Chris Pearson: 
 

Ignorance?

That is where I’m heading, the 10 mm cable is okay, possibly over the top, but not an issue.

It is the 32 amp circuit breaker I have an issue with.

Sparkingchip: 
 

Chris Pearson: 
 

Ignorance?

 

That is where I’m heading, the 10 mm cable is okay, possibly over the top, but not an issue.

It is the 32 amp circuit breaker I have an issue with.

 

 

I agree. Even if the shower is rated 8.5 kW at 240 V, you still have to account for a load of 35.4 A. (If it's 8.5 kW at 230 V, that's 37 A.)

It certainly is unlikely to meet the loading factor criteria for the CU.

The Breaker will be running on its curve. It might run forever but not a good idea

Perhaps the maker's instructions suggested a 10.00mm2 cable. The B32 is too low in rating, but as showers are only brief activities it may work o.k. for a long time. It may never trip off except if it is located in a very hot location.

It is commendable that you could confirm the power rating of the shower Sparkingchip. I find the stupid little labels with tiny print, affixed under the shower unit normally behind pipes, a real pain to read.

Z.

Why worry, it is not dangerous, and the breaker will almost certainly never trip. It may seem to be an odd design, but perhaps a 40A breaker was not available, and close fusing of this kind is not a problem in principle.  Study fig 3A4 carefully, a B32 will carry 37A for at least 10,000 seconds, which is quite a long shower. If it was a fuse it would never blow. 

Yeh, but what about 433.1 and 433.1.1 (i)? I know that the cable can't be overloaded, but the M.C.B. may run hot which is not desirable.

Z.

Study fig 3A4 carefully, a B32 will carry 37A for at least 10,000 seconds

Those graphs only show the maximum permitted disconnection time - any real device will be faster in practice. The only reassurance the standard gives is that the MCB won't trip within conventional time (1h) at 1.13x In - so a slightly enthusiastic B32 could validly trip after 3600s at just 36.16A.

  - Andy.

I have ordered a replacement MCB to be posted to the customers home from Screwfix, because in addition to being rated at 32 amps it was not a original equipment manufacturer device being   a MCG MCB in a Chint consumer unit with the busbar pin cut down to get it onto the busbar and the slots don’t really line up. So when other remedial work is done if I don’t get back the customer will have the correct device ready for whoever turns up to install on a spare untouched pin.

So in this particular instance the job just wasn’t good enough, but I see some really good installations installed in a really professional manner by electricians who use a 32 amp protective device for a 8.5 kW shower overloading it by around 20% which stresses the MCB and overheats the MCB and the adjoining devices.

I cannot get a grip on their rationale.

davezawadi (David Stone): 
 

Why worry, it is not dangerous, and the breaker will almost certainly never trip. It may seem to be an odd design, but perhaps a 40A breaker was not available, and close fusing of this kind is not a problem in principle.  Study fig 3A4 carefully, a B32 will carry 37A for at least 10,000 seconds, which is quite a long shower. If it was a fuse it would never blow. 

Just because something works ……………………………….

“Building voids” possibly suggest a Flat. The presumed void could be clipped direct, but is there a ceiling isolator at the end? Circuit length? In many instances 6mm may not be sufficient.