My Daughter has just moved into a new house and wants to change her existing electric shower from an 8.00kw ( which is working fine, just a bit old fashioned design) to a 8.5kw one, this is being feed of of an MEM (Memera) consumer unit of 80amp 250v AC on a 30amp rewireable type fuse using 6mm wire over just under a 5 metre distance feed behind a wall and up into the loft to reach the shower and does not have a 30ma RCD in the circuit. Is it OK to change this shower to the new one without doing anything else? Any help with this is appreciated. Thanks.
Electric Shower
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The protective device (fuse) is a little on the small size for you to get optimum performance out of a 8.5kw shower, so that would need upgrading.
Cable sizing is not quite as simple as that, because the rating will change depending how the cable is installed and if it is covered by insulation.
You could be stretching the use of 6mm cable already.
An 8.5 kW load will draw about 37A, so 10mm cable will be required, the circuit protection will need upgrading to 40A (assuming the existing consumer unit can take a 40A device) and an RCD installing.
You could be stretching the use of 6mm cable already.
An 8.5 kW load will draw about 37A, so 10mm cable will be required, the circuit protection will need upgrading to 40A (assuming the existing consumer unit can take a 40A device) and an RCD installing.
Thanks for your reply, would the RCD have to be an 80A 30ma RCD or would some other type also be suitable for this, thanks again.
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There are two figures here. One is the current rating, or how much electricty can safely flow through the RCD. 80A would be OK, but 40A would be sufficient for the shower. For an RCD, this figure being higher than necessary doesn't compromise safety, beause the fuse / MCB in the consumer unit protects against overload.
The other figure 30ma is the important one, this represents the current flowing to earth in the event of a fault, causing the RCD to trip. This is correct for a shower
The other figure 30ma is the important one, this represents the current flowing to earth in the event of a fault, causing the RCD to trip. This is correct for a shower
It needs to be 30mA, but the load rating can be lower than 80A. 63A is a commonly available size.
Or if you can't get a 40A fuse into the existing CU and have to have a separate unit then you could have an RCBO instead.
But TBH, as you're getting into the realms of new cable, RCD, higher rated protective device etc, I'd suggest having the electrician install a full sized CU, mostly empty, rather than a mini-unit (assuming there's space, of course), as this will almost certainly be of use to start populating later, and to go for a 10.5kW shower.
Or if you can't get a 40A fuse into the existing CU and have to have a separate unit then you could have an RCBO instead.
But TBH, as you're getting into the realms of new cable, RCD, higher rated protective device etc, I'd suggest having the electrician install a full sized CU, mostly empty, rather than a mini-unit (assuming there's space, of course), as this will almost certainly be of use to start populating later, and to go for a 10.5kW shower.
He hasn't got MCBs...
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