I have an existing 8kw shower with 6mm cable. However I am now fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the wall, then into empy ceiling joists, then inside metal covering chased into a solid brick wall, then into the roof resting on the surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm


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wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the wall, then into empy ceiling joists, then inside metal covering chased into a solid brick wall, then into the roof resting on the surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm



We need to know how long the cable is and what sort of protection is at
the consumer unit (fuse, circuit breaker, etc).

wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now
fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the
wall, then into empy ceiling joists, then inside metal covering
chased into a solid brick wall, then into the roof resting on the
surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm

Your cable is installed as per reference method C and so will allow a 47A
current.

If the shower is RCD protected then the maximum length of your 6mm cable is
40m for a 40A fuse or MCB.

--
Adam

ARWadsworth wrote:
wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now
fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the
wall, then into empy ceiling joists, then inside metal covering
chased into a solid brick wall, then into the roof resting on the
surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm

Your cable is installed as per reference method C and so will allow a 47A
current.

If the shower is RCD protected then the maximum length of your 6mm cable is
40m for a 40A fuse or MCB.


40m? Did you forget voltage drop?

Dave Osborne wrote:
ARWadsworth wrote:
wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now
fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the
wall, then into empy ceiling joists, then inside metal covering
chased into a solid brick wall, then into the roof resting on the
surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm

Your cable is installed as per reference method C and so will allow
a 47A current.

If the shower is RCD protected then the maximum length of your 6mm
cable is 40m for a 40A fuse or MCB.


40m? Did you forget voltage drop?

No. The voltage drop is the limiting factor for a 40A 6mm RCD protected
supply.

Voltage drop = (mV/A/m) x I x L / 1000

Where voltage drop is 11.5V (5%), I = 40, and mV/A/m ~ 7.0 for 6mm T&E.


But I did not do any calculations. I just looked it up in the OSG and
checked it with

http://www.tlc-direct.co.uk/Technica...ltageDrop.html

Far easier:-)

--
Adam

ARWadsworth wrote:
Dave Osborne wrote:
ARWadsworth wrote:
wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now
fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of the
wall, then into empy ceiling joists, then inside metal covering
chased into a solid brick wall, then into the roof resting on the
surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm
Your cable is installed as per reference method C and so will allow
a 47A current.

If the shower is RCD protected then the maximum length of your 6mm
cable is 40m for a 40A fuse or MCB.

40m? Did you forget voltage drop?

No. The voltage drop is the limiting factor for a 40A 6mm RCD protected
supply.

Voltage drop = (mV/A/m) x I x L / 1000

Where voltage drop is 11.5V (5%), I = 40, and mV/A/m ~ 7.0 for 6mm T&E.


But I did not do any calculations. I just looked it up in the OSG and
checked it with

http://www.tlc-direct.co.uk/Technica...ltageDrop.html

Far easier:-)


OK. I got max. length = 38.6m when I did the calc. based on 40.8A
(=9800/240) and 7.3mV/A/m (table 4D2B). shrug.

Dave Osborne wrote:
ARWadsworth wrote:
Dave Osborne wrote:
ARWadsworth wrote:
wrestler wrote:
I have an existing 8kw shower with 6mm cable. However I am now
fitting a shower rated at 9.8kw at 240v or 9kw at 240v.

The 6mm cable runs from consumer box, clipped to the surface of
the wall, then into empy ceiling joists, then inside metal
covering chased into a solid brick wall, then into the roof
resting on the surface of the roof insulation.

What installation method from the 17th regs is this ??

Then ultimately can I still use the 6mm or do I need 10mm
Your cable is installed as per reference method C and so will allow
a 47A current.

If the shower is RCD protected then the maximum length of your 6mm
cable is 40m for a 40A fuse or MCB.

40m? Did you forget voltage drop?

No. The voltage drop is the limiting factor for a 40A 6mm RCD
protected supply.

Voltage drop = (mV/A/m) x I x L / 1000

Where voltage drop is 11.5V (5%), I = 40, and mV/A/m ~ 7.0 for 6mm
T&E. But I did not do any calculations. I just looked it up in the OSG
and
checked it with

http://www.tlc-direct.co.uk/Technica...ltageDrop.html

Far easier:-)


OK. I got max. length = 38.6m when I did the calc. based on 40.8A
(=9800/240) and 7.3mV/A/m (table 4D2B). shrug.

