Shed/workshop electrics

Discussion in 'Electricians' Talk' started by Dominic Baines, Mar 31, 2015.

  1. Dominic Baines

    Dominic Baines New Member

    Our house and starting to look at what is where and updating etc.

    The garage is 5m from the main distribution board, fed with a single 3 core 10mm armoured cable.
    A shed/workshop is 50m from the garage.

    The garage is fed from a 40A RCBO from the main distribution board (which is not wired 17th Ed but does have a single RCD and unit was put in during 2008). Inside the garage there is a 63amp RCD 3 way 'shed/garage unit'. One 16A MCB has six metal 13amp sockets on a spur (all wiring in metal conduit) not a ring in the garage, one 6A MCB to some fluorescent lights.

    The workshop is fed from the garage unit using a third 32A MCB to a spur (10mm armoured cable) witha 'cooker switch', effect is dual LN can be disconnected, from this the cabling to the workshop is TWO lengths of 4mm T&E in a buried c18inches 50mm PVC pipe all the way to it. There is also a 6mm earth wire running along side the others in the same pipe, they were tied together at regular intervals before being fed in (it is a straight run). There is earth bonding at the point the wiring leaves the garage and again when it arrive in the workshop (10mm earth leads used to what looks like 1/2 copper pipe). I've looked the pipe is sealed and nice and dry, there is an IP55 inspection box about 1/2 way down.

    In the workshop there is a modern (17th Ed) domestic consumer unit with dual 32A RCD, 2 16A MCB (sockets) and 1 6A MCB (lights) on one and 1 10A MCB (A/C heater/cooler of sorts) on the other. There are other MCBs in the unit (were the originals and were left in place) but not used. Was swapped last year for the old one. No external sockets or feeds.

    The voltage drop at the workshop end I measured at about 9V so have the electricians.

    If the garage trip goes it feeds the workshop not an issue really and if any of the RCD trip in the workshop the garage stays up.

    Asked two different sparks, one said seems fine leave it alone as everything is protected and not changing anything other said I should rewire the main household fuse box to 17th Ed and change the cable to the workshop to 10mm armoured.

    What would you do?

    Dom
     
  2. unphased

    unphased Screwfix Select

    The whole setup is badly designed. You have far too many RCDs and they are all in series so no discrimination. First thing to do is remove the front end RCBO in your main fuseboard and swap it with a 40A mcb. This will take out the RCD upstream of the garage. Next remove the 30mA RCD main switch from the gargae unit and install a standard 100A main switch. Swap the 16A mcb and the 6A mcb for RCBO's and put another 32A mcb (not RCBO!) to feed the shed. In the shed keep the CU as it is. The pair of twin 4mm2 T&E cables are effectively a ring circuit feeding the main switch in the shed. This is perfectly adequate for the length of run because you are using conductors in parallel and reducing the impedance of the sub-main by doing so.

    You are misunderstanding volt drop. Volt drop is dependent upon the load. If there is no load then the volt drop you are measuring is just the difference in the voltage at the shed to the house NOT the volt drop. VD is measured in millivolt per amp per metre and each cable size has a tabulated value for this. You need to use those figures in the equation mV/A/m substituting the estimated load for the 'A'.

    The impedance of the 4mm2 ring circuit is going to be about 1/4 the value of the equivalent radial. So you would need to work this out before applying vd calculations.

    I think you are focussing on the wrong factors, here. Forget vd and sort out the RCD mess. The circuits are incorrectly designed.
     
    FatHands likes this.
  3. Dominic Baines

    Dominic Baines New Member

    Tnx for answer.
    1. The MU 40A RCBO to 40A MCB easy swap.
    2. In the Garage unit remove the main RCD and replace with a standard 100A switch.
    3. In the Garage unit replace the two MCB with RCBO (16A and 6A)

    ... makes sense as keeps them under RCD and independent of the shed. The shed already is just an 32A MCB so do not have to change that.

    Just asking as thinking.... doing 1 and 2 doesn't that remove the RCD protection between house and garage as the RCBO in the house MU was going to protect that bit up to the Garage Unit? Admittedly rare the armoured cable will have an issue and in a pretty static/stable location and not driven over etc.... but I guess it happens... leaving the MU RCBO in place if don't do 1 a problem.

    The pair of 4mm T&E cables to the are used in parallel, not a ring so BOTH fed at same time, was expecting them to act as if were one 8mm cable or this not the case? ... didn't HAVE a 50m run of 10mm cable originally or would have used that. Yes understand there is a danger if one cable fails then other takes whole load. (50m of 10mm T&E probably about £250 these days and rising)

    The voltage drop for the cable size was under max load (6KW so c 26A) sorry... should have said. So I calculated 6mm or more... happy to be told wrong as long as explain :)

    Dom
     
  4. unphased

    unphased Screwfix Select

    Hi Dom,

    Thanks for clarification on the measured vd. :)

    There is no need for RCD protection to the sub-main circuits. Indeed it is preferential no to so you can allocate RCD protection to circuits served without affecting the upstream devices.

    The pair of 4mm2 cables, if used together as one circuit and terminated together (as I assumed) are in fact technically a ring circuit. I know they are side-by-side but the conductors will behave as parallel conductors which increases their capacity to carry load by virtue of reduced impedance. The conductors in parallel are not the same as twice the csa as a single (8mm2) because they behave as conductors in parallel and are better than one single 8mm2 (though the nearest stock size would be 10mm2 as you know). This is the principle of the UK standard ring circuit wired in 2.5mm2 T&E on a 32A breaker. A single 2.5mm2 T&E is not up to carrying 32A but a parallel pair of 2.5mm2 conductors increases their capacity.

    Regards
     
  5. Dominic Baines

    Dominic Baines New Member

    Tnx.

    I'll time table the rcbo/mcb and mcb/rcd swaps soon, once find them available at OK price.
    I guess I was thinking protect the circuits not just the person ho hum.

    tnx fer clarification of the paired 4mm cables vs 8mm. I assumed (dangerous) the pair would be better but not quite as good as the 10mm option but it was what I had to hand at the time and as both cables in same trunk chance of single failure pretty low.

    And yes before asked off line ... I will get an official in to charge me to test the circuits so I get the bit of paper I'll copy, laminate and stick by the MU in the workshop. Although technically I could just disconnect the workshop feed, 32A MCB and cables and it stays as just a simple garage.

    Years ago was all just run with an extension lead and a 4 way socket, how times move on!

    Dom
     
  6. Risteard

    Risteard Screwfix Select

    It would be a parallel circuit - NOT a ring circuit. There are certain rules which must be followed for a parallel circuit.

    Incidentally, I wouldn't be in favour of the distribution circuit being fed from an MCB as this will not discriminate correctly with downstream MCBs.
     
    Lectrician likes this.
  7. unphased

    unphased Screwfix Select

    You know best, Risteard.
     
  8. Dominic Baines

    Dominic Baines New Member

    I was trying to find the details... please can you expand on what they are?

    Dom
     
  9. seneca

    seneca Screwfix Select

    Both conductors should be of the same size and length and it should not be likely that one of them could become disconnected at some time as overloading of the other one could occur. That's the way I see it anyway!
     
  10. Dominic Baines

    Dominic Baines New Member

    Tnx... they are identical cables (were from two reels obtained at same time fed togther I recall), following exact same route and the only connections/breaks were at either end and in the middle (inspection pit) and they were crimped together before being inserted into any necessary connection not just twisted and squashed under a screw thread.

    Dom
     
  11. seneca

    seneca Screwfix Select

    Sounds fine then Dom.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice