10.8 kw shower cable size

[Zebsport]

Hi.
I've just bought a Mira Sport 10.8kw shower. The house has a RCD protected consumer unit.

I've fitted 10mm cable, about 10 metres in total length, with 2.5 metres chased in a wall.

1. Is this cable size adequate a friend keeps saying I should have put 16mm in?
2. 10800watts divide by 230v equals 46.9 amps. I can only buy a 40 or 50 amp fuse for our CU, which one do i go for?

Any help much appreciated.

 

[Softus]

I've just bought a Mira Sport 10.8kw shower. The house has a RCD protected consumer unit.

I've fitted 10mm cable, about 10 metres in total length, with 2.5 metres chased in a wall.

Please be aware that this work is notifiable under Part P of the Building Regulations.

1. Is this cable size adequate

Yes.

a friend keeps saying I should have put 16mm in?

Does he have a reason for advocating 16mm² cable?

2. 10800watts divide by 230v equals 46.9 amps. I can only buy a 40 or 50 amp fuse for our CU, which one do i go for?

I'd put money on the Mira rating being at 240V, not 230V, so the design current works out to be 45A, so you need a 50A MCB.

45A MCBs exist, but you shouldn't run a breaker at it's rated tripping current.

 

[GaryMo]

45A MCBs exist, but you shouldn't run a breaker at it's rated tripping current.

Have you got anything to confirm a 45A MCB will trip at 45A?

 

[Softus]

Have you got anything to confirm a 45A MCB will trip at 45A?

Could you translate that into English?

 

[SeanT]

Surely the rated tripping current is considered to be 45A, in that any load above 45A runs some risk of tripping the breaker, regardless of whether it actually ever does? A better expression might be rated design current I suppose?

 

[Softus]

I don't see how you can improve on the expression "rated tripping current" by calling it "rated design current", since they are two distinct concepts and, by necessity, two different values.

 

[equitum]

I don't see how you can improve on the expression "rated tripping current" by calling it "rated design current", since they are two distinct concepts and, by necessity, two different values.

An improvement on all the above "expressions" would be to state for an overcurrent protective device

i) The nominal current rating of the device (In) is not less than the design current (Ib) of the circuit
ii) In does not exceed the lowest of the current carrying capacities (Iz) of any conductors in the circuit and
iii) The current (I2) causing effective operation of the protective device does not exceed 1.45 times Iz

In this case Ib is circa 45A, which is needs to be less than (or equal to)In , so a 50A (and possibly 45A) breaker satisfies the first requirement.
10mm2 cable reference method 1 willl handle 64A thus greater than Iz
so the second part is OK

However, I2 for a type B 50A device is 250A (page 199 16th Edition) which exceeds 1.45 Iz.


Fortunately, where the device is a 60898 (MCB) complying with part (ii) also complies with part (iii)

 

[GaryMo]

Have you got anything to confirm a 45A MCB will trip at 45A?

Could you translate that into English?

As opposed to this being in Spanish? A 45A MCB will not trip at 45A.

 

[Softus]

Have you got anything to confirm a 45A MCB will trip at 45A?

Could you translate that into English?

As opposed to this being in Spanish? A 45A MCB will not trip at 45A.

Ah, I see your point now - you were pointing out an error in my post.

Quite right - I shouldn't have written "tripping current", but "nominal current" instead.

You could have been more direct about it, but thanks for pointing it out nonetheless.
_______________________

I believe that new MCBs are rated at 230V, so at 240V a 45A MCB would, with both shower elements switched on, be running above its nominal rating, which would be an incorrect installation.

The issue often gets fudged because the duration of usage is usually short, i.e. much less than one hour, but a 50A MCB would be appropriate for a 10.8kW shower installed at the end of 10.0mm² T&E cable.

 

[Spark123]

However, I2 for a type B 50A device is 250A (page 199 16th Edition) which exceeds 1.45 Iz.

The MCB needs to operate on the magentic part within the specified time (0.1secs) at 250A (Ia) which is to provide fault current protection, it is the thermal part of the MCB which provides the protection against overload. iirc the device needs to disconnect within some much bigger time period i.e. 1hour @ 1.45x its rated current on overload.

 

[Zebsport]

Thanks everyone for your advice.

The 16mm cable advise was only given because the Mira handbook says the terminal could take upto 16mm.

Electrics are to be checked/tested by building control on completion of our new extension. I spoke to building control about the cable size but they were a little unsure of showers over 10kw, not the answer I was after!!

On your advice i'll stick with the 10mm cable and a 50amp fuse. I've chased out the wall section much deeper and wider then needed so there'll be some air flow over the cable when in the wall. Sorry to continue this post but how much does a cable being in the wall contribute to problems. I presume it stops the cable from losing heat or is there a more exact science to it?

 

[equitum]

Being buried in a (non insulated) wall does not cause any "problems" for the cable, as plaster/brick/masonary etc is not thermally insulating.

This is deemed to be reference method 1, and is the same for the purposes of current carrying capacity of the cable as being clipped onto the surface.

 

[equitum]

However, I2 for a type B 50A device is 250A (page 199 16th Edition) which exceeds 1.45 Iz.

The MCB needs to operate on the magentic part within the specified time (0.1secs) at 250A (Ia) which is to provide fault current protection, it is the thermal part of the MCB which provides the protection against overload. iirc the device needs to disconnect within some much bigger time period i.e. 1hour @ 1.45x its rated current on overload.

Spark123, please read my post in full some of which I have pasted below, which is a summary 16th reg 433-02-01 and 17th reg 433.1.1 PROTECTION AGAINST OVERLOAD CURRENT . At no point do these regs mention 1 hour, or indeed thermal operation of the MCB.


i) The nominal current rating of the device (In) is not less than the design current (Ib) of the circuit
ii) In does not exceed the lowest of the current carrying capacities (Iz) of any conductors in the circuit and
iii) The current (I2) causing effective operation of the protective device does not exceed 1.45 times Iz

The figure of 250A from appendix 3 gives characteristics of OVERCURRENT PROTECTIVE DEVICES and does not mention fault current
other than for fault currents in excess of causing instaneous operation of the device manufacturers data should be used.