lakelandfolk

Cable size help please

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We are planning a kitchen revamp and will need to run some extra cables.  We are doing the first fix but would like to be sure we use the correct size of cable. Can anyone advise?

Combi Micro Wave. catalogue says 16 amp

Oven.  catalogue says 16 amp

Induction hob.  catalogue says 32 amp        Each run to CU will be circa 9 metres.

 

Many thanks

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insulation is your enemy when running cables in,

do the runs pass through any?

 

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will the cable have to pass through the insulation on the way up, or back down again?

 

 generic CCC for your runs would be 2.5mm radials for the 16A, and a 6mm for the 32A , and you have some headroom if the actual current usage of what you buy is increased on the 16A circuits,

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From recollection of looking at this in the past I would oversize for any high power appliance - only because there is a reasonable payback on the reduced power loss from lower resistance cable. This is especially true if there is a reasonable distance from the meter to the appliance. The obvious candidate for thicker cable is the oven. Hobs and microwaves have more intermittent use so the economics are less clear.

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5 minutes ago, Logix1 said:

From recollection of looking at this in the past I would oversize for any high power appliance - only because there is a reasonable payback on the reduced power loss from lower resistance cable. This is especially true if there is a reasonable distance from the meter to the appliance. The obvious candidate for thicker cable is the oven. Hobs and microwaves have more intermittent use so the economics are less clear.

Where on earth have you got that information from about cable sizing,?

Thats plain and simply misinformed.

Cable sizing calculations are all based on the resistance of the cable, volt drop, and power requirements already. 

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Guest Alphonsox
8 hours ago, Steptoe said:

Thats plain and simply misinformed.

 

That's plain and simply physics - Oversizing a cable will reduce its resistance and hence reduce the I2R heat losses due to current flow. All the regs are providing you with is a minimum for safe operation. Using a 10mm feed for an oven where a 6mm would meet minimum requirements is perfectly acceptable. I do however have some doubts about the economic payback but this would depend on things like run distance and the cost of copper/electricity dependent.

 

@Logix1 Welcome to the forum, please feel free to introduce yourself on the Intro board.

 

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8 hours ago, Steptoe said:

Cable sizing calculations are all based on the resistance of the cable, volt drop, and power requirements already. 

Can you post up the tables that show the sizings.  My OnSite Guide is out of date (and I don't know where it is now).  I seem to remember that is was pretty simple to work out any derating because of insulation.

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10 minutes ago, SteamyTea said:

Can you post up the tables that show the sizings.  My OnSite Guide is out of date (and I don't know where it is now).  I seem to remember that is was pretty simple to work out any derating because of insulation.

 

No offence but anyone who blatantly posts these up is at risk of breaching copyright and the wrath of the IET.

 

My latest OSG was £27 from memory.

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25 minutes ago, SteamyTea said:

Can you post up the tables that show the sizings.  My OnSite Guide is out of date (and I don't know where it is now).  I seem to remember that is was pretty simple to work out any derating because of insulation.

 

Installation method 103 in table 4A2 of the regs covers a cable embedded fully in insulation in a stud wall, but there's no specific case in that table that covers cables embedded in thicker insulation within an enclosed space, as may well be the case with the requirement for acoustic insulation in ceiling voids.  Often the acoustic insulation is mineral wool or similar and so is a pretty good thermal insulation, but realistically I think the de-rating factor given for Method 103 is probably good enough, given the U value that was used for the testing to derive the factor.  Method 103 gives a current de-rating factor of 0.5 for T&E (as per table 4D5) embedded in insulation with a minimum U value of 0.1 W/m².K and not touching a surface.  That's straight from my copy of BS7671 17th Ed, which is not the most recent, it's the amendment 2 version, not that I think those tables will have changed.  If they have changed with amendment 3 than perhaps someone with a copy can correct me.  I'm not going to buy another copy of the regs now our build is completed.

 

As above, I'm not going to breach the copyright by posting the tables, either!

Edited by JSHarris

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1 hour ago, Alphonsox said:

 

That's plain and simply physics - Oversizing a cable will reduce its resistance and hence reduce the I2R heat losses due to current flow. All the regs are providing you with is a minimum for safe operation. Using a 10mm feed for an oven where a 6mm would meet minimum requirements is perfectly acceptable. I do however have some doubts about the economic payback but this would depend on things like run distance and the cost of copper/electricity dependent.

