CC45 Posted June 16, 2019 Share Posted June 16, 2019 Electrical challenge number 2 - what size of cable for the range cooker I'm told we want? TLC calc https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html used and for the following criteria this is what it suggests: SWA (2 core) PVC, 14m and load of 15kw (looked up the installation instructions) suggests 10mm - does this sound right? The cable will run in the ceiling void with 50mm of insulation above it ( about 150mm above it). Will check with the sparky who signs my work off but always worth an ask here... This should be the last challenge wiring wise! Desperate to get some outside work done - I need a change from inside. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted June 17, 2019 Share Posted June 17, 2019 First off, 15 kW seems high, is that allowing for diversity? Normally a domestic cooker would be rated at 10 A + 30% of the full load current for the appliance from the MIs. If there is a socket outlet on the cooker switch then add another 5 A. Secondly, unless there are some specific circumstances that require a high level of mechanical protection, then you wouldn't use SWA indoors for something like this, but twin and earth. Allowance needs to be made for the installation method and insulation, but the cable size required is best obtained from the tables in BS7671, rather than the TLC calculator, as the latter tends to under-estimate the cable size required a fair bit of the time (it doesn't take proper account of derating or the installation method, AFAICS). If the cooker is rated at 15 kW at 230 VAC, then the full load current would be 65 A and the load for calculating cable size, accounting for diversity, and assuming no additional outlet at the cooker point, would be 10 A + 19.6 A = 30 A for calculation purposes. Using reference method 100 (T&E cable running across a ceiling with insulation above) gives a cable size of 6mm² (34 A max) and a voltage drop of 7.3mV/A/m which is ~3.1V for 14m (1.35%) which is within the maximum allowable. Increasing the cable size to 10mm² gives a capacity of 45 A and a voltage drop of about 0.8%. My inclination would be to use 10mm² T&E to give a bit of additional headroom, but 6mm T&E would be OK. 2 Link to comment Share on other sites More sharing options...
ProDave Posted June 17, 2019 Share Posted June 17, 2019 +1 to the above. Even though it should just be okay, I would not even consider 6mm for this. I would be putting 10mm t&e without a doubt on a 40A MCB Keep it as far away from the insulation as you can. 2 Link to comment Share on other sites More sharing options...
CC45 Posted June 17, 2019 Author Share Posted June 17, 2019 Cheers both. 15kw is the rating (my addition) when all the induction hobs are on boost + oven on. With all the induction hobs on max + oven going = 12kw. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted June 17, 2019 Share Posted June 17, 2019 20 minutes ago, CC45 said: Cheers both. 15kw is the rating (my addition) when all the induction hobs are on boost + oven on. With all the induction hobs on max + oven going = 12kw. The figure to use in the calc above is the 15 kW maximum, with an assumed supply voltage of 230 VAC, so 15 kW / 230 VAC = 65.2 A. This assumes that the maximum load in the MIs for the cooker is actually 15 kW (worth checking, as it may be that the maximum load is limited in some way - our induction hob has a rated maximum load that is lower than the sum of all the rings, for example). Applying diversity gives 10 A + (65.2A x 30%) = ~ 30 A. This is the current to use when selecting the cable. Next the cable table in BS7671 has to be checked for a T&E cable running across a ceiling, with insulation spaced above it, which is reference method 100 in this case. The table shows that a 6mm² run of T&E is just acceptable, with a rating of 34 A, but, as above, it's a very good idea to allow a margin, and the next cable size up is 10mm² T&E which would be what I'd use (and is actually what I did use for our cooker cable). 1 Link to comment Share on other sites More sharing options...
CC45 Posted September 24, 2019 Author Share Posted September 24, 2019 On 17/06/2019 at 07:30, JSHarris said: First off, 15 kW seems high, is that allowing for diversity? Normally a domestic cooker would be rated at 10 A + 30% of the full load current for the appliance from the MIs. If there is a socket outlet on the cooker switch then add another 5 A. Secondly, unless there are some specific circumstances that require a high level of mechanical protection, then you wouldn't use SWA indoors for something like this, but twin and earth. Allowance needs to be made for the installation method and insulation, but the cable size required is best obtained from the tables in BS7671, rather than the TLC calculator, as the latter tends to under-estimate the cable size required a fair bit of the time (it doesn't take proper account of derating or the installation method, AFAICS). If the cooker is rated at 15 kW at 230 VAC, then the full load current would be 65 A and the load for calculating cable size, accounting for diversity, and assuming no additional outlet at the cooker point, would be 10 A + 19.6 A = 30 A for calculation purposes. Using reference method 100 (T&E cable running across a ceiling with insulation above) gives a cable size of 6mm² (34 A max) and a voltage drop of 7.3mV/A/m which is ~3.1V for 14m (1.35%) which is within the maximum allowable. Increasing the cable size to 10mm² gives a capacity of 45 A and a voltage drop of about 0.8%. My inclination would be to use 10mm² T&E to give a bit of additional headroom, but 6mm T&E would be OK. just in the process of doing this - just thought this was a very generous piece of advice - thanks @JSHarris Link to comment Share on other sites More sharing options...
CC45 Posted September 26, 2019 Author Share Posted September 26, 2019 guessing @ProDave is the man for this query... Going to order some Hagar RCBO's, I need an additional two for ring mains (32a) & one for the cooker discussed above (10mm2 cable) - 45A. The two 32a seem simple enough since the sparky put some in last time he checked my work (https://www.edwardes.co.uk/products/hager-ada332g-32a-30ma-b-curve-rcbo-type-a-ac-and-pulsating-dc-sensitive-) and I will just buy two the same. The cooker - I was going to fit a 45A RCBO - is this the correct one? https://www.edwardes.co.uk/products/hager-ada145g-45a-30ma-b-curve-rcbo-type-a-ac-and-pulsating-dc-sensitive- Just getting unsure because the numbers are a bit different to the 32A ones. Can anyone confirm the 45a one is the right one? Cheers Link to comment Share on other sites More sharing options...
ProDave Posted September 26, 2019 Share Posted September 26, 2019 Your link did not work, try this https://www.edwardes.co.uk/products/hager-ada145g-45a-30ma-b-curve-rcbo-type-a-ac-and-pulsating-dc-sensitive- What is your concern with that? 1 Link to comment Share on other sites More sharing options...
CC45 Posted September 26, 2019 Author Share Posted September 26, 2019 hi @ProDave thats the one I tried to link to - its just the others had 3XX with XX = amps and this one is 145 thats all. Will get it ordered now. Cheers. Link to comment Share on other sites More sharing options...
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