by ericmark »
Tue Dec 21, 2010 7:56 am
My shower is gas powered at 27kw and in comparison 8.5kw is small. Even so we note difference summer to winter due to temperature of water arriving at the house.
It takes about 3 minutes to boil a kettle and you want half that temp and using 3 time the power so about 3 kettles of hot water per minute. That is not much water so first question are you asking too much?
The more water through the shower the cooler the water will be.
As to cable even 6mm will not have a volt drop large enough to reduce output. When fitted the electrician will have measured the prospective short circuit current. With a B40 MCB this must be over 200A and this means the voltage must be 192v at the shower (allowing for reduced output due to volt drop) but that is absolute min and in real terms he must consider volt drop. He should have calculated max volt drop in the premises of 5% with together with the 6% supply authority is allowed give 204.7v at the shower. This would mean an 8.5kw shower really works at 7.6kw. If measured as loop impedance 0.8 ohms and as short circuit current 290 amp this will be written on the installation certificate issued.
At the 37A rating it should be on at least a 40A MCB with 10mm cable again all will be on installation certificate. So assuming you are just within the permitted values you could lose 1kw due to cable losses. If you assume one changes 10mm cable for 16mm cable and you are on limits in first place you could gain 18v that would lift it from 7.6kw to 8.2kw and that is very little really.
More likely than a cable fault is some plumbing fault. The most common is wrong rose on shower. With my 27kw the shower head is big and loads of high pressure water comes through it. With an electric shower the rose is much smaller to get some pressure.
To me all electric showers seem like a trickle the only way to use electric for a shower is with an immersion heater so it has chance to pre-heat the water. Then use a power shower to give it some pressure. However is seems to be the plumbers way out when fitting combi boilers to tell customers to use electric when the boiler can run a shower direct and it is just laziness on part of plumber.
