big new summer house - tiny little cable

[Welchyboy]

I'm refurbishing a large house and part of the refurb is a huge summer house at the end of a long garden

The summer house is to have all electric heating and lighting and a small kitchen area, estimated MD of around 50a at least

Unfortunately the foreign workers who have built the cabin are also installing the electrics

The Run from house supply to summerhouse cu is around 125m, they have been digging a trench to the house for the water and electric this week with a mini digger

I have only been doing the main house so far but they have left a big coil of swa at the house end after backfilling the trench and said as I have been doing the house I can run the rest of the swa back and connect to the main supply

I thought cheeky gits it's your job, had a quick look at the cable, 10mm 3core!
'get the mini digger back out again lads!!!'

 

[SPARTYKUS]

Not big enough for that demand and length. Maybe ok for one socket & a light :drunk:

 

[telectrix]

it's OK for the demand, but VD at 40A would be 22V. uh,oh.

 

[the oracle]

I take it they have not inspected, tested and certified the installation in the Summer House either? It would seem that they are leaving you with all the problems, very nice of them, and oh so typical of an awful lot of the crap that we electricians are facing these days!
I would suggest you approach the customer with a written report, outlining;
a/. the lack of an inspection and test certificate b/. the undersized cable for the assessed demand, which will probably need to be replaced, unless they reduce the load required! c/. Did the Summer House Gang notify their 'bit of work' to the local Building Control (required under Building Regs., d/. Were they paid for the complete job, including the connection at the house end and e/. If you connect the distribution circuit you will then become responsible for energising an unknown/unproved installation. You have no option but to test and inspect it all first, which will be at an additional cost. You will need to provide an EIC for the Summer House AND an EIC for the new distribution circuit, so that will take you a while.

Make sure the customer is well aware of these issues, perhaps next time they will get you in to do ALL the work so it is done properly. I would also suggest the customer writes to the firm installing the Summer Houses, informing them of how they are presently breaking the law, as these issues are definitely covered by the Electricity at Work Regulations, 1989.

We need to start stamping our feet and getting angry about this type of thing, there is SO much of it going on, jobs that competent electricians should be getting!:furious3:

 

[Elmer BeFuddled]

If I was inspecting I would give an Immediate C1 and not even go there! Do your own stuff and get the client to get them back and certify their work.

 

[Octopus]

I'm refurbishing a large house and part of the refurb is a huge summer house at the end of a long garden

The summer house is to have all electric heating and lighting and a small kitchen area, estimated MD of around 50a at least

Unfortunately the foreign workers who have built the cabin are also installing the electrics

The Run from house supply to summerhouse cu is around 125m, they have been digging a trench to the house for the water and electric this week with a mini digger

I have only been doing the main house so far but they have left a big coil of swa at the house end after backfilling the trench and said as I have been doing the house I can run the rest of the swa back and connect to the main supply

I thought cheeky gits it's your job, had a quick look at the cable, 10mm 3core!
'get the mini digger back out again lads!!!'

Can't you simply speak / write to the client with your recommendations? Also, if you are doing the main house why are the others doing the summer house?

 

[alasdairp]

I assume you mean 10mm2 copper conductors?
That would be fine current-wise (Table 4D4A for a trench installation rating is 60 A if single phase and the third conductor is used as the safety protective earth). Even 10 mm2 Ally conductors would be ok.
However, the voltage drop would be unacceptable.
I read Table 12A as saying max Vdrop on a 125 m feed from the mainbox to a subbox (private supply) as 6.125% if it is to feed lighting circuits.
Voltage drop is listed as 4.4 mV/A/m = 125 x 4.4 x 50 = 27.5 volts at 50 A (unacceptable).
6.125% is allowable which = 0.06125 x 230 = 14.1 volts
I make it that the Vdrop on the 10 mm2 cable would be ok if fused at supply end at 25 A, but that is half the 50 A you say is needed.
If you ignored lighting then Vdrop could rise to 8.125% = 18.7 volts so it could be fused at 32 A
Also, if you connected it up you would be taking responsibility for their downstream work, I believe.

