Logix1

Steptoe, my comments about oversizing come from the experience I have in my existing home - the voltage drops 6V (using an accurate appliance monitor, 8V in the kitchen and 2V at the meter) in the kitchen when the 2.4kW oven (which I assume is a largely resistive load) is on, which suggests about 60W (2.5KW * 6V/ 240V) is being lost between the meter which is in the garage, and the kitchen about 6m away. Unfortunately the cable goes underground so there is little I can do about it. However if you assume your 2.5kW oven is perhaps on for 1 hour a day, then its annual consumption is about 900 kWh/year (probably an over-estimate), which costs £140/year (at 15p/kWh), over a 25 year investment period for the cable that's £3,500 of electricity used by the oven, my 6V drop on 240V is a 2.5% drop, so £90 over 25 years being lost in the cable, which probably represents a worthwhile investment in additional copper if it were not for the difficulty of replacing the underground cable. I suspect there may be some other issues going on in my 40 year old home to cause this sort of drop.... Anyway, if you were starting from scratch and comparing 3 different cable types 1.5mm2, 2.5mm2, 4mm2 of 10m in length for the same oven, with resistances of 12, 7 and 5 mohm/metre, then the voltage drops would be 1.2V/0.75V/0.5V respectively or 0.5%, 0.3%, 0.2% - and the 25 year cost of the losses in these cables would be £14, £8 and £5 respectively, and the cost of the 10m cable would be £3.60, £5 and £8.00 respectively. So the energy loss saving of going from 2.5mm2 cable to 4mm2 cable would be £3, and the additional cost of the cable would be £3, so the breakeven seems to be around the 2.5mm2 to 4.00mm2 range on a 25 year investment horizon. Anyway, I suspect my views are tainted by my experience in my existing home - which was all as far as I know installed professionally. The previous owner had the house built and was not at all practical - so wouldn't have done the electrical work himself, and my impression of him was that he would have hired a recommended 'professional' electrician. Perhaps there is something else going on in my home, but a 6V drop, and 60W cable loss, to me is unacceptable, given a professional installation? There may also be something wrong with my schoolboy maths above - comments please? But if the numbers are correct, and I was installing new cabling I would generally round up the required cable size to the next available size if it were a long term investment?

From recollection of looking at this in the past I would oversize for any high power appliance - only because there is a reasonable payback on the reduced power loss from lower resistance cable. This is especially true if there is a reasonable distance from the meter to the appliance. The obvious candidate for thicker cable is the oven. Hobs and microwaves have more intermittent use so the economics are less clear.