Thats cheating. 5% voltage drop of 240V is 12V.

--
Adam

On 21/11/2010 19:01, ARWadsworth wrote:

Your cable is installed as per reference method C and so will allow a 47A
current.

Surely not, because of

then into the roof *resting on the surface of the roof insulation*.

None of the Appendix 4 methods exactly matches this. The anally minded
would say use ref. method 100 or 101, depending on the insulation
thickness, giving ratings of 34 or 27 A respectively [table 4D5A]. For
the new shower Ib is 39 A so method 100 would need 10 mm^2 cable and
method 101 would need 16 mm^2!

That's a bit OTT though and I'd suggest that Method A and Table 4D2A
would be OK - see the 6th note in section 7.2 of appendix 4 - so the
ratings become 32 A for 6 mm^2 and 43 A for 10 mm^2 and the latter size
is still required.

The best option though is to re-route the loft section so it's not in
contact with thermal insulation - see reg. 523.7, first paragraph. Then
revert to method C, table 4D5A and 6 mm^2, voltage drop and Zs permitting.

--
Andy

Andy Wade wrote:
On 21/11/2010 19:01, ARWadsworth wrote:

Your cable is installed as per reference method C and so will allow
a 47A current.

Surely not, because of

then into the roof *resting on the surface of the roof insulation*.

None of the Appendix 4 methods exactly matches this.

It is one that came up at work. The NICEIC were quite happy with method C as
the majority of the cable was in free air. ie only one side was in contact
with the insulation. Obviously there were no clips!


The anally
minded would say use ref. method 100 or 101, depending on the
insulation thickness, giving ratings of 34 or 27 A respectively
[table 4D5A]. For the new shower Ib is 39 A so method 100 would need
10 mm^2 cable and method 101 would need 16 mm^2!

That's a bit OTT though and I'd suggest that Method A and Table 4D2A
would be OK - see the 6th note in section 7.2 of appendix 4 - so the
ratings become 32 A for 6 mm^2 and 43 A for 10 mm^2 and the latter
size is still required.

The best option though is to re-route the loft section so it's not in
contact with thermal insulation - see reg. 523.7, first paragraph. Then
revert to method C, table 4D5A and 6 mm^2, voltage drop and Zs
permitting.

Yes, ideally the cable would not be in contact with the insulation, or in a
position where it may be totally covered with insulation later eg more loft
isulation added.

--
Adam

On 22/11/2010 13:54, ARWadsworth wrote:

It is one that came up at work. The NICEIC were quite happy with method C as
the majority of the cable was in free air. ie only one side was in contact
with the insulation. Obviously there were no clips!

I'm amazed by that. Prior to the 17th ed. 'one side in contact with
thermal insulation' meant a 25% derating. I suppose though that if it
snakes unclipped across the loft much of it we be in clear air above the
insulation, just touching in places. Now suppose someone goes in the
loft and deposits a box or old duvets (etc.) on top of the cable...

Yes, ideally the cable would not be in contact with the insulation, or in a
position where it may be totally covered with insulation later eg more loft
isulation added.

Quite.

--
Andy

Andy Wade wrote:
On 22/11/2010 13:54, ARWadsworth wrote:

It is one that came up at work. The NICEIC were quite happy with
method C as the majority of the cable was in free air. ie only one
side was in contact with the insulation. Obviously there were no
clips!

I'm amazed by that. Prior to the 17th ed. 'one side in contact with
thermal insulation' meant a 25% derating. I suppose though that if it
snakes unclipped across the loft much of it we be in clear air above
the insulation, just touching in places. Now suppose someone goes in
the loft and deposits a box or old duvets (etc.) on top of the
cable...

Have you got a reference you the 25% derating. All I can find is sheathed
cables clipped direct or lying on a non-metallic surface is method 1.

There is always the danger of stuff being piled on top of the cable. Not
only could this act as thermal insulation it could also bury the cable into
the insulation by pushing down on it so that it sinks into the insulation.


Yes, ideally the cable would not be in contact with the insulation,
or in a position where it may be totally covered with insulation
later eg more loft isulation added.

Quite.

And there seems to be little regulation in the insulation business with
regards to covering cables. With all the insulation that is thrown into
lofts these days via the grant schemes I suspect that I will see many
overheated cables in the future.

--
Adam