10.5kw shower installation

[Stepho]

Hi all. I've just bought a 10.5kw shower to replace my old 9.5kw that is on its last legs. The current cabling installed for the old shower is 6mm with a 32A MCB, both of which I know have to be upgraded. I live in a top-floor flat and the cabling runs out of the consumer unit through about half a metre of conduit trunking (bunched with other wiring) until it reaches the attic space where it's then just surface clipped for about 10 metres until it gets to the pull-cord isolator switch on the bathroom ceiling. There is then another 2 - 3 metres of cable from the switch through the attic space to the shower unit itself, with the last half a metre inside conduit trunking down the bathroom wall.

Based on this layout, should the wiring be replaced with 10mm or 16mm cabling? (the shower's instruction booklet has said the MCB has to be upgraded to a 45A but all it says about the wiring is that it should be at least 10mm "depending upon circumstances" - but I don't want to spend more on cabling than I have to.)

 

[Lectrician]

Is the cable going to be under, in, on thermal insulation?

 

[Stepho]

Once the cable goes through the ceiling of my flat it then runs through a bit of loft insulation, but is (for the most part) on top of the insulation because it runs over the joists to get to the bathroom ceiling and then back down through the insulation again. In total, I'd say about a metre of the entire cable length is either under or running through insulation.

 

[davelx]

Once the cable goes through the ceiling of my flat it then runs through a bit of loft insulation, but is (for the most part) on top of the insulation because it runs over the joists to get to the bathroom ceiling and then back down through the insulation again. In total, I'd say about a metre of the entire cable length is either under or running through insulation.

Through or Under makes a big difference. If it's running for 500mm or more through insulation then you have a 0.5 derate factor and even 16mm is too small. In fact, even 50mm would have approx 0.89 derate factor (I only have the derates for up to 10mm to hand, so it may be slightly different), giving 16mm cable a CCC of just over 42A so it's still too small.

If it's running under the insulation and in contact with the plasterboard ceiling then 16mm should be OK.

 

[Stepho]

Through or Under makes a big difference. If it's running for 500mm or more through insulation then you have a 0.5 derate factor and even 16mm is too small. In fact, even 50mm would have approx 0.89 derate factor (I only have the derates for up to 10mm to hand, so it may be slightly different), giving 16mm cable a CCC of just over 42A so it's still too small.

If it's running under the insulation and in contact with the plasterboard ceiling then 16mm should be OK.

So let me get this right. Once the cable reaches the attic, it can run along the plasterboard but under the loft insulation until it reaches a wooden joist and then it can go up the side of joist? Does it have to be laid so that it is never completely surrounded by the loft insulation at any one time? (i.e. in between insulation and plasterboard or insulation and wooden joist is fine?)

 

[davelx]

So let me get this right. Once the cable reaches the attic, it can run along the plasterboard but under the loft insulation until it reaches a wooden joist and then it can go up the side of joist? Does it have to be laid so that it is never completely surrounded by the loft insulation at any one time? (i.e. in between insulation and plasterboard or insulation and wooden joist is fine?)

Yes, that's the way I read it. That would then be installation method 101 assuming the insulation is >100mm thick, and the 16mm cable is then rated 46A ( from table 4D5) which is > 45A of the MCB so OK.

 

[sparky1505]

DONT FORGET THAT CABLES CAN NOT SIT DIRECTLY ON THE PLASTERBOARD CEILING AS THIS IS NOT WITHIN A PRESCRIBED CABLE ZONE SO WHY NOT TRY TO KEEP THE CABLE ABOVE THE INSULATION AND ALWAYS REMEMBER TO USE RCD PROTECTION.

 

[DetlefSchmitz]

DONT FORGET THAT CABLES CAN NOT SIT DIRECTLY ON THE PLASTERBOARD CEILING AS THIS IS NOT WITHIN A PRESCRIBED CABLE ZONE SO WHY NOT TRY TO KEEP THE CABLE ABOVE THE INSULATION AND ALWAYS REMEMBER TO USE RCD PROTECTION.

