Wiring between CU or Meter and New Garage
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digitalgraham
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Wiring between CU or Meter and New Garage

by digitalgraham » Sun Aug 07, 2022 5:27 am

So I've a dilemma and therefore a technical query, I don't intend to do all the work myself part of it will be an Part P reg person. Anyway I've read, re-read the IET tech book and fail to see which route of cabling I should go with meter or CU connection.

There's a 100A fuse coming into the house, then a meter (therefore no isolator switch installed), hence me getting a registered person to do the final connection to the supply. The distance from CU & Meter to the remote CU for the garage is 40m. I'm wanting fairly decent supply to ensure welders, lathes etc work at the same time and don't trip the RCD, hence the use of 16mm2 tails or cabling cables.

My questions/options that I've come up with are:
What cabling method do I choose,

16mm2 twin & earth clipped to loft boards or wall throughout the 40m run
OR
2x 16mm2 cores and a 16mm2 earth cable all individually clipped to loft boards & wall.

Do I run the feed between CU and remote CU, or Meter to remote CU?

Or alternatively, have I completely misread the regs and therefore can someone please offer some further advice. TIA

ericmark
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Re: Wiring between CU or Meter and New Garage

by ericmark » Sun Aug 07, 2022 10:05 pm

There seems to be a requirment for a single point isolation, but never found the regulation, likely fire or building regs, rather than BS 7671.

It has been said the DNO should provide an isolator, but it seems very rare to find that, again not BS 7671 but HSE who require the electrical supply to be isolated else where, to the board your working on, and unless there is an isolator, you could only do this by drawing the DNO fuse, and two wrongs don't make a right.

Usually the way forward is to get an isolator fitted, then who ever does the work can do it dead.

As to where to take supply from, I would personally use either a MCB or RCBO, even if a 45 amp one.

But first question is how your going to earth the new garage. So your looking at design, there is an IET wiring matters about wiring out buildings, a bit dated, but free down load and worth a read.

Distance between building matters, not just your buildings but any other which may have metal outside bonded to the supply which you can touch at same time as some metal on your building.

I would aim today to make a garage TT if possible to make EV charging easier in the future.

But I could only decide that after visiting the site, basic it is a risk assessment, no right or wrong, the advent of the double pole single width RCBO has made it easier, I would not today consider a double RCD consumer unit, it would be all RCBO, but go back 20 years, and we did not have that option, so many people have to work with less than ideal equipment.

So design is not always simple, and whoever does the design has to sign the installation certificate to say they did the design, and although the forms can have three signatures, the forms used by the scheme providers only have one, so you would need to go through LABC.

The cost of using LABC can get silly, it depends on what you have, when I looked at doing my mothers house I estimated it would cost between £500 and £1000 to get the test equipment and pay LABC to DIY, and so although an electrician I decided it was cheaper to pay some one else to do the job.

I would suggest you get a quote first, then decide if DIY is worth while, and likely they will say how they would do the job so gives you some pointes.

digitalgraham
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Re: Wiring between CU or Meter and New Garage

by digitalgraham » Sun Aug 07, 2022 10:33 pm

Thanks for your comments and suggestion, you're right about the isolation switch as this would be the safest option to ensure total disconnection. However, most (if not all) DNO's recommend the isolation, but they do not give it for free and is classed as very low priority so no rush to install.

I think it's time to call an expert, although that isn't a simple and straight forward task.

ericmark
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Re: Wiring between CU or Meter and New Garage

by ericmark » Tue Aug 09, 2022 7:06 am

This https://electrical.theiet.org/media/106 ... cbr+%2f%3e from the IET tells you how to take a supply to an outbuilding, it is out of date, but the main things are talked about.

It also talks about TN and TT supplies.

Sorry was using tablet before so hard to find link.

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