MCB on old Memera CU

[deeks]

I am a complete novice, so please bear with me. We have an old Memera 3 CU and have recently had a new electric shower fitted (10.5 KW) and it keeps blowing the 30A fuse. So instead of replacing the fuse wire each time, I've just bought a 40A plug-in MCB from the local eletrical wholesaler. The pins were the wrong way, but when unscrewed I have rotated them through 90 degrees and have managed to fit it on the CU. After switching on and testing for a few minutes I switched off again and the MCB had become very hot. There was a slight smell coming from the CU too. Doesn't take a genius to work out that there's something not quite right here, but would anyone out there expect the MCB to get hot like that? Is my best option to get the whole CU replaced with trip switches?

 

[davy_owen_88]

10500w / 240v = 43.75A

You need a 45A MCB, but first find out what size cable is feeding the shower and don't use it again until you do. Also tell us how long the cable run is and what route it takes from the CU to the shower (through insulation etc...).

P.S What you did was reckless, you should NEVER upgrade the fuse/MCB without checking things out first and ensuring the cables/accessories are fit for the increased load.

 

[deeks]

Thanks Davy - I'll check it out. At a guess I'd say the cable run is around 10m from the CU

 

[JohnD]

Although for reasons of age it would be advantageous to replace the whole CU, and bring all the circuits and bonding up to standard, your immediate problem is with the shower circuit.

It is possible to obtain a small CU just for the shower, with an MCB and an RCD (which reduces the risk of electric shock).

As Davy has in mind, you also may need the existing shower cable to be replaced with a heavier one.

You need to have this installation done by an electrician who is a member of a self-certification scheme and will give you the correct documents on completion. Ask round friends and neighbours for recommendations, and ask each if he is a member of a self-certification scheme, and which one.

 

[deeks]

Thanks John. What size should the cable be as a minimum?

 

[JohnD]

see Davy's q.

 

[deeks]

Cable appears to be 16mm. I'll get an electrician to take it from here. Thanks for the advice John and Davy

 

[Adam_151]

Cable appears to be 16mm. I'll get an electrician to take it from here. Thanks for the advice John and Davy

16mm² is big for a shower even by todays standards, hold on... how are you determining the size of this cable?

 

[deeks]

Just measured the diameter. As I said, I really don't know what I'm doing, so will get a professional in

 

[RF Lighting]

STOP RIGHT THERE

Memera 3 CUs have a maximium outgoing rating of 30A

You are also tyring to bodge a WYLEX MCB into an MEM fuse box.

As you have an old MEM fuse box, it is unlikely you have any RCD protection for your shower.

You need to either have a seperate CU installed for the shower, or have your existing CU replaced.


In the mean time DO NOT USE THE SHOWER. You say it started burning after just a few minutes. Think what will happen if it is run for much longer than this (Clue: it's not a good thing)