 

@Logix1 Welcome to the forum, please feel free to introduce yourself on the Intro board.

 

The regs don't quote the minimum required, they have 'headroom' allowed, as they work to 80% capacity Zs, going much beyond this is simply an excerise in wasted money, for any form of minimal gain you might think you will get. 

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28 minutes ago, SteamyTea said:

I used to be able to post stuff up as it was for education and research, but technically not allowed to now.

An online calculator is probably good enough.

http://myelectrical.com/tools/cable-sizing-calculator

 

The way that BSI have protected their copyright and charged a lot of money for any safety-related standard (not just electrical stuff) has always slightly irked me.  The consequence is that a lot of the very useful and important information in those standards is not available to the majority of the population, and I think that's not conducive to making people more aware of risks and hazards.  One consequence is that information ends up getting disseminated like this, in an exchange on an internet forum, and with the best will in the world that information is likely to have the odd error, or be out of date when someone reads it years later.  There's also a very real risk that misinformation can be spread in this way, too.

 

For all my working life I had free access to all BS and EN standards, as a part of my job, but since retiring I've lost that, so had to buy one or two, and they are far from cheap, which highlighted the access problem - I'd never really thought about it before.

Edited by JSHarris
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15 hours ago, Steptoe said:

The regs don't quote the minimum required, they have 'headroom' allowed, as they work to 80% capacity Zs, going much beyond this is simply an excerise in wasted money, for any form of minimal gain you might think you will get. 

Agreed. Plus you can allow for diversity ( unless that's already been factored in by the regs which I'm unsure of ). 

 

On 23/02/2017 at 23:12, Steptoe said:

Cable sizing calculations are all based on the resistance of the cable, volt drop, and power requirements already. 

Again, ageeed. 

NOTE : Each install is unique. ;) 

 

Let's remember to dispense advice accordingly, in a positive manner, and get the op to as good a resolve as possible. 

Cable sizing should only be summised under qualified supervision, IF you do not deem yourself competent to do so. 

We should also advise that for a 3rd party sign-off ( certificate ) said qualified person will need to witness the cable installation, from start to finish, in order to ascertain its compliance, and satisfy themselves that the type and size of cable used is completely fit for such purpose. 

@lakelandfolk, who will be signing-off the work?

Edited by Nickfromwales
Second re-quote content amended after review
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1 hour ago, JSHarris said:

One consequence is that information ends up getting disseminated like this, in an exchange on an internet forum, and with the best will in the world that information is likely to have the odd error, or be out of date

+1, which is why I've added the 'disclaimer' in my last.

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14 minutes ago, Nickfromwales said:

Agreed. Plus you can allow for diversity ( unless that's already been factored in by the regs which I'm unsure of ). 

 

Again, ageeed. 

NOTE : Each install is unique. ;) 

 

Let's remember to dispense advice accordingly, in a positive manner, and get the op to as good a resolve as possible. 

Cable sizing should only be summised under qualified supervision, IF you do not deem yourself competent to do so. 

We should also advise that for a 3rd party sign-off ( certificate ) said qualified person will need to witness the cable installation, from start to finish, in order to ascertain its compliance, and satisfy themselves that the type and size of cable used is completely fit for such purpose. 

@lakelandfolk, who will be signing-off the work?

 

 

I hadn't seen those other comments that you quoted, Nick, but there is also another factor that is important to self-builders - VAT.  My experience was that very few electricians around here were registered for VAT, and that means that you would be daft to get them to supply any materials, as you'd not be able to reclaim the VAT, so everything would automatically be 20% more expensive.  Add in that trade prices for electrical items from local electrical factors are very often a lot higher than online prices for the same items, and for a self-builder it just doesn't make sense to have a non-VAT registered electrician buy the materials for your build.

 

Also, it doesn't make sense, economically, for a self builder to buy reels of every size of cable, it's often cheaper to buy a reel of the biggest cable needed and then use it where it isn't really required.  As an example, I needed a fair length of 6mm², and a short length of 4mm², but opted to use 6mm² in place of the 4mm² just to avoid buying another reel, or an expensive short length, of cable.

 

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.

 

Edited to add:

 

Sorry, Nick, I was typing as you posted.

Edited by JSHarris
typo, and added comment in light of Nick's post whilst I was typing
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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/0.75V/0.5V 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?

