Sub main cable sizes?

[Curlewhouse]

Hi, I'm building my own house, and currently pricing electrics for the building society spreadsheet (one of the joys of a self-build). I've looked online for the calculation for this but get some odd results, and I've seen others asking this question on some forums and getting replies asking them what the load is etc. and no further - but someone building a house cannot know everything which is going to be plugged in over the life of that house, so there must surely be "standard" allowances. So it's a 3 bedroom detached house, 1 leccy shower, gas cooker, the other shower running off the DHW (LPG boiler) - normal amount of TVs' computers and so on (2 of each) - so nothing out of the ordinary. Also there is a detached garage, which will serve as a workshop and occasionally be running a MIG welder which takes about 20 amps.

Mains comes onto the site in a standalone cabinet (that way we got to have a supply on site without additional cost of a "temporary" connection later being made a permanent one, by having a permanent one straight away) - from which one sub main SWA cable will run underground 10 metres, terminating at the house consumer unit and another SWA cable goes 25 metres (also direct from the standalone cabinet) to the garage/workshop which is well away from the house. My question is what size underground cables do I need for each? Obviously the job is being certified, but the guy is away on holiday for a month so I can't ask him, and need to work the cost in for our building society figures - for which I need to know what size to price up.

So - 1 cable underground 10 metres feeds the 3 bedroom house only and 1 cable underground for 25 metres to supply the garage/workshop.

Cheers.

 

[EFLImpudence]

The electrician who has a month off at Christmas. Is it the same one who has six months off in the Summer?

Depends. On lots of things.
Size of supply and incoming cable.

Cost for 25mm² armoured cable - SWA.
It'll hardly make a difference to the overall cost.

Best wait and let the electrician decide.

 

[Iggifer]

We've got a TT submain to pull in first couple of weeks back, overhead TT supply, 50m ish from the cabinet to the house. All electric heating & hot water. That is gonna be a BIG cable

But as above, 25mm² vs 16mm² will make little to no difference in terms of rough costing.

The switchfuses will cost you nearly as much as the 10m of SWA so I wouldn't worry too much - I assume there's room in this cabinet for two metal switchfuses and metering equipment?

 

[Curlewhouse]

The electrician who has a month off at Christmas. Is it the same one who has six months off in the Summer?

Depends. On lots of things.
Size of supply and incoming cable.

Cost for 25mm² armoured cable - SWA.
It'll hardly make a difference to the overall cost.

Best wait and let the electrician decide.

Really useful answer thanks. Just the sort of sarcastic answer I noted other enquirers getting. Funny, because my dad was a sparky and was a nice guy, so it's definitely not a trade thing, and checking some of your responses to other people clearly you have ego issues. Unfortunately if I wait, we can't apply for the mortgage as I need proper figures - don't assume everybody is up to something, it a good rule not to judge others by ones own standards I find. Checked my own posts and there's nothing about her (yes it's a her) being away last summer, so I think you may be confusing me with someone else.

 

[Curlewhouse]

We've got a TT submain to pull in first couple of weeks back, overhead TT supply, 50m ish from the cabinet to the house. All electric heating & hot water. That is gonna be a BIG cable

But as above, 25mm² vs 16mm² will make little to no difference in terms of rough costing.

The switchfuses will cost you nearly as much as the 10m of SWA so I wouldn't worry too much - I assume there's room in this cabinet for two metal switchfuses and metering equipment?

Thank you for that. That's helpful. Yes, we had to put 2 cabinets in, one for the board for their meter etc and one right alongside it for our equipment (I know people have had problems where the board insists on a seperate cabinet, so we built it that way from scratch). Dad having been a sparky I grew up working with him (crawling under the floors with cable on rewires lol) so a qualified friend is overseeing my basically labouring work of running cables etc and they are doing the techy bits and certifying, so I really don't want to hassle them on holiday with questions ..

 

[EFLImpudence]

don't assume everybody is up to something, it a good rule not to judge others by ones own standards I find.

Were I doing that, I would not suspect anything, would I?

so a qualified friend is overseeing my basically labouring work of running cables etc

Q.E.D.
How come he doesn't know?

and they are doing the techy bits and certifying,

It doesn't work like that.

so I really don't want to hassle them on holiday with questions ..

Of course not. Maybe could have thought of it before, then.

 

[RF Lighting]

I've had to make a few assumptions as I'm not on site.

