Cable size help please

[Jeremy Harris]

1 minute ago, ProDave said:

I am minded to leave my floor boards lose laid for the time being upstairs, so after I have plasterboarded the ceilings, I can fit any accoustic insulation from above before the floor goes down for good.
 

 

 

I would!

 

I didn't have that option, as our flooring was all laid before the first floor and roof part of the house frame was lifted into place.

[Nickfromwales]

2 hours ago, JSHarris said:

 

 

I would!

 

I didn't have that option, as our flooring was all laid before the first floor and roof part of the house frame was lifted into place.

I don't see how you can leave the floorboards out of the first floor with a TF build . You not using T&G Dave? 

[ProDave]

On 24/02/2017 at 17:00, Nickfromwales said:

I don't see how you can leave the floorboards out of the first floor with a TF build . You not using T&G Dave?

 We had temporary OSB flooring to work from but that has mostly gone from upstairs now (in use downstairs now) so I now have bare joists. Rather than fix the t&g flooring properly I will probably loose lay it for now as a temporary work platform and only fit it properly once the ceilings are up and the accoustic insulation rolled out.
 

[Nickfromwales]

Just all the TF builds I've been on have had to have the deck down first so the stud frame / cassettes can sit on top of them. Trying to work out how your dealing with that but needs must!

[Nickfromwales]

Best to get back to cable sizing 

[ProDave]

4 minutes ago, Nickfromwales said:

Just all the TF builds I've been on have had to have the deck down first so the stud frame / cassettes can sit on top of them. Trying to work out how your dealing with that but needs must!

The joists go in as the frame goes up, so the upper floor frame sits on the joists. Same is true of the internal load nearing walls. I have chosen to build the non load nearing walls straight off the joists so the flooring will go in around them. A lot of joiners will put the floor in first and build the non load bearing walls off the floor as it's less cutting, but I like being different.
 

[Steptoe]

7 hours ago, Logix1 said:

Steptoe, my comments about oversizing come from the experience I have in my existing home - the voltage drops 6V (using an accurate appliance monitor, 8V in the kitchen and 2V at the meter) in the kitchen when the 2.4kW oven (which I assume is a largely resistive load) is on, which suggests about 60W (2.5KW * 6V/ 240V) is being lost between the meter which is in the garage, and the kitchen about 6m away. Unfortunately the cable goes underground so there is little I can do about it.

 

However if you assume your 2.5kW oven is perhaps on for 1 hour a day, then its annual consumption is about 900 kWh/year (probably an over-estimate), which costs £140/year (at 15p/kWh), over a 25 year investment period for the cable that's £3,500 of electricity used by the oven, my 6V drop on 240V is a 2.5% drop, so £90 over 25 years being lost in the cable, which probably represents a worthwhile investment in additional copper if it were not for the difficulty of replacing the underground cable. I suspect there may be some other issues going on in my 40 year old home to cause this sort of drop....

 

Anyway, if you were starting from scratch and comparing 3 different cable types 1.5mm2, 2.5mm2, 4mm2 of 10m in length for the same oven, with resistances of 12, 7 and 5 mohm/metre, then the voltage drops would be 1.2V/00.5V respectively or 0.5%, 0.3%, 0.2% - and the 25 year cost of the losses in these cables would be £14, £8 and £5 respectively, and the cost of the 10m cable would be £3.60, £5 and £8.00 respectively. So the energy loss saving of going from 2.5mm2 cable to 4mm2 cable would be £3, and the additional cost of the cable would be £3, so the breakeven seems to be around the 2.5mm2 to 4.00mm2 range on a 25 year investment horizon. Anyway, I suspect my views are tainted by my experience in my existing home - which was all as far as I know installed professionally. The previous owner had the house built and was not at all practical - so wouldn't have done the electrical work himself, and my impression of him was that he would have hired a recommended 'professional' electrician.

 

Perhaps there is something else going on in my home, but a 6V drop, and 60W cable loss, to me is unacceptable, given a professional installation?

 

There may also be something wrong with my schoolboy maths above - comments please? But if the numbers are correct, and I was installing new cabling I would generally round up the required cable size to the next available size if it were a long term investment?

