I had a phone call from my son yesterday, the shower in his house had broken and not being a plumber/electrician got a couple of quotes. The old one was 6.5 kW with 6mm cable. The plumbers all seemed to think that the cable and MCB should be replaced as the current rating was inadequate for a replacement, the MCB is 40A. I had a quick check what could be fitted as there are many more powerful ones available and this weather the water is not wonderfully warm! I have not really thought about shower cables for a long time as they are almost always in good condition. Table 4D2 6mm clipped direct says 46A, continuous rating. A 10.5kW shower takes 45.65A on 230V so is quite satisfactory on the 6mm cable. In fact, the model I was looking at (Mira fancy thermostatic, expensive) would only take this power at full flow and temperature rise as it has electronic thermostatic proportional control and normal running current would be modulated and somewhat less. I consider this to be perfectly satisfactory, but the plumbers obviously haven't read the regulations. I often find the same with cookers, various sellers suggest 10mm cable, which may well not be the existing size. Should any change be made, and why do would you do this?
If my back-of-the-envelope sums are right, a 15m run of cable for a 45A shower will give 1.7% power losses for 6mm2 and 1% for 10mm2. The larger cable will cost an extra £10 approx, so will pay for itself after about 70 kWh of reduced losses, or 10,000 kWh of shower usage, or about 1000 hours of use. Not sure what that is in units of Teenage Daughters.
I get slightly more. The difference in resistance between the two cable sizes is 4.05 mΩ/m so 61 mΩ. Power = I²R = 123 W extra. For a 15 min shower, that is 1/8 x 1/4 = 1/32 kWh. At 16 p that is a ha'penny per shower, so pay back is after 2000 showers or about 2 years for a hygienic family of three.
If my back-of-the-envelope sums are right, a 15m run of cable for a 45A shower will give 1.7% power losses for 6mm2 and 1% for 10mm2. The larger cable will cost an extra £10 approx, so will pay for itself after about 70 kWh of reduced losses, or 10,000 kWh of shower usage, or about 1000 hours of use. Not sure what that is in units of Teenage Daughters.
I get slightly more. The difference in resistance between the two cable sizes is 4.05 mΩ/m so 61 mΩ. Power = I²R = 123 W extra. For a 15 min shower, that is 1/8 x 1/4 = 1/32 kWh. At 16 p that is a ha'penny per shower, so pay back is after 2000 showers or about 2 years for a hygienic family of three.