Cable 1 - house supply:


Cable type: 70 deg.C Multi (armoured)
Fixing method: In ground (direct)

Protection device: BS1361
Protection rating: 100 A

Cable size: 50 mm
CPC size: 25 mm
Current carrying capacity of cable in this instance: 140 A

Final circuit: No
Ambient temperature: 30 deg.C
Number of cables grouped: 1
The cables are spaced


Voltdrop: 0.82 %
Allowable voltdrop: 3.00 %

Length: 20.0 m
Calculated R1+R2: 0.022 ohms
Calculated Zs: 0.02 ohms
Maximum Permitted Zs: 0.29 ohms

Type of system: TN
Supply: Single phase at 230 v
Ze: 0.001 ohms
Fault current: 9.88 kA

Applied correction factors:
Ca: 0.89
Cg: 1
Ci: 1
Cc: 0.9
---------------
Ct: 0.801




Cable 2 - Outhouse:


Cable type: 70 deg.C Multi (armoured)
Fixing method: In ground (direct)

Protection device: BS1361
Protection rating: 60 A

Cable size: 16 mm
CPC size: 16 mm
Current carrying capacity of cable in this instance: 78 A

Final circuit: No
Ambient temperature: 30 deg.C
Number of cables grouped: 1
The cables are spaced


Voltdrop: 2.56 %
Allowable voltdrop: 3.00 %

Length: 35.0 m
Calculated R1+R2: 0.080 ohms
Calculated Zs: 0.08 ohms
Maximum Permitted Zs: 0.56 ohms

Type of system: TN
Supply: Single phase at 230 v
Ze: 0.001 ohms
Fault current: 2.82 kA

Applied correction factors:
Ca: 0.89
Cg: 1
Ci: 1
Cc: 0.9
---------------
Ct: 0.801

 

[ban-all-sheds]

Thank you for that. That's helpful. Yes, we had to put 2 cabinets in, one for the board for their meter etc and one right alongside it for our equipment (I know people have had problems where the board insists on a seperate cabinet, so we built it that way from scratch). Dad having been a sparky I grew up working with him (crawling under the floors with cable on rewires lol) so a qualified friend is overseeing my basically labouring work of running cables etc and they are doing the techy bits and certifying, so I really don't want to hassle them on holiday with questions ..

Have you thought about what you might do when Building Control refuse to give you a completion certificate?

 

[ericmark]

A consumer unit can only go to 125A and in the main they are rated at 100A so in real terms a normal house is limited to a 100A supply. The lowest I have seen supplying a house is 60A so your supply will be 60, 80 or 100 amp unless there is something very special.

Clearly the local authority building control (LABC) will be involved, I personally had a real struggle to get them to agree to allowing either me or my son to sign the certificated for my fathers house. I wanted my son to sign as he had insurance as he was a self employed electrician at that stage, even with a C&G2391 they would not accept his signature, he said what is the point in having some one with less qualifications inspect my work, if there is any dispute then you will need to accept the person with the higher qualification. But the LABC inspector was not budging. So then my son spouted what if my father signs it, he has a degree in electrical engineering, and finally the inspector agreed that I could do the inspection and testing and submit the results to the LABC.

It seems the LABC often employ electricians to do an Electrical installation condition report (EICR) and will issue a completion certificate if the report is satisfactory, but the electrician is selected by the LABC not you, and you have no control over the price charged.

After the problems first time around getting the LABC to play ball, when I wanted to rewire the house I employed a scheme member to do it simply because of the hassle trying to DIY within the bounds of the law, had I intended to live in the house myself I would have just done it, and claimed it was planned before 2004 if anyone asked, but we thought we may need to rent out the house. So all t's needed crossing and all i's dotted.

You are in a similar situation because the LABC is involved you have no option but to satisfy their demands. When it says the person who designed the installation you can't enter "The lads on the forum" so you need some one who can not only work out what you need, but the LABC will accept their signature. As said even with a family of electricians it was not easy, Dad was Electrical Engineer in charge of a Steel works power station, Father-in-law electrical project director for Liverpool Hospital board, I was electrical engineer for local concrete works, son was a sole trading electrician mainly working in shops, if we had problems consider what problems you will have. And yes we had all the meters, and all in calibration at that time.

I don't want to put you off, but I do want you to go into the project with yours eyes open.

 

[OwainDIYer]

Cable size: 50 mm
Length: 20.0 m

about £12 a metre

Edit to add: in cost terms this means that if you use too small a cable, and it has to come out and be replaced by a new run of larger cable, that's a lot of money wasted.

If you use too large a cable, you're wasting the difference in cost between what you bought and a smaller cable that could have been designed for the installation.

Therefore this part of the installation does actually need to be designed rather than based on assumptions and standard circuits.

Cost of trenching and ducting will not be insignificant,either.

 

[John D v2.0]

True Eric mark, it must depend on the local authority. Ours said they need a part p certificate for the electrical work when we submitted full plans for the extension, then I just said there won't be one as I'm doing it myself, at which point they didn't seem bothered. Not sure what would happen if we rented it out, it was rented out before with problems that would be code 1 eg unfused lighting circuit and unenclosed live parts.

 

[Curlewhouse]

Have you thought about what you might do when Building Control refuse to give you a completion certificate?