 

I don't know what your appliance monitor is, but unless its a calibrated volt meter I wouldn't have much faith in it being anywhere near accurate, I only have a basic volt meter and it is calibrated to 2 decimal places, (although it reads to 3,)

All your calcs are wrong, they should be made wrt 230volts, not 240

You have a bad install it seems, badly designed (perhaps not designed at all, just done because its always done like that), and badly installed, if a properly designed circuit has a volt drop of over 2% in 10 metres I don't think anyone could honestly say it had been designed properly. I assume your oven is on its own radial circuit,?

Also, are you saying your install is 40years old, as in your 40 year old home,?

If that is the case then it may just be a matter of end of life deterioration

It would he very interesting to know some continuity reading on these circuits as there is very clearly some issue with the figures you are reporting , assuming they are actually accurate that is.

 

6 hours ago, JSHarris said:

 

 

It may well be that we do start to see ring finals in 4mm² before too long, especially in larger houses, as more and more cable ends up embedded in insulation.  Almost all of our cables have some runs in insulation, because of the need to fit acoustic insulation between floors, and as acoustic insulation is often good thermal insulation.  In our case, allowing for the number of outlets per ring final, plus diversity, plus de-rating for the between floor runs, we were OK with 2.5mm², but had we had more outlets we'd have had to consider splitting the house into smaller rings, upping the cable size to 4mm², or, the better solution, IMHO, switching to a radial wiring scheme.  Even with a radial scheme we may well have had to up the size of the longer runs, or increase the number of radials, as de-rating for the long runs in insulation takes a 2.5mm² radial down to around 18A.   

 

FWIW I did the same and ran 10mm² to the kitchen hob etc, where it wasn't strictly required, but in my case it was partly because I had a long enough length of 10mm² left from another job that would have sat unused otherwise!

 

 

I'd wholeheartedly agree, but there are probably quite a few on this forum who have the knowledge and experience, and who don't happen to hold a current competency chit, who would be quite OK to determine cable sizes, and may well be able to do this earlier in the design phase, as they know where the cables are going to have to run, a bit like me checking the de-rating impact of the acoustic insulation in our ceiling/first floor void and the internal mineral fibre filled stud walls.  If they then pass that information on to the competent person that's going to do the installation and sign it off, it reduces the risk of there needing to be spec changes later in the build.  One advantage of this is that there are significant savings to be made by buying everything needed in as few orders as possible.

 

I think that there is a different perspective on some aspects of specifying things like cable sizes for self-builders, both for the reasons given earlier and because often self-builders may be thinking ahead about how extras might possibly be added in future, something that any trades person tasked with installing a system may well not be aware of unless they are told, and that means the customer has to have awareness of the implications of what is initially installed in order to tell them.   In our case I put several extra ducts in, to places where I thought we might possibly want power or water at some future date, but even then I've found that I didn't put one in that I'm still kicking myself about, and that's a water supply to the garage.  It's far too disruptive to even think of doing it now, though.

 

RFCs are an ourdayes throwback to the days of copper shortages, 

One of their biggest limitations is area covered is limited so of no use in larger installs. 

Radials have been the future for the past 20 years at least, it really seems to be only mainland UK that is very slow on the uptake of a lot of things, whilst the rest I'd the world moves on.

10mm for kitchen hobs has probably been more norm now than not for the past 10 years or so, people with large electric ranges, inductance hobs etc,

 

 

 

5 hours ago, Nickfromwales said:

@ProDave / @Steptoe 

What about putting a piece of soil pipe in that 300mm section to allow room for convection air flow? Could you have mitigated the 6mm cable by ducting through the insulation?

I don't see why not, if using the correction factor for ducting , most islands I do normally end up with a bit of 3" duct as that was what the builder had when the floor guys were there.

[lakelandfolk]

Oh dear, I seem to have started something unexpected.  A local qualified electrician will connect to CU, inspect and sign off when he is happy.

Thanks for all your comments/recommendations.