 

[Welchyboy]

I assume you mean 10mm2 copper conductors?
That would be fine current-wise (Table 4D4A for a trench installation rating is 60 A if single phase and the third conductor is used as the safety protective earth). Even 10 mm2 Ally conductors would be ok.
However, the voltage drop would be unacceptable.
I read Table 12A as saying max Vdrop on a 125 m feed from the mainbox to a subbox (private supply) as 6.125% if it is to feed lighting circuits.
Voltage drop is listed as 4.4 mV/A/m = 125 x 4.4 x 50 = 27.5 volts at 50 A (unacceptable).
6.125% is allowable which = 0.06125 x 230 = 14.1 volts
I make it that the Vdrop on the 10 mm2 cable would be ok if fused at supply end at 25 A, but that is half the 50 A you say is needed.
If you ignored lighting then Vdrop could rise to 8.125% = 18.7 volts so it could be fused at 32 A
Also, if you connected it up you would be taking responsibility for their downstream work, I believe.

Mate where are you getting your figures from????

It's 3% for lighting from the origin of the install
thats 6.9v from house supply to end of furthest lighting circuit in the summer house

So let's say we allow 5v drop across the sub main to allow a further 1.9v for the lighting sub circuit, that allows you about 12 odd amps on a 10mm swa!

I priced the job first to install a 25mm 4core and was going to use it as a parallel supply

 

[Richard Burns]

I assume you mean 10mm2 copper conductors?
That would be fine current-wise (Table 4D4A for a trench installation rating is 60 A if single phase and the third conductor is used as the safety protective earth). Even 10 mm2 Ally conductors would be ok.
However, the voltage drop would be unacceptable.
I read Table 12A as saying max Vdrop on a 125 m feed from the mainbox to a subbox (private supply) as 6.125% if it is to feed lighting circuits.
Voltage drop is listed as 4.4 mV/A/m = 125 x 4.4 x 50 = 27.5 volts at 50 A (unacceptable).
6.125% is allowable which = 0.06125 x 230 = 14.1 volts
I make it that the Vdrop on the 10 mm2 cable would be ok if fused at supply end at 25 A, but that is half the 50 A you say is needed.
If you ignored lighting then Vdrop could rise to 8.125% = 18.7 volts so it could be fused at 32 A
Also, if you connected it up you would be taking responsibility for their downstream work, I believe.

Remember this is not a private supply, it is from the public supply network, so whilst you can add 0.125% for the 25m over 100m you are adding this to the 3% for lighting to give 7.18V
Similarly for the non lighting you could add 0.125% to the 5% to give 11.78V

 

[drew35]

"Unfortunately the foreign workers who have built the cabin are also installing the electrics"

Do they still make Cowboy Traders???

 

[alasdairp]

I see it as a difficult issue to resolve reasonably. I would feel quite differently if this was a feed to a house extension - then I agree the 3% is the figure to work with.
I agree that 10 mm2 is too light weight for such a long run. I wonder, too, if they burried it deep enough and with appropriate marker tape.

You are formally correct that ideal figure is the 3% one and you are on site and have calculated the effective load for the summer house.

25 mm2 4-core in parallel may be ultimately correct, but we do have to remember what this feed is supplying otherwise it can be unecessarily priced out.
Also you need to watch the Earth loop impedance and I would always go for separate Earth conductor in such a long feed rather than relying on the armouring.

Do they really need 12.5 kW of mostly heating load in the summerhouse? That is a lot of electrical heating - could it be reduced?
Is some of it to be off-peak (when diversity of loading with time comes into play)?

From the few details we have, I would probably have gone with 16 mm2 (with off-peak storage heating) or 25 mm2 3-core (L, N, E, + armour) and fused accordingly.
It would depend, of course, on exactly what heaters and timing they were installing in the summerhouse.

7671:Section 525 is remarkably open about the practical reasons for the need to avoid voltage drops. This is a feed to an outbuilding which probably has mostly resistive loads (space heaters and electric kettle and non-voltage fussy lights (they should use CFLs or preferably LEDs, i.m.h.o. now), so tight voltage stability at the far end of the cable is unlikely to be essential for safety or user convenience.

