Shower power wattage

shouldbeinbed

Technical question for you.

The electric shower that came with the house has finally died.

I've bought a replacement 9.5kw one but have just found the power rating on the old one is 7.5kw. There is a 8.5kw one that'd fit the hole too.

I'm assuming that the wattage is either the heater or the pump flow power. Is a 2kw jump a bit much given that the old one was fine for pressure & heat. & is there much difference in the running costs? It'll be used for 4-6 showers most days.

Cheers

freddiegrubb

:oops: Basic electric shower facts - to heat the water you have to slow down the water as it passes over the heating element so the the higher the wattage the greater the heat transfer for a given water flow. If you want a hot shower -slow the flow down or increase the wattage of the shower . Now you have a problem increased wattage = increased electrical current (amperes) your old shower was 7.5 kw. & this will require a current of approx. 32.6 amps. at 230 volts, so the cable supplying this current must be capable of supplying this without overheating so it was probably wired in 6mm. twin &earth. The max. current carrying capacity of 6mm.in a perfect installation exceeds this 32.6 figure so your okay . Using the the same formula for the 9.5kw. & the figure is 41.3 amps, so your wiring has to be capable of carrying this extra load or it seriously overheats. Check your cable size & the current carrying capacity of the shower pull cord switch (if it has one) you may have to upgrade to 10mm. cable to carry the extra load (amperes). As for running costs of the bigger shower , forget it unless you have daughters!!! Also think about fitting an earth leakage trip safety device (elcb) for peace of mind, you may need to consult an electrician for the above.

mattshrops

dont know the electrical technicalaties but can confirm as a plumber
6mm cable up to 7.5kw.
over 7.5kw -10mm cable required.

golfergmc

From an electrician, you need a 45 amp fuse/breaker, 30ma trip switch, 45 amp pull cord switch and 10mm twin and earth cable. For every hour you use the 9.5kw it will cost approximately 30 pence more than the 7.5kw.

shouldbeinbed

Brilliant info thanks all, looks like i may as well keep this one and call in an electrician to be legal on the wiring & ensure pull switch is right (unlikely)

Drysuitdiver

make sure they are part P registered sparks. it will be notifiable as its

a) in a bathroom
b) change of protective device
c) a new circuit

9.5 Kw will need 10mm cable
an 8.5 can just get away with 6mm but only if the Consumer unit is almost back to back with the shower and the cable run is clipped direct ( very unlikely) 47A rated.

when you take Volt drop into account then 10mm is going to be the way forward. you will also need the circuit protected by a 30mA RCD see section 701 of BS7671 : 2008 ( 17th edition wiring regs)

oh and your supplementary bonding ( earthing) will likely need upgrading in the bathroom as well.