Installing a feed to new building - confused!!

[cornburn]

Hey yall!


I've been asked to look at a job where there's a new building going up. As part of the build the other contractors are installing all the electrics and other services within the building, but leaving the actual hooking up to external contractors (which is why I've been asked to look at it). Is this normal? It seems very odd to me as basically they're going to stick in whatever they like, then it's up to me to calculate maximum demand afterwards and put a feed in for all of it. I also can't see how on earth they're going to certificate all the work they've done without any power hooked up!

Any suggestions appreciated, is it one to steer clear from or should I just ask them what size they want putting in and put a note that certificating all the work they've done inside will be their responsibility, so they'll have to come back to do Zs and all their certs. Seems like they're just cherry picking the nice jobs tbh as the armour will have to be run under a car park through to the building.

It will only be a 63A single phase as only a small office, so if I go ahead with the quote I guess I'll just size the feed to the main switch rating (63A) and connect the gas and water bonds locally to earth within the CU instead of running back to MET, which is about 150 meters away.

 

[UNG]

Why is it I hear warning bells ringing

I assume this is a brick building if it is I've never come across this method before. Somebody else installing the supply is only usual if it is a prefab portacabin type building

It's not clear whether you are installing the CU or the other contractor

I would suggest if they cannot tell you the maximum demand then it may be better to walk away as you will possibly be putting a supply to a dodgy installation and may cause you hassle long term

 

[malcolmsanford]

What you need to do it itemise your role. You have been asked to run a cable from somewhere to a new building and hook up the new installation.

There are obvious things you will need to know like all designers the most important is the load to that building. You should be given a loading ie the cU there is a 6 way with 2 lighting circuits, 2 ring final circuits and perhaps 2 spare. From this you can say that they will have 2 x 6amp 2 x 32amp with a 16 and 32 amp spare.

To get your rule of thumb add them up so 6+6+32+32+16+32 = 124 x 0.6 for an office is 74amps. In fact you only really need to add up the actually load but I always add the spares for future proofing. You could decide a 63amp supply with just the available load and leave a couple of 16amp spares in the board.

You then need to design your circuit to that. Is there enough spare capacity in the existing installation, your cable sizing, your method, RCD protection or not for the sub mains etc.

Once you designed and installed your part of the installation you come to hooking up and testing. Me personally I woulds want to see a part filled EIC with the "Dead " results on it for the new building. This is similar to what the DNO do. Once your satisfied that the installtion is safe you can energise the installtion and let them do the "Live" tests.

If this is not available then you can do your EIC for the sub main, making sure the CU in the new build is isoalted and locked off. After you done your tests for that subman you leave the installation isolated and with your EIC and results for the sub main only advise the client that you have isolated the new build due to lack of information on it from the Electricians that installed it. Provide that on a cover letter.

 

[cornburn]

Thanks that's all very helpful. It is a pre-fab type building, will be permanent but it's built up in sections and the electrics are installed afterwards, including a CU with all the internals in (not sure about bonding). There will be a channel installed to pull the new feed through under the car park so it's just baffling me why their guys aren't doing it along with everything else. Other than if the electrics are pre-installed miles away in a factory before it's slotted together. I just don't want to get burned on it while their guys make life nice and simple. I need a lot more info from them I think, build is supposed to be starting in a week and no one has a clue what's going on!

 

[Guest123]

Are you able to speak to the other contractors an get an estimated demand form them???

I do a lot of demountable classrooms in schools in my area and these all come prewired by their own contractors. In the spec I get it always has the demand they want on it, most of the time it isn't available to them and they get what I can give.

If you allow for whats available i.e 63A in this case then you should be fine as no more is available anyway.

I cert my work but only power up when I get to see their cert with dead tests on.

 

[cornburn]

Cheers Lenny

This sounds like the same sort of thing. What happens about the cert for all the bits in the new build though? They won't have a cert with any Zs values for any of the circuits, RCD test results etc. Surely you don't start completing their blank boxes do you once it's finished?

 

[UNG]

They should return to site to complete their cert or if you are working together you may have the supply powered up before they finish

 

[malcolmsanford]

No what they should do as we said are the "DEAD" tests so you will have the continuity, insulation resistance, polarity results availalbe for you. This is what a lot of DNO want when you request a hook up by them.

Once you have these results you can activate the installation and they can finish the EIC by then doing the "LIVE" tests Polarity again, EFLI Zs, PSCC for the local dB if they wish or they can do at origin same as the Ze.

All I would want to see though are the dead tests and make a copy of it for my records,

 

[cornburn]

ok, so take a photocopy of the dead tests, leave the supply isolated then call them to finish off and power their bit up. That makes sense Cheers fellas

 

[marksparky]

on a seperate note sorry for butting in on ur thread, i was asked to quote for a 4 way consumer unit with main switch and tails feeding it, wen i asked wat else was required i was told that is it, he went on to say we have our own sparks that do the installation work but they rnt qualified to connect to the mains? iv never heard of a spark that is not qualified to connect to the mains! anyone else ever heard of anything like this?

 

[Deleted member 9648]

on a seperate note sorry for butting in on ur thread, i was asked to quote for a 4 way consumer unit with main switch and tails feeding it, wen i asked wat else was required i was told that is it, he went on to say we have our own sparks that do the installation work but they rnt qualified to connect to the mains? iv never heard of a spark that is not qualified to connect to the mains! anyone else ever heard of anything like this?

You will probably find their 'spark' is actually the gardener.

 

[Sintra]

Personally i would install the sub main and fit an isolator and test and certify to that point. Then these other guys only have to tail into isolator and power up their installation. This would mean that you have no direct involvement in the powering up or certification of the other installation.

 

[cornburn]

Personally i would install the sub main and fit an isolator and test and certify to that point. Then these other guys only have to tail into isolator and power up their installation. This would mean that you have no direct involvement in the powering up or certification of the other installation.

That's a very good idea cheers, I'll quote for that and if they don't complain just go ahead with that. It's just a distribution circuit to an isolator then and would get rid of all the concerns. They're not making life easy for anyone!

EDIT:

Forgot to double check, the gas can just be bonded locally in the new room to the new board can't it, as it's "effectively" connected to the MET via the earth in the SWA running back to a board in the main building. I'm sure there's no need for a separate core for each bond but wanted to make sure. Thanks

 

[malcolmsanford]

No that gas bond will have to go back to the main earth terminal. Reg 411.3.1.2. So you will have to run a 10mm^ back to the MET

 

[cornburn]

So basically I'd better hope that all the incoming services are in plastic, as the MET is about 200m away through about 20 rooms (no joke), and the feed for the new building will be from a sub dis board nearby. If the services are copper the earth could be used, providing the earth path back through the new cable and the sub dis board back to the MET was large enough to act as both an earthing conductor and bonding conductor.