix Select

    I aint got a clue m8 - 10mm overkill? 32ma rcd mainswitch? I don't know tbqh.
     
  5. JP.

    JP. Screwfix Select

  6. pete4690

    pete4690 New Member

    No ive done away with the 40a 32ma rcd up the shed end and been replaced with a 100a mainswitch as the circuit this will be on is already rcd protected at the house CU and the 32a mcb in the shed replaced for a 20a rcbo as i will only be having 2 double sockets on a ring off it and then lights off of the 6a mcb apart from that there wont be anything used up there that draws major amounts of current
     
  7. JP.

    JP. Screwfix Select

    If the shed sub circuit is cabled in swa directly from the cu (or proposed) then why does it need to be rcd'd at house end? surely a henly and switchfuse sub main would be far better? - that's why I said 10mm, this will allow discrimination.
     
  8. pete4690

    pete4690 New Member

    The mcb it will be hooked up to at the house end is on the rcd protected side so thats where my theory of not needing the rcd up the shed comes from am i right to think that?
     
  9. nigel willson

    nigel willson Screwfix Select

    You need rcd in shed not the house! SWA does not need rcd protection. You dont want circuits outside tripping supplies in the house , or having to run back to the house to reset.Shed RCD and earth rod IE TT. regards nigel
     
  10. MGW

    MGW Screwfix Select

    Taking a design current of 37A then 4mm will have too much volt drop at around 9 volt and permitted is 6.9 volt. At 6mm it will work. You can get 24 meters at 38A with a volt drop of 6.4 expect a drop of around 0.17 ohms so if house is 0.35 you will expect shed to be around the 0.52 ohm mark. I have used correction factors so you are right on the edge for volt drop.

    I have not got your figures to work with so there is a little guess work but should not be too far out. Ambient temperature I took as 30 degs C and Max permitted operating temp tp 70 degs C Installation method C I got a Correction factor Ct of 0.9538252602987778 so 6.962924400181078 mV/A/M as the volt drop.

    Clearly you need to do your own calculations but it would seem OK. I have my shed on house RCD clearly there are problems with this in that with a fault in shed it can trip house but I don't use shed much as there is an advantage knowing when there is a fault. I will use a FCU for mine but at moment just plugged in. I do feel being able to switch shed off is an advantage including the neutral.

    As to TT where house is TN and all gardens back onto each other I see no problem using the TN supply. However where nothing but countryside beyond the shed then may be a TT is required. If I was supplying a boat car or caravan from shed then yes look at TT but in the main TN is good enough.

    As I reviewed what you have said I see the design current has dropped so you will have to recalculate. At design current of 26A I get volt drop to 6.24 volt with 4mm cable. No guarantee my calculator is correct http://gw7mgw.co.nf/Electric-Cal.html but it should not be far off.
     

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