[Jeremy Harris]

6 minutes ago, lakelandfolk said:

Oh dear, I seem to have started something unexpected.  A local qualified electrician will connect to CU, inspect and sign off when he is happy.

Thanks for all your comments/recommendations.

 

Please don't worry about it, threads here inevitably end up with a life of their own, and often are more useful and educational or many because of that.

[Steptoe]

Don't worry @lakelandfolk

There is a lot of misconception regarding electrical installations,

Rather than take advice from a lot of people on the internet that like to think they know more about electrical installations than qualified, competent, registered and experienced professionals, I'd recommend your best course of action is to engage the services of a good reputable competent electrician that will be able to guide you in what he will be happy to connect, test, and sign off as being compliant.

This is an area where a little knowledge can be very dangerous, as some peoples interpretations of the regulations, and their installation practices clearly show, 

[Nickfromwales]

1 hour ago, Steptoe said:

RFCs are an ourdayes throwback to the days of copper shortages, 

One of their biggest limitations is area covered is limited so of no use in larger installs. 

100m2 is not quite so limited . 

Also, , for large installs you simply run multiple ring mains accordingly  How many rewires have you done in a medium sized house that have a ring for the kitchen / utility sockets, a ring for upstairs sockets, and a ring for downstairs sockets? More than one?

( I know I'm going to regret asking, but I'll take what's coming ) 

[ProDave]

12 minutes ago, Nickfromwales said:

100m2 is not quite so limited .

Also, , for large installs you simply run multiple ring mains accordingly  How many rewires have you done in a medium sized house that have a ring for the kitchen / utility sockets, a ring for upstairs sockets, and a ring for downstairs sockets? More than one?

( I know I'm going to regret asking, but I'll take what's coming ) 

Indeed. I could do the entire ground floor of our house as one ring final and still be within the limit. But I won't it will be split in two, then a third for the garage.  Our present house (remember it's a B&B) has separate circuits for the guest rooms so if a guest plugs in something dodgy, it does not trip our personal circuits.
 

[Steptoe]

12 minutes ago, Nickfromwales said:

100m2 is not quite so limited . 

Also, , for large installs you simply run multiple ring mains accordingly  How many rewires have you done in a medium sized house that have a ring for the kitchen / utility sockets, a ring for upstairs sockets, and a ring for downstairs sockets? More than one?

( I know I'm going to regret asking, but I'll take what's coming ) 

It is with Zs,

 

Quite a few at the cheaper end of the market, mostly for private rental,

Most owner occupiers are willing to pay the little bit extra and have a properly designed and installed radial system,  

Radials can often be much quicker and less cable consuming to install, it's the extra fitted equipment that ramps up the cost slightly, and obviously dependant ok no the number of radials the client wants, then sometimes the cable saving goes the other way, I've done some with 2 radials to a room as they never wanted power to trip out a whole room, or more usually, 2 rooms split across 2 radials, slightly less expensive. I've done some quite modest 4 bedroom houses with close to 30 circuits in them.

And some fairly flash 6 bed+ with fewer than 15, horses for courses. 

 

I have a very modest 2&1/2 bed semi with 14 circuits, not including shed. 

But, tbh, that's ridiculous, and only because I could, ( "for free" )

 

[Nickfromwales]

Game, set and match 

 

[Steptoe]

2 minutes ago, Nickfromwales said:

Game, set and match 

 

I suppose my point is,

There is no one size fits all,

Radials are more and more becoming the accepted norm, it's just,

how many,?

For a cheaper install, then yes, RFCs are still very viable, a normal 2 , 3 bed semi, 1 x RFC for kitchen/utility, 1 for rest of house is still very viable.

1 lights up, 1 down, shower, cooker, that's it, 6 circuits, cheap cheerful install, typical rental, 

That same install could turn into 14 circuits easily for segregation purposes using radials. 

Horses for courses, or how high spec an install you want done, and are willing to pay for.

 

[Nickfromwales]

2 hours ago, lakelandfolk said:

Oh dear, I seem to have started something unexpected.  A local qualified electrician will connect to CU, inspect and sign off when he is happy.

Thanks for all your comments/recommendations.

Relax. It keeps us out of trouble ??

