I'm having a shower moved and the electricians have just finished their 'first fix'. The rating of the shower heater is 9.8kw at 240v and the original circuit was run entirely in 10mm cable. It looks like the elctricians might have used part of the original circuit because the cable to the pull-cord isolating switch is 10mm. However, the circuit between the switch and the shower unit has been run in 4mm cable.
This doesn't seem right to me.
Why would they do this, especially when the original circuit was entirely in 10mm cable?
Given a load of 9.8kw, is it OK to have part of the circuit in 10mm cable while part (probably about 2.5m of it) is in 4mm cable.
I wondered if thinner cable was used in order to ease the problems of making the connection to the shower unit. It's possible to do it in
10mm cable ( that's how it was originally), but it isn't very easy. I have a copy of the regs, but I'm not sufficiently familiar with them to be able to find the relevant paragraphs. If the rated load is 9.8kw are there any circumstances under which part of the circuit is permitted to be run in 4sqmm cable?
Firstly are you sure he has dropped to 4mm sq and not 6mm? They are not always easy to tell apart visually. Use the following as a guide:
CSA (CPC) Strands Overall Current A mm^2 No./dia. mm approx. 6** 15** 1**
--------- ------- ---------- ---- --- ---
1.0 (1.0) 1/1.13 4.5 x 8.2 11.5 12 16
1.5 (1.0) 1/1.38 4.7 x 8.2 14.5 15 20
2.5 (1.5) 1/1.78* 5.3 x 9.9 20 21 27
4.0 (1.5) 7/0.85 6.1 x 11.4 26 27 37
6.0 (2.5) 7/1.04 6.8 x 13.1 32 35 47
10 (4.0) 7/1.35 8.4 x 16.8 44 47 64
16 (6.0) 7/1.71 9.6 x 19.5 57 63 85
or 7/0.67
** Fixing method:
6 Enclosed in conduit in an insulated wall
15 Installed directly in an insulated wall
1 Clipped direct
If it is 4mm^2, then no - there is no circumstance where any amount of
4mm^2 cable could be acceptable for this application.
If it is 6mm^2, then maybe _just_ with provisos...
Having that, there is no real excuse for using a smaller cable even if it can be technically justified. If it were my place I would insist that the whole run was completed in 10mm^2 since it gives more options in the future, and changing the cable at a later date may be far more disruptive.
How you might justify reducing to 6mm^2:
Your total shower load is a little under 41A, so the de-rating factors (installation method, ambient temperature, grouping with other cables etc) would have to be such that they did not reduce the maximum current carrying capacity of the 6mm^2 cable (47A) to below this.
The protective device would need to be sized to protect the smaller cable and not the larger one (otherwise you would need a further protective device and the transition from the bigger to smaller cable)
The length of the run would need to be short enough to not exceed the voltage drop limitation (typically 6% or 13V in this case) however if you are only talking about two and a half meters of the thinner cable, this will only add about a third of a volt drop compared to 10mm sq.
Thanks for your comprehensive reply. It's actually 6mm^2 cable, and not 4mm^2 as I first thought. About 1.25m of the 6mm^2 cable is loose laid in the ceiling - no heating pipes nearby, no insulation and no other cables, so I guess that bit is OK. The other 1.25m is buried in the wall under plastic capping, plaster and tiles. The wall isn't insulated as such, so does that count as 'clipped direct' for de-rating purposes or is it still to be treated as if it were 'installed directly in an insulated wall'? If it's the latter, the 35amp derated capacity of the cable will not be up to the job - is that correct?
Either way, as you suggest, it limits future options when compared to the original 10mm^2 circuit. It beats me why supposedly 'professional' people are so quick to cut corners for the sake a few quids worth of materials. I've seen this time and time again on this project. Part P or not, it will be strictly DIY for me in the future.
In plaster is not the same as in insulation... A cable embedded directly in building materials counts the same as "method 1 - clipped direct". Also where the de rating applies to only a section of the cable then its overall effects are reduced. So in this case he should be ok on basic current carrying capacity.
You would have to have a maximum MCB size of 45A however.
Is it on an RCD protected circuit? Also do you know what type of earthing arrangement your house has (TT, TN-S, TN-C-S)
if the terms mean nothing see:
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on the answers to the above, and also knowing what the Prospective Short circuit Current for the circuit is, there are some other sums that ought to be done to make absolutely sure it is ok.
If as you suggest he seemed to know what he was doing, then it may be it all checks out ok. Hmmm, "hapeth of tar" and all that!