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14 minutes 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/0.75V/0.5V 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?

Thanks for the very informative post ;)

Personally I feel that most would put those small losses down to eventuality, and accept that there will be some drop in performance but not enough to re-write the books. I certainly don't see anyone rushing to do a ring in 4mm2 cable but I do always run a 10mm2 for every instance that requires only a 6mm2 as the sliding scale of performance is most significant with higher loads. 

The savings stated may have to be toned down slightly to allow for additional labour / time for working with ( terminating ) larger cables, fitting deeper boxes etc so I really think this should be considered as 'case-specific'.

I maintain that it's down to the commissioning agent to ascertain sizes / suitability of cable runs, so it is in fact a little strange that the question has been asked in the first place TBH, unless the op is wiring themselves and has already agreed to get a third party to sign off. In which case, again, they should have already designed, and specified all of this criteria from the outset. 

Lets wait for @lakelandfolk to rejoin for further input and clarity of the situation :) 

 

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23 minutes ago, Nickfromwales said:

 I certainly don't see anyone rushing to do a ring in 4mm2 cable but I do always run a 10mm2 for every instance that requires only a 6mm2 as the sliding scale of performance is most significant with higher loads.

 

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!

 

24 minutes ago, Nickfromwales said:

I maintain that it's down to the commissioning agent to ascertain sizes / suitability of cable runs, so it is in fact a little strange that the question has been asked in the first place TBH, unless the op is wiring themselves and has already agreed to get a third party to sign off. In which case, again, they should have already designed, and specified all of this criteria from the outset. 

Lets wait for @lakelandfolk to rejoin for further input and clarity of the situation :) 

 

 

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.

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Modern houses with a sealed envelope and all services within a void inside the insulated shell do not pose any issues.  Regarding accoustic insulation, to be perfectly frank that's a building regs requirement that I am paying lip service to.  I don't want it, it gets in the way, it's expensive and horrible to install. but I must have it.  I will absolutely not pass cables through accoustic insulation, where a cable has to go, there will be a gap in the accoustic insulation. End of.

 

I have installed a cable this week for the kitchen island. It will only serve one of those pop up socket things, and the ignitor for a gass hob, and will be a radial circuit on a 16A rcbo. But because it passes down through the 300mm floor insulation, along under the floor, then back up through the 300mm floor insulation, I have run it in 6mm. This will be the only cable passing through the building insulation as it's the only route to the kitchen island (well apart from burying it in the screed with the UFH pipes!!!!!)

 


 

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@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?

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10 minutes ago, ProDave said:

Modern houses with a sealed envelope and all services within a void inside the insulated shell do not pose any issues.  Regarding accoustic insulation, to be perfectly frank that's a building regs requirement that I am paying lip service to.  I don't want it, it gets in the way, it's expensive and horrible to install. but I must have it.  I will absolutely not pass cables through accoustic insulation, where a cable has to go, there will be a gap in the accoustic insulation. End of.

 

I have installed a cable this week for the kitchen island. It will only serve one of those pop up socket things, and the ignitor for a gass hob, and will be a radial circuit on a 16A rcbo. But because it passes down through the 300mm floor insulation, along under the floor, then back up through the 300mm floor insulation, I have run it in 6mm. This will be the only cable passing through the building insulation as it's the only route to the kitchen island (well apart from burying it in the screed with the UFH pipes!!!!!)

 


 

 

I agree about service voids, we have them on all external walls, but all the internal stud walls are filled with rockwool, and there are a fair few cable runs through them, where the cables are clipped to the studs but surrounded by tightly packed rockwool.  The ceiling/first floor void is filled with 200mm of rockwool, more than required by building regs, but my other half was concerned about noise transmission from the living room to the bedroom above (she goes to bed early and gets up at 5 am for work).  It wasn't checked by building control, though, so could have been left out if we weren't concerned about noise transmission.

 

I'd also agree that the acoustic insulation is a bloody nuisance and it was one of the most horrible parts of the build, too.  I don't think I will easily forget the time spent packing 200mm thick rockwool into the gaps between the floor joists, as I had to do it from underneath, resting the stuff on my head as I went along each run.  Even with a Tyvek suit on, with a hood, and gloves and ankles taped to the suit, I still ended up itching like mad.

Edited by JSHarris

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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.
 

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