 

[Jeremy Harris]

One thing well worth thinking about is that no trades person is a mind reader, and many will quote on the basis of what they can see, what they are told and what they can guess based on their experience.  Perfectly understandable, but for a lot of people self-building or doing major renovations, it's important to remember that only you have all the information about how the finished structure is going to end up. 

 

Also, it's still perfectly legal and OK to DIY plumbing, electrics and even gas, if the person doing the work is competent and if building control have someone competent to sign it off.  BC have an obligation to offer this service, but we found that they just simply didn't have, or know of, anyone they could use to inspect, test and sign off either unvented plumbing work or electrical installations, and asked me, very nicely, if I could make their life a bit easier by using a Part P approved and registered electrician.  Personally I'd NEVER DIY gas work, but, believe it or not, it is allowed in your own home, something I find staggering, given the other safety-related constraints we're under.  I'd also never suggest that anyone undertakes any plumbing or electrical work unless they are competent to do so, and just reading how to do it on the internet, no matter how reliable the source, does not make anyone competent.

 

As an illustration of how important it is for you, the self-builder or renovator, to understand how each trade best uses their skill and experience, so that you can specify a job as thoroughly as possible, I can give an examples of where competent (as in registered and qualified) trades people can be misled by not fully understanding what the building is going to have done to it after they've done first fix.

 

I had quotes from 4 electricians to do the first and second fix internal electrical installation for our new build.  All were given detailed and scaled electrical installation plans, with all outlets and key positions of things like the incoming supply, position of the CU etc, marked, plus additional key information, like the acoustic insulation in the stud walls and ceilings required to meet building regs.  None of them had made any allowance for that insulation, shown clearly on the plans, when they got back to me, because when they walked around the inside of the house frame to price the job up it hadn't yet been fitted, for the simple reason that it's a pain fitting plumbing, wiring and ventilation stuff in around insulation, plus the internal wall insulation can't go in until one side of a stud wall is boarded at second fix.

 

These weren't by any stretch incompetent people, in fact two of them I knew already and would trust implicitly.  None of them had ever seen a passive house structure before, which isn't at all surprising, and all were surprised when I stressed that they could not make any holes in the external wall Durélis board inner lining, or clip cables to it; all cables had to be clipped up the side of the 50mm x 50mm service void battens.  I also told each that I'd be fitting glued-on, over-sized, ply backing boards to space out back boxes to all the fittings that were going on the inside of an external wall, and that all such fittings were positioned on the plans so that they were adjacent to a vertical batten, to remove the need to clip cables to the airtight/vapour tight Durélis board inner lining.

 

The guy I chose in the end was the one who was the most willing to work on a house that was a very different to anything he'd seen before and was also keen to learn about how a passive house is built and how it works.  He was also more than happy to have me working as his labourer, and the two of us ended up getting on well together.  One chap put himself out of the running, not because his price was too expensive, but because he was just opinionated and inflexible (and happened to be the oldest of the four, I think), and wanted me to change my spec for a few things, including not fitting a big combination plate for the TV "as it was against the regs", which was just untrue, plus stating that it wasn't an electricians job to wire 'phone extension cable, Ethernet cable or satellite TV cable, so I'd need to get someone else to do that.   Curiously, he was also the one that argued with me over the phone when I asked him a question about the cable size he'd quoted for the run of high current cable, partly in insulation, with him arguing that it was fine to use 4mm² to feed a 40A RCBO protected radial to a cooker outlet, even when three quarters of that run was embedded in insulation.  I didn't want to make things worse by pointing out it wouldn't even be OK for 40A if clipped direct.  The other two I didn't end up using were OK, except one who insisted that he'd have to buy all the materials for the installation to be compliant, even when I explained that if he did that we'd not be able to reclaim the VAT (none of them were VAT registered).  I think the reality was he wanted to make a bit on supplying the materials, which is fair enough, but he just didn't understand, or perhaps even know about, the self-build VAT reclaim rules.