If the owner fitted an electrcity meter and charged the occupant of the summerhouse (say his wife) for electricity - is that still a "public supply" as it originates on the public DNO supply, or does it effectively become a "private supply". I would be interested to read where this is defined as I can't see it defined in BS7671.

I am still on a learning curve and the discussion is helpful. Thanks.

 

[Knobhead]

Welchyboy why all this worry about something that is nothing to do with you? Let the foreigners get on with it.

 

[Silly Sausage]

...and refuse to connect up their cable. It's their install, it's their job.
And note all the Summerhouse works as 'Exemptions' on your paperwork.

 

[yellowvanman]

Not condoning whats been done, but everybody gets worked up about the 3% and 5% voltage drops.

If you read 525 it does say that if the product spec does allow a lower voltage limit then its Ok to have a greater VD than 5% or 3%, or in the absence of a spec the lower voltage should not impair the safe functioning of that equipment.

So if in the summer house the appliances have a spec of lower than 218.5V then its ok eg. 210-250V. If the heating is electric and you have an electric cooker and simple halogen/tungsten lamps would the lower voltage impair their safe functioning?

With that I'll get my coat!

 

[Welchyboy]

Welchyboy why all this worry about something that is nothing to do with you? Let the foreigners get on with it.

tony, im not worried, if anything im feeling a little....smug i think, i hope it all goes ---- up for them, makes me laugh how they under cut me, then try to get me to connect it up!

the cheek of it

i will have nothing to do with it! i havent even walked down the garden to look inside the place yet so god knows whats it actually looks like inside, i dread to think

the sheath for cpc reading of a 25mm 4core at 125 meters should be around .11 ohms with XPLE SWA no need for separate CPC

 

[alasdairp]

Thanks for your comments!

 

[alasdairp]

Yes, no need for a separate CPC - on install...

I am an electrical engineer who qualified in the early 1970s. I now sort difficult EMF (magnetic field) problems in homes and businesses for private clients and for DNOs. You might be very surprised about just how many faults there are in the UK's LV Distribution networks with failed CPC that used the armour on (particularly XPLE) cables for combined N&E on 3-phase LV networks. It is a surprisingly common problem and causes large net currents to flow around the networks due to PME connections and link-boxes between substations. Water gets in and the (so-called plated) steel rusts away. I have loads of case studies which found this to be the cause of large net currents in LV distribution systems. The ENA recognoise it now as a significant problem and their members now specify a separate Neutral conductor in their composite LV cables. As N&E are connected together every 100 metres of so around the network under PME rules, this is a satisfactory solution if the armouring gets dissolved away.

I even found a substation in Sheffield where the DNO had forgotten to reconnect the Neutral after commissioning and the large building complex had been running with all currents returning via Earth and the building steel frame for 4 years! It did cause real problems with flicker on old CRT type VDU computer displays in the offices, yet most equipment (including in a large supermarket and a cinema complex) worked OK.

So I like to see a real internal insulated CPC as well as the armouring. If the armouring gets eaten away, due to minor cable damage and water getting in, the remote end will loose its CPC.

I am completely supportive on wishes that we could get rid of cheap, usually not very technically good, foreign electricians and 5WWs undercutting people who do a good job at a realistic price by undercutting and doing a naff and potentially unsafe job. I was speaking with a BT engineer a couple of weeks ago who say that Polish people (and so called telecomms qualified) are apparently being employed by job agencies (as a cost saving measure by BT) to work on the BT system - they are apparently causing many problems by not fully understanding what they are doing and by cutting corners.

It this case, of course throw the problem back to the undercutting company.

 

[Welchyboy]

Very informative post, I can believe that just by looking at the state of some of the DNO cables that are above ground that are still in service

 

[SYKRAPS]

They can use the SWA as a heating element as it enter the summer house lol

 

[Knobhead]

tony, im not worried, if anything im feeling a little....smug i think, i hope it all goes ---- up for them, makes me laugh how they under cut me, then try to get me to connect it up!

Wasn’t having a go at you. Hope you get to see the end results of their work.
Then it’s time