Re-wire questions

Discussion in 'Electricians' Talk' started by Griff, Oct 12, 2013.

  1. Griff

    Griff New Member

    Looking for advice on a house rewire I am doing
    I am an electrician but have not been involved in installs for over 20 years (now in factory maintenance)
    • Installing a single oven which requires a 13A supply so am planning to simply use the kitchen ringmain via a plug as is standard on single ovens these days (we've had 2 like this) - The socket that will be used to power the oven will be hidden behind the cupbard next to the oven (not behind) so am planning to use a fused spur at worktop level to power this socket for ease of isolation in case of a fault. Is this ok?
    • Installing an electrical hob which requires a 32A supply so am planning a seperate 32A supply from the consumer unit via a 32A RCD or RCBO? (see last question) to a 32A DP switch to a cooker outlet point and using 6mm TW&E. The hob will then wire into the cooker outlet point.- Is this ok
    • For the earth bonding it appears that there are different trains of thought. If RCBO's are used then suggestions seem to be local bonding (baths, sinks, pipes etc) is not required. Is this so? If not what is the current requirement for earth bonding wrt cable sizes. Seems to be a question of 6mm or 10mm.
    Incedentally once complete a certified part P electrician will be employed to test and provide certificate but I want to get it right first time without the need to rip up floor boards again.
  2. unphased

    unphased Screwfix Select

    Hi Griff
    I would recommend keeping your hob and single oven on the same 32A radial circuit. Wire the circuit in 6mm2 T&E to a 45A cooker control and drop a 6mm2 in to a cooker outlet plate. Put your oven and hob tails on to that. You are allowed up to 15kW on a standard 32A cooker circuit. Remember, hobs and ovens are fixed loads so no overcurrent protection is necessary. It is not good practice to put plugs on single ovens. 2kW loads are okay but 3kW (13A) should not be put on a plug. Okay for kettles (3 to 4 minutes) but ovens can be on for hours.

    Standard bonding arrangements for a TN earth system are 10mm2 to gas and water and, yes, if using 30mA RCD protection all circuits, no supplementary bonding is necessary.

    Use 16mm2 for your main earth conductor.

    Use 25mm2 meter tails.

  3. J.P.

    J.P. New Member

    Why has he go to use 25mm2 meter tails Un?
  4. unphased

    unphased Screwfix Select

    Standard practice JP. 16mm2 require 80A or 60A BS1361 and no guarantee of there being one. its pretty much standard practice.
  5. J.P.

    J.P. New Member

    Thanks Un
  6. Gas bonding = (within 600mm of gas meter)

    Un would it be better to keep the oven on the ring supplied by a 20A DP switch. If the oven is supplied by13A supply, increasing it to 32A will not protect the oven should fault develop. Both appliances can be isolated seperatley.

    Edit: mistake  'oven' should read 'flex'
  7. Griff

    Griff New Member

    Thanks all for the feedback.
    My install scenario is that the oven will be on a wall directly opposite an island where the hob will be so whilst I take Un's point I think Jitenders suggestion suits my application better, in that the tails cannot reach the same outlet.
    Also Un, can I just clarify that RCD protection is sufficient and that I dont need to upgrade to RCBO's.
    Thanks again
  8. Look at either a 17th ed split load or high integrity (i.e. Dual RCD + upto 3 RCBO's) con. unit both having Dual RCD protection which meet  the requirements.

    An expensiver option is to use full RCBOs with main switch - this is my preferred option.

    DNOs can fit a 100A DP isolator. BG = £48, Eon = free (make sure BG! upgrade tails their end to 25mm) instead of organising temp disconnection and re-energizagion.
  9. If the cooker is supplied with a fitted plug it can be assumed that a 13A supply is suitable - in this case use the cooker with the plug incorporating a 13A fuse. I don't see the point in installing a FCU as there will be no discrimination between these two fuses - therefore use a 20A DP switch.

    If the cooker comes supplied without a plug wire using a 20A DP switch and flex outlet.

    <strong style="font-size: 10pt;">But do check man. literature to ascertain witch method is recommended as well as power rating - as if not followed it could invalidate the warranty.[/b]
  10. unphased

    unphased Screwfix Select

    I would still separate the oven supply from the ring circuit. The simple way to do that is to put the cooker outlet plate in the island under the hob and just extend a second 6mm2 T&E cable from the cooker outlet plate back to the wall and then either put another outlet plate in the cupboard next to the oven, or just connect it direct to the oven. It would be perfectly acceptable to put a junction box on the end of the 6mm2 and connect flex from that to the oven if the oven comes with flex attached. Jetinder's point about 32A not protecting the oven is not correct. As I said, the oven is a fixed load so it cannot overload the circuit. In the event of a fault such as short circuit the current generated will easily trip a 32A mcb before any further damage can occur to the oven or its flex. A common misunderstanding so worth pointing out. MCBs are matched to the rating of the cables. The RCD will handle any earth faults. The difference between an RCD and RCBO is just that an RCBO combines the function of a fuse and an RCD in one unit. An RCD is just an earth fault device that won't do the job of a fuse and protect against overload or short circuit. As long as the circuit is protected by both then it matters not whether it is RCD and mcb, or RCBO. 30mA is the required rating.
  11. seneca

    seneca Screwfix Select

    Spot on UP, what a load of carp being spoken about the 32a breaker not protecting the oven, it's not intended to for christ's sake!
  12. Thanks Un for clarifying the fixed load part.

    If the oven is suppied with a plug atached it is most likely to be 1.5mm flex (i.e 16A rated) - in this scenario would you still rec. 32A?
  13. seneca

    seneca Screwfix Select

    I would because it's a fixed load so can't cause an overload and s/c protection is afforded adequately by the 32a mcb.
  14. Thanks Sen

    Sorry not trying to be funny, but we knew from the start that the cooker required a 13A supply, hence a flex of no more than 1.5mm (16A) would have been fitted to oven.
  15. Without seeing the kitchen and measurements. I would go towards seperate supply, as my only concern is providing a means of local isolation. A swith shoud be within 2m of appliance.
  16. seneca

    seneca Screwfix Select

    Just as a point of interest even when an oven does come pre-fitted with a 13amp plug it isn't good practice to put it on the ring circuit, especially if over 2 kW.
    Cooking appliances of more than 2 kW should be on a dedicated circuit.
  17. I agree with seperate supply if 3kW+ as Un mentioned. I am just thinking use as the original manufacturer intended thats all. The Op is installing a 'kitchen ring' - again without ratings cant comment any further.
  18. seneca

    seneca Screwfix Select

    No, JG, >2kW is the recommendation not 3kW.
  19. Could Griff provide max. power rating for both oven and hob.

    On second though it may be better to go with seperate radial circuit, if they decide to upgrade to higher capacity cooker in future.

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