 

The key thing here is the point I've made elsewhere; only you know what you want, and what work is going to be done after someone has left the site and that might very well impact on the decisions they made, in good faith, when they did their work.  It's the reason that I think it's very useful for anyone specifying work to have as great an understanding of what is needed as possible, as that way it reduces the chance of errors and increases the probability that you'll end up with exactly what you want, without things needing to be re-worked later.  Even if you're not going to DIY anything at all, it's still very beneficial to have some understanding of what each trade needs to know in order to reduce the risk of later problems.  I doubt there's anyone here who hasn't heard of, or experienced, a builder-related dispute, and I'd happily bet that the majority of those are because the customer didn't fully understand what they were asking for, or what the basis of a quote was.

 

Edited February 25, 2017 by JSHarris
typo, "position" when I meant "positioned"

[Guest Alphonsox]

Excellent post Jeremy. Personally I have tried to understand every aspect of our house build to a level where I feel happy discussing details with the various trades involved. My feeling is that the more you understand the better the end result will be. Last week I was trying to convince a very skeptical plumber that a large house could be heated by a 3kW immersion and that I hadn't been taken in by snake oil salesmen when he first saw the SunAmps. In both cases a good understanding of the issues at least left him convinced I might know what I was taking about.

[Jeremy Harris]

4 hours ago, Alphonsox said:

Excellent post Jeremy. Personally I have tried to understand every aspect of our house build to a level where I feel happy discussing details with the various trades involved. My feeling is that the more you understand the better the end result will be. Last week I was trying to convince a very skeptical plumber that a large house could be heated by a 3kW immersion and that I hadn't been taken in by snake oil salesmen when he first saw the SunAmps. In both cases a good understanding of the issues at least left him convinced I might know what I was taking about.

 

I've had much the same with heating.  When I was looking around for an ASHP, many suppliers wouldn't supply just the heat pump, but insisted on selling a supply and install service.  As I had cable ducts in place for power and control cables, two bits of 22mm pipe sticking out through the wall and a concrete pad for the thing to fix down to, I wasn't too bothered by paying what I thought would be a modest sum for a very simple install.  All of the suppliers, without exception, wanted to massively over-size the unit and none would believe the heat loss calcs that showed that the house needed, at most, around 1.6 kW to heat when it was - 10 deg C outside, and that most of the time the heating requirement in winter was perhaps a couple of hundred watts, if that.

 

I can understand this, as 99% of the time trades people will be working on houses that do have massively greater heating requirements, or that don't have things like very airtight/vapour tight external walls.  Self-builders tend to be a minority, and many are focussed on building a home that not only meets their needs but that is almost certainly built to a higher performance specification than the majority of new homes today.  The Bovis homes problems highlighted in another recent thread are pretty typical of the way the majority of new homes are built, I'm sure.

Edited February 25, 2017 by JSHarris
typo, missed out the word "only"

[curlewhouse]

On ‎24‎/‎02‎/‎2017 at 11:31, JSHarris said:

".....The supervision of installation methods is a very good point to highlight.  My way around that was to work with our electrician as his labourer, so he could ensure that all cables, boxes etc that I helped put in were to the regs and that he was happy to sign off on that basis, having seen everything go in with his own eyes."

 

 

 

Pretty much what we are doing. My father is a retired electrician and I grew up learning from him, (in his 70s he even still keeps his copy of the part P regs updated out of pure interest) so we naturally want to do the work ourselves. I got slagged to bits on another forum and accused of all sorts for saying we are doing this (it was pretty unpleasant actually) . Told it's illegal, I must be lying, only a part P qualified electrician can do any of the work etc etc ! I suspect by guys who don't like the idea of people realising they can perfectly legally do the work themselves if overseen by a qualified person -  after all, it would be mad if only part P qualified people could run cable or hammer in cable clips for example, or pull the other end of the cable from the floor above or whatever.  We originally had a retired electrical lecturer who still has her tickets and membership going (has a little business doing the odd solar panel work etc) to oversee our work and certify us, but they are often away being retired, and since then my nephew who oversees and signs off all his electrical contractor firms work has offered to do it for me  which is perfect.

[curlewhouse]

Oh, I've found this calculator to be a little easier to use   http://www.doncastercables.com/technical-help/