DIY Doctor

Supply to workshop. Addvice needed

Postby Gary B » Sat Jan 16, 2010 2:53 pm

Hi folks.

I have just built a workshop and I am now in the possition to supply a sub-main from my house to the workshop, (W/S). In the W/S I will have:-
a) 1 x Lighting circuit. (12 x 58W flurecent tubes).
b) 1 x Ring main.
c) 2 x Dedicated 16A, (I believe, Radial circuits), 1 for table saw, 1 for bandsaw.

For ease of threading cable under the house I have oppted for 10mm2 T&E from the house CU to an external waterproof junction box joining to 3 core 10mm2 SWA. The SWA will then go at least 600mm underground to another external waterproof junction box on the ouside of the W/S, and joined to 10mm2 T&E, which will then go into the W/S and terminated at the W/S CU. The cable length will be, (max distance). T&E from house to SWA = 11M; SWA = 8M; T&E from SWA to W/S CU = 2.5M. Total distance from house to workshop = 22M

Workshop power consumption; (items running at the same time).
Lights - 58W x 12 = 696W/230V = 3.02A
Radio - 9.5W/230V = 0.04A
Dust extractor - 1500W/230V = 6.52A
Bandsaw - 2200W/230V = 9.56A
Table saw - 2200W/230V = 9.56A
TOTAL = 6605.5W/230V = 28.72A

In the house I have a split CU with a 100A main switch, and a RCD, not sure of value. On RCD is writen: "Inc 10,000A" - "I triangle, (I think the symbol for Delta) n 0,03A" - "In 8,0A" - "Un 230V". I hope that makes sence to someone.
I also have for the W/S a split load CU which contains a main switch of 100A, and a RCD 80A 30mA

I have been advised to come off the house CU on the RCD UN-protected side so as not to interfere with the lights if the house RCD trips. Is that what you electrisians would do? If so, would it be preferable to use a RCBO, (I think) rather than a MCB? Also, what would you put in the CU at the workshop end?

Any help or advice will be most welcome.
Thank you for your time.
Gary B
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Simply Build It

Postby collectors » Sun Jan 17, 2010 7:00 am

Must admit i think a 3 core 10mm SWA cable direct from the main intake would have been a better job with no joints & a small independent 2-3 way fuse board at the house end.
The house fuse board would need a 60amp RCD with a 40 amp mcb to control the WS run, leaving a spare way for something in the future.
At the WS end an 6 to 8way box with a 100 or 60amp main switch as the whole WS is controlled for safety via the RCD at the house end.
With the radial circuits running to the 3 machines i would use type 2 fuses on these circuits as these will take up the initial serge of the machine when they start up.
On the lighting i would of put this over 2 or 3 switches as there can be a surge on some flory’s when they start up.
This is all subject to being installed with accordance to the new regs.
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Postby ericmark » Sun Jan 17, 2010 12:53 pm

22 meters is quite a run. From your figures still within volt drop considerations but earthing methods need considering. In 2005 Wiring matters the IET magazine published a very good guide called "Electrical installations outdoors: a supply to a detached outbuilding" Google "" and from that you will see it is not just a matter of exporting the house TN-C-S system.

You will need a completion certificate and unless you are a member of one of the clubs then that's £100+ to council who will likely want to inspect. The testing seems to be quite contentious with many councils wanting you to provide the test results. Personally I think that is reasonable as unless you can test you should not be doing the work. However the test equipment at £750 approx to buy is beyond what most DIY people will have. Even to hire is a problem as wrongly used it can be lethal.

If using a TT system I am not sure what council will allow. Norm is to power up then test but in theory you should test first which needs yet another meter.

Sometimes I am really envious of my son with no BS7671 to worry about and even the RCD (Recreational Craft Directive) does not cover his narrow boat. So he can do what he likes. Diodes in earth cables etc. However most of us have many sets of rules to follow. As a result I would not consider what you are doing as DIY.

You seem to have done well so far. The "Inc 10,000A" refers to the maximum current that under fault conditions the unit can handle. The ELI meter also measures the PFC (Prospective Fault Current) two measurements are made line - neutral and line - earth and largest is used and should be below the 10,000A shown. Or the "Let Through" value of previous protection device needs to be below 10,000A. It is these sort of details which means when a PIR (periodic inspection report) is completed as required every 10 years or change of occupant, you have a whole list of faults.

On a MCB you will often see B32 and the "B" means the magnet part needs 5 times the rated current to operate. So it that case 160A to allow that to flow ohms law at 230 volt means whole circuit needs to have a resistance of 1.44 ohms. Although you can calculate this with parallel paths and cable joints really it does require the expensive meters.

If you think you can do this great. But I would not be helping if I didn't tell you it's not just the physical bit that needs doing.

This forum is slow because of the process used and I can see there is a reply pending. Do remember I have not seen that reply when writing this.
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Postby Gary B » Sun Jan 17, 2010 1:29 pm

collectors, and ericmark.

Thanks for your replies.
One thing I forgot to say, my house earth is TN-S.

I have spoken to my local building regs people, and at the moment I am not sure if I am going to do it myself or get a sparks in.

Now I am going to re-read your replies and try to digest it all, and make sense of it.

Thanks again.
Gary B
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Postby ericmark » Mon Jan 18, 2010 12:05 pm

With a TN-S system the loop impedance only needs to be 0.8 ohms this means any type D MCB's need to be 25A or less. And even type B by time you add local loop resistance may be very limited in size.

With modern consumer units where all outputs are RCD protected this is not likely to be a problem but as distance to the boards earth increases so does the problem of a voltage gradient and at the distance you are talking about it may well be better to use a TT system?

In a housing estate normally the TN-C-S system means that all gardens are surrounded by earths and so there is no problem. However in the country the voltage gradient can be steep and without out being on site I would not like to advise as to earthing methods.

Often electricians don't like digging holes. Therefore you may find an electrician who will sub contract you to dig holes. With the £100+ charges from council for Part P and earthing problem I would suggest you at least get a quote from local electrician able to sign completion certificate as you may find very little price difference between both routes and just not worth hassle to fully DIY.
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Postby moggy1968 » Sat Jan 23, 2010 11:33 pm

Two routes I would go would be to split your incommer into a henley block, 1 feeding the main board and one feeding a sub board fitted with a 64amp MCB and time delayed RCD. Alternatively if you have an unprotected way on your main board use that to supply the sub board. this will satisfy the problem of disconnection times and provide RCD protection for your main cable.
then use a sub board in the WS with RCD protection on all circuits as described. All circuits will need CDs because the WS will need to be earthed by an earth spike or similar (TT system) as it is a remote outbuilding.
this means if you trip an RCD in the WS it probably won't trip the main switch in the house. otherwise if you use a standard RCD at the house end evertime a circuit trips (which is quite likely given the useage) both the house and WS RCDs would trip, meaning traipsing into the house to reset it every time.
Also, I would use 2 seperate lighting circuits otherwise there is a chance that evertime a bulb blows the whole workshop will be plunged into darkness!
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Postby Gary B » Sun Jan 24, 2010 3:06 pm

Hi everyone. Sorry for delay. Vissiting the In-laws.

Heres what I was thinking :-
As I have a split load consumer unit, that I would go from the RCD NON-protected side, (so if house trips, shed wont be effected), using a MCB, 10mm2 T&E to an external waterproof box joining to 10mm2 SWA, which will be at least 600mm underground, leading to another waterproof box on outsid wall of shed, joining to 10mm2 T&E leading into shed and terminating at shed consumer unit. The reason for using T&E as well as SWA is ease of threading through house and shed.
Gary B
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