This is simply not true, and has no basis in BS7671.

Also, posting in upper case is simply annoying and pointless.

 

[riveralt]

DONT FORGET THAT CABLES CAN NOT SIT DIRECTLY ON THE PLASTERBOARD CEILING AS THIS IS NOT WITHIN A PRESCRIBED CABLE ZONE SO WHY NOT TRY TO KEEP THE CABLE ABOVE THE INSULATION AND ALWAYS REMEMBER TO USE RCD PROTECTION.

This is simply not true, and has no basis in BS7671.

Also, posting in upper case is simply annoying and pointless.

I am in learning mode today - so can you tell me which bit is not true. Putting aside the RCD issue - is it the case that cables can sit directly on ceiling plasterboard - because surely that would be less than 50mm deep?

 

[PrinceofDarkness]

The O/P, 'Stepho', has a 9.5kW shower on a 32A/6mm feed, and he'd like to replace it with a 10.5kW unit - a matter of just a 4A increase........


So, for this, he should change the 6mm cable to 16mm? Oy Vey!

Cables are allowed to run through short distances of thermal insulation without the application of de-rating factors - and even so, why not move that section of cable from the insulation?

There's never any reason to use 16mm cable on a shower circuit, whatever the circumstances - unless you happen to be a hapless text book hero......


Lucia.

 

[DetlefSchmitz]

is it the case that cables can sit directly on ceiling plasterboard - because surely that would be less than 50mm deep?

The regs only say that it must be 50mm away from the plasterboard when it passes through a joist. Presumably that's where the nails are likely to go. Cable in contact with the plasterboard is a defined installation method. To be honest it originally surprised me too.

 

[bernardgreen]

There's never any reason to use 16mm cable on a shower circuit, whatever the circumstances - unless you happen to be a hapless text book hero......

Reducing voltage drop and power disapated along the cable to the minimum ?

Larger contact area of conductor to clamp in connections ?

 

[ban-all-sheds]

The fact that 10mm² installed to Ref Method 103 is only enough for a shower rated at 7.4kW at 230V?

 

[sparky1505]

DONT FORGET THAT CABLES CAN NOT SIT DIRECTLY ON THE PLASTERBOARD CEILING AS THIS IS NOT WITHIN A PRESCRIBED CABLE ZONE SO WHY NOT TRY TO KEEP THE CABLE ABOVE THE INSULATION AND ALWAYS REMEMBER TO USE RCD PROTECTION.

This is simply not true, and has no basis in BS7671.

Also, posting in upper case is simply annoying and pointless.

I am in learning mode today - so can you tell me which bit is not true. Putting aside the RCD issue - is it the case that cables can sit directly on ceiling plasterboard - because surely that would be less than 50mm deep?

This is true, cables concealed within the fabric of the building ( which includes a ceiling) of a depth less than 50mm from the surface must not be routed outside the prescribed zones without additional mechanical protection. unless under the supervision of a skilled or instructed person which is clearly not someone who can`t read upper case . please be caerfull who you seek advice from, always consult a qualified person. for reference on permitted cable route zones see (pocket guide 6) . for cables passing through or over joists check out pocket guide 21.

 

[ban-all-sheds]

This is true, cables concealed within the fabric of the building ( which includes a ceiling) of a depth less than 50mm from the surface must not be routed outside the prescribed zones without additional mechanical protection. unless under the supervision of a skilled or instructed person which is clearly not someone who can`t read upper case . please be caerfull who you seek advice from, always consult a qualified person. for reference on permitted cable route zones see (pocket guide 6) . for cables passing through or over joists check out pocket guide 21.

I don't know who you are quoting there, but they are wrong. The regulations about 50mm depth and prescribed zones apply to walls and partitions, not ceilings.