For 2.5 meters the difference in price is probably less than a quid. He
*might* save an extra ten mins terminating the thinner cable. Probably more a case of he had some 6mm in the back of the van!
Sadly it is the only way you seem to be able to guarantee quality job these days.
BTW, part P does not prevent you doing it yourself anyway, although it would require you to pay the money and jump through the building notice hoops should you choose to do it legit.
It's probably not material cost, although it could possibly have been availability on the van. It is more likely that 10mm cable is a pig to get into the shower terminals. The 6mm cable allows a 45A MCB, which is good to
10.3kW@230A, which would be sold as a 11.3kW@240A shower.
Just what I was thinking. Table 7.1 of the On-Site Guide does not cover circuits in 6 mm^2 exceeding 32 A so I think the OP should be asking to see the design calculations for the circuit - in particular for earth fault loop impedance (Zs) to ensure < 5s disconnection time and for CPC ('earth') size. 6 mm^2 T&E only has a 2.5 mm^2 CPC so one does need to be a bit careful when using it on circuits above 32 A. Ask the electrician if he has checked for "adiabatic compliance of the CPC."
What earthing arrangement, protective device and Zs value are documented on the minor electrical installation works certificate that the OP will have received on completion of the job? If there are no calculations and no certificate then loud alarm bells should ring.
Also (and probably the most important safety point of all) is there a supplementary bonding connection between the shower CPC and the other items (if any) in the room that should be bonded? (Such other items being metal plumbing & taps, a metal bath, and the CPCs of any other electrical circuits in the room, including lighting, that are 'within the zones'). Bonding conductors should be a minimum of 4 mm^2 unless protected in conduit etc.
"You would have to have a maximum MCB size of 45A however. Is it on an RCD protected circuit? Also do you know what type of earthing arrangement your house has (TT, TN-S, TN-C-S)"
The job isn't complete, so I don't have any certificates, etc, yet. I will ask the electrician why he's not used 10mm^2 throughout. I suspect it is all to do with the hassle of terminating in the shower unit. It is tricky (I did the original installation), but doesn't, in my view, really justify downgrading the circuit to 6mm^2. As a DIYer whos not that familiar with handling10mm^2 cable, it probably took me
45 minutes to get the cable lying 'right' in order to be able to make the connections in the shower unit.
The circuit is RCD protected and the MCB is 45amp. Earth system is TN-S. The earth bonding within the shower room has all been done as I would expect and, yes, he's used 4mm^2 cable.
"... 6 mm^2 T&E only has a 2.5 mm^2 CPC so one does need to be a bit careful when using it on circuits above 32 A. Ask the electrician if he has checked for "adiabatic compliance of the CPC."
The electrician has installed a load of earth bonding cables that link all the exposed metal items in the room and connect back to the earth on the distribution unit. Included in this bonding is a wire at the shower position that connects to the incoming water supply pipe and has a tail that looks as though it is destined for the earth terminal in the shower unit itself.
Doesn't all this additional earthing capacity 'count' as far as your statement above is concerned about the size of the earth conductor in a
6mm^2 T&E, or do the rules require that only the earth wire in the supply cable be considered in the calculation?
On Fri, 19 May 2006 09:38:17 +0100 someone who may be John Rumm wrote this:-
The Wiring Regulations being a legal requirement via Building Regulations has long been the case in Scotland, two decades at least. I have no idea about Northern Ireland. It's a very sensible idea.
What was stupid was the way the officials did this in England and Wales. Presumably they asked vested interests to draw it up. The IEE were entirely ineffective on the matter and only seemed to wake up late in the day.
So the water main comes up in the shower room, as in a downstairs bathroom?
The latter. Bathroom supplementary bonding doesn't specifically need to be earthed, although it often is via other connections to the main earth terminal - in this case both via the shower CPC and via the main bonding connection (>= 10 mm^2) to the water main.
"So the water main comes up in the shower room, as in a downstairs bathroom?"
No - the water main enters the house elsewhere. I was referring to the copper water feed into the shower unit. The main bonding between incoming earth, gas and water pipes already exists and uses 10 sqmm cable. However, the electrician has taken the 4 sqmm supplementary bonding cable back to the main earth as well.
Now I know that I'd mis-identified what I thought was 4 sqmm T&E cable (it's actually 6 sqmm), I don't doubt the capabilities of this guy. He was very particular about the positioning of items in the room with respect to the various zones that apply in a shower room. My only reservation is that he's replaced a 10 sqmm circuit with one that is limited by his 6 sqmm run between the isolating switch and the shower. If I'd been doing the job myself again I would have stuck with 10 sqmm throughout.
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