Shower circuit design.

Over current is the current that exceeds the design current, so on a radial circuit supplying a fixed appliance such as a shower should never exist, because no diversity is allowed in the circuit design unlike a cooker circuit. Over current on a shower circuit including the MCB is not permissible it is a design failure.

The circuit is the complete assembly including the circuit protective device, so a MCB supplying shower must never be overloaded, even if it is not an immediate danger.

You cannot treat every circuit as a socket circuit and assume that unknown loads will be connected resulting in an overload and say it will be all because the MCB will trip, neither can you under specify the circuit, which includes the MCB, in the first place.

davezawadi (David Stone): 
 

I am also very unhappy with the idea presented here that MCBs can overheat due to “overload”. 

MCBs can overheat running at their rated capacity and may need voids ways left on either side of them for ventilation to ensure they work as intended or a MCB with a higher rating may be required, as detailed in the Hager documents I posted earlier.

If MCBs can overheat running at their full stated capacity it’s a foregone conclusion that they can overheat when overloaded.

That picture you posted Andy is nonsense, the failure was caused by a loose connection! Just who or what are you trying to fool? I do not understand why everyone is trying to prove that there is something in the regulations which doesn't cater for circuit overload. Perhaps you do not understand how MCBs work? It is surely obvious to you that the normal operation of the thermal element must be designed to not cause danger or failure? A product that failed could not pass the approval tests and would be unsaleable. 

You are now suggesting that MCBs need to be spaced to prevent overheating. Whilst that might be true at very high ambient temperatures in particularly demanding circumstances, why do you think such spacing is NOT designed into CUs or any other circuit protection system? In a domestic CU, the average consumption is very small. How long do you think it would take to make the whole box at an excessive temperature, given the very small overall dissipation of the very few MCBs?

Not me, I didn’t post it and it’s actually a video, posted by Zoomup.

davezawadi (David Stone): 

You are now suggesting that MCBs need to be spaced to prevent overheating. 

I have indeed done that in off-peak storage heater consumer units where the MCBs will be running fully loaded for possibly more than five hours, maybe possibly even  up to seven Hours.

The alternative would be to increase the rating of the MCB and omitting overload protection on the storage heate circuit, it would be a braver man than me who does that with a storage heater circuit having seen some storage heater control failures, the heaters have thermal fuses in addition to their thermostats, but I would not consider fitting anything other than close protection on the circuit, those circuit conductors and MCBs can get pretty hot after running at full tilt for seven hours before the meter removes the supply.

davezawadi (David Stone): 
 

You are now suggesting that MCBs need to be spaced to prevent overheating. Whilst that might be true at very high ambient temperatures in particularly demanding circumstances, why do you think such spacing is NOT designed into CUs or any other circuit protection system? In a domestic CU, the average consumption is very small. How long do you think it would take to make the whole box at an excessive temperature, given the very small overall dissipation of the very few MCBs?

 

David, I pointed out in an earlier post that manufacturers may say their CU is only suitable for fractional loading

I went to edit a spelling mistake and a post disappeared  altogether, the gist of it was that an E7 storage heater could have over 21 kW hours or more run through it in a heating period, so say you have three heaters plus an immersion in a big tank you are now heading up towards over 80 kW hours being distributed through circuits that could be protected by B16 MCBs and have 1.5 mm twin and earth running up insulated 4” stud walls and under 4” of loft insulation.

The MCBs and cables have to warm up, you cannot assume that appliance will only run for a short period like an electric shower generally will, running maybe around 2 kW hours through a much larger cable.

Which is why EV granny leads restrict charging to less than the 13 amp socket capacity as they may run over 30 kW hours through the socket and the circuit, which may be of unknown providence with an unknown design.

gkenyon: 
 

Legh Richardson: 
 

Colin Haggett: 
 

I know one thing, if there’s a couple of teenage girls in the house it will definitely trip. 

Yes, I was called out to investigate a shower that had blown a 30A wylex fuse. The lady in question said that her niece had been enjoying the new water power spray for about ¾ of an hour when it stopped. It appeared the woman had had installed a new electric shower the day before.

Legh

BUT … is that a failure of DESIGN (in relation to the characteristics of the fixed appliance and the OCPD) or OVERLOAD PROTECTION?

I really think it's NOT 433 …

… BUT as I've said all along in this discussion thread, I don't think that's a reason to say it's OK and complies with BS 7671! 

Both!

 8.4kW electric shower replacement for an 8.0kW shower supplied from a 6.0mm2 cable on a 30A fuse. 

Poorly designed in the first place, then upgraded to a less safe design and therefore over loading the circuit.

She seemed  to think the council would pay for the work. Not to cast aspersions but you really need to be careful when dealing with certain types of tenant.

There was some talk about DIY gaswork.

Needless to say, I did not progess with this job any further.

Legh

Legh Richardson: 
 

gkenyon: 
 

Legh Richardson: 
 

Colin Haggett: 
 

I know one thing, if there’s a couple of teenage girls in the house it will definitely trip. 

Yes, I was called out to investigate a shower that had blown a 30A wylex fuse. The lady in question said that her niece had been enjoying the new water power spray for about ¾ of an hour when it stopped. It appeared the woman had had installed a new electric shower the day before.

Legh

BUT … is that a failure of DESIGN (in relation to the characteristics of the fixed appliance and the OCPD) or OVERLOAD PROTECTION?

I really think it's NOT 433 …

… BUT as I've said all along in this discussion thread, I don't think that's a reason to say it's OK and complies with BS 7671! 

Both!

 8.4kW electric shower replacement for an 8.0kW shower supplied from a 6.0mm2 cable on a 30A fuse. 

Poorly designed in the first place, then upgraded to a less safe design and therefore over loading the circuit.

She seemed  to think the council would pay for the work. Not to cast aspersions but you really need to be careful when dealing with certain types of tenant.

There was some talk about DIY gaswork.

Needless to say, I did not progess with this job any further.

Legh

Very wise.

Z.

So, having read through this thread, does this mean that 6.00mm twin and earth is too small for a 8.5KW shower? And to protect the cable,  a 32A mcb should be fitted?

A 40A mcb would exceed the current carrying capacity of the cable?

8.5KW at 230V =37A.

Current rating for 6.00mm twin and earth pretty much only seems to be complaint if the cable is clipped direct. 

Given that most installations now contain thermal insulation to some degree or other, has 6.00mm had it's day?