No reason why they wouldn't providing work is properly certified - I've made a point of going through everything with him to avoid issues later as much as I can (if you check out the self build forums, lots of folks wish they'd talked to their BR before doing things as they tend to pay a LOT more attention to self builders work). He knows and is perfectly happy the work is being overseen properly and will have a part P cert at the end before he signs off - the regs say the work needs to be done to code and certified as such. The regs don't say that a certifying sparky must physically do every last cable clip or dig the trenches him or herself - else there would be not apprentices for example until they'd done their ticket. The regs were set quite properly to stop cowboys, (improving safety for the consumer of course by ensuring installations are done properly - I know some of the horror stories dad used to come across and have to fix before the regs came out, done before by people calling themselves electricians, some it quite literally made your hair stand on end! funny if it were not also so dangerous sometimes. ) and have gone a long way to ensuring things are done properly. I've never understood why people would cut corner with electrical or gas installations (worth looking at the youtube channel of plumberparts - some of the stuff there is scary!) themselves, but as we know they really did!

 

[ban-all-sheds]

No reason why they wouldn't providing work is properly certified

Have you done what they are expecting regarding design and construction? i.e. have you done exactly what you said you, or what by default you allowed them to assume you, would do to ensure compliance with Part P?

Given the question you have come here to ask I do not see how the electrical work could ever be properly certified.

- I've made a point of going through everything with him to avoid issues later as much as I can (if you check out the self build forums, lots of folks wish they'd talked to their BR before doing things as they tend to pay a LOT more attention to self builders work).

Indeed they do. Show me this year's self-builder who ploughed on via a Building Notice instead of Full Plans and I'll show you a short-listed nominee for next year's Darwin Awards.

Probably.

He knows and is perfectly happy the work is being overseen properly

But it isn't. You've ended up on an internet DIY advice forum asking random strangers to do the design of your electrical installation.

and will have a part P cert at the end before he signs off

There are two classes of people, and only two, who can certify to Building Control that the work complied with Part P:

1. A registered electrician who does the work himself
2. A registered third-party certifier

I may have misunderstood, but it doesn't seem as if you are going down either of these routes.

the regs say the work needs to be done to code and certified as such.

Actually they don't - this is what the Building Regulations say:



No mention of "code" (aka BS 7671, the Wiring Regulations, if that's what you meant).

The regs don't say that a certifying sparky must physically do every last cable clip or dig the trenches him or herself - else there would be not apprentices for example until they'd done their ticket.

This is what someone who signs an EIC is certifying (with my emphasis):

I being the person responsible for the design, construction, inspection & testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design, construction, inspection & testing hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:​

An electrician can be responsible for the work of an apprentice because there is a formal employment & supervisory relationship between them. There is no such relationship between you and any electrician, or between you and a bunch of random strangers on an internet DIY advice forum.

I've never understood why people would cut corner with electrical or gas installations

And I've never understood why people would think that they were competent to do electrical installation design work when in reality they need to ask a bunch of random strangers on an internet DIY advice forum to do it for them.

From my POV, I really DGAT what you do regarding all this, but I do GAT about giving good advice, so the $64K question is what exactly did you tell your LABC would be the way that you would ensure that the electrical work would comply with Part P when you sought Building Regulations approval for the building of the house? If you do something different you might find yourself in a mixed metaphor of rocks and hard places without a paddle.

 

[Curlewhouse]

If instead of getting all protective about the trade some folks had actually read my question, you'd realise that I'm currently pricing the list I've been given to buy, but unfortunately the plan they've done doesn't show the rating of the buried cable (though it does for the 2.5, 1.5 t&e etc) and I'm trying to get the pricing done ASAP rather than wait another week. But as an aside there's definitely nothing in the regs that says the electrician has to dig the trench or hump the cables upstairs themselves - the sort of stuff I'll be doing. Can't really been bothered to explain in huge detail or try and justify myself as the only people who needs to be happy is the part P qualified person doing all the design (done) and technical bits and our BR - and they are. Checking through dads 17th edition book and it definitely doesn't say only a qualified sparky can carry cable reels upstairs or dig holes - if someone can show me where I missed that sentence I'll stand corrected! But thank you for the constructive answers as at least I've now got the ballpark figure in that part of the spreadsheet.

 

[EFLImpudence]

It's not a question of being protective of the trade but being used to people coming on here saying what you say when it is clearly blatant lying. That does not mean nor imply that you are - but the fact that you took umbrage so quickly when I said nothing to warrant it was and is suspicious.

The fact that you are only asking for costing purposes makes the actual size required irrelevant. Just price for 25mm² or larger if you want. If you end up requiring smaller than that, what difference will it make compared with the total price?
That the size of this cable is not on your list could mean that it has not yet been determined so how can we be expected to know?
Nor do you need to know the size to dig a trench and carry it upstairs. ???

There is nothing in the regulations to say that you, personally, cannot do ALL of the work.
There is no such thing as Part P qualified. You have to satisfy the Local Authority Building Control.

So, you say a ball-park figure is satisfactory, after all. What sized ball-park did you use?