80A MCB


Postby robert_bullimore » Mon Apr 13, 2009 6:47 pm

I like to think that i'm a competent and qualified electrician - but with no experience!!!!!
I'm Installing a Sub distribution board in my utility room for a kitchen extension. Connected to the main board by a 16mm Twin + Earth Cable. (Which i have calculated to be the correct size following the tables and formulas in BS7671)

I was planning to take the 16mm cable from an 80A MCB wich will be protected by one of the 30mA (80A) RCD's of the split load main distribution board.

My first problem is that 80A mcb's don't seem to be readlity avaliable from any major electrical outlet (or am i just not looking hard enough)
The largest i can find easily are 50A and 63A.

Also would a 100A DP isolator be acceptable in the sub board and will it require addtional RCD protection or is the RCD in the main board enough?

Thanks very much
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Postby sparx » Mon Apr 13, 2009 8:43 pm

Hi,
Loads of points here,
First, you say you like to think you are 'competent & qualified' electrician, but with no experience!
I would suggest you read the top paragragh of page 22 of the red book!!

Now, sub board means a new circuit ie notifiable to building control first, unless you are 'part p ' registered, which clearly you are not.
If you wire the ring/sub main etc. no other electrician can issue test certs.
To the practical side, if you run the 16mm any way other than 'clipped - direct' its rating will be at best 63Amps which happens to match the largest size MCB available for a domestic consumer unit.
Any isolator larger than the supply MCB rating will suffice, and if coming from an existing RCD board then a second one is pointless, also the first one will usually trip first anyway.
I really don't think you should be undertaking this work untill you have talked to a leckie registered with a 'competent persons scheme' who may be happy to allow you to do some of the 'donkey work' under his direction , as he will have to sign for the work as if he has done it himself.
regards SPARX
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Postby rosebery » Mon Apr 13, 2009 9:03 pm

"I really don't think you should be undertaking this work untill you have talked to a leckie registered with a 'competent persons scheme' who may be happy to allow you to do some of the 'donkey work' under his direction , as he will have to sign for the work as if he has done it himself."

I seem to remember that this is old ground for you, Robert, going back to sometime in November last year.

Cheers
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Postby BLAKEY1963 » Tue Apr 14, 2009 7:54 am

ROBERT
I have looked back to last november and rosebery has a point.
This i think could be the same project u r on.
AS sparx and rosebery has pointed out u need 2 urgently get in a part p spark.

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Postby robert_bullimore » Tue Apr 14, 2009 8:28 pm

First of all, my apologies, i was wrong to deem my-self competent as i haven't got the relevant experience to be competent. (my definition of competent was "someone possessing sufficient technical knowledge and/or experience" which is not what the big red book says.)

When i first started enquiring about my installation i took on all the advice and got an electrician in to do the job.
I wasn't happy with the rubbish he talked such as... and i quote "we'll just stick a bigger cable in - 10mm will do"

So i got another one in and he wanted to cut out more holes and knockouts in 150 year of beams and joists which really do NOT want anything more happening to them.

So after that I decided, like most things in life, if you want a job doing properly do it yourself...

I already have a degree in Electrical and electronic engineering

so i went back to college and the did the 17Th edition thing and the inspecting and test thing (I was 1 of 3 that passed in a class of 10 "electricians")

Part "p" is on the way, hopefully. I spoke to the man who checks the quality of your work , he is happy with my qualifications but obviously like to see another installation or two so I'm going get some hands on with a local electrician.

I'll also like to add I'm only do all this for my own personal achievement and my own installation. I do think by any means that i could advertise and start working as an electrician. (but compared to some of the people i have met in the electrical trade its scary to think about some of the things they could be doing)

Like i said my one of my downfalls (i have many) is experience. Such as not knowing that 63A is the max mcb for domestic consumer units.
My 16mm cable is going clipped direct in the roof, way above the insulation, and down inside a non-insulated cavity wall. There is a de-rating factor of 0.79 or so for ambient air. The cable does go through a couple of meters of plastered wall (can i treat this as neglible in reality?)

I know some of my questions are stupid, but i find if you ask stupid questions you get simple answers
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Postby sparx » Wed Apr 15, 2009 8:26 pm

Hi R..B,
well done on the quals. plus good luck with the part p .
BTW who are you registering with? just nosey.
I would suggest that 63A will be adequate allowing for diversity of load, and this will allow for some part of the run to be other than 'clipped direct'
regards SPARX
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Postby robert_bullimore » Thu Apr 16, 2009 7:57 pm

Thanks.

I think i'm going to go with Elecsa, I might have this wrong - but i think its cheaper to join their scheme. I spok to the NICEIC people too, but the i prefferred talking to the Elecsa man.



Another possible stupid question.... Is it possible to terminate a SWA cable to plastic consumer unit (using a gland of course) Or would i save myself a lot of bother by getting a metal cased consumer unit?
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Postby kbrownie » Fri Apr 17, 2009 1:01 pm

Hi robert_bullimore;
This will help "competentperson.co.uk"
Some of the bodies do require different qualification and as you state prices are different too.
But the general rule is at least a level 3.
Some don't consider 2381/1 0r 2391 as acceptable even though niccy insist that you gain 2391 within 18 months or 2 years can't remember which!
KB
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Postby ban_all_sheds » Fri Apr 17, 2009 10:17 pm

hi RB

Why are you needing a sub board for a kitchen extension, unless you are considering future further expansion. A photo of your existing cu would be good. I commend you on gaining the qualification but why pay £400 quid to join a scheme plus the indemnity insurance and the cost of test equipment if you you not wish to further your career in this direction. getting some more quotes and finding a solution you are happy with is surely a lot cheaper. just interested.

good luck.
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Postby sparx » Sun Apr 19, 2009 11:17 pm

Hi, RB,
armoured to plastic ok as long as good connection from gland,
I have been using 'P-Nuts' for some time, (google 'Pirahna earth nuts').
so much better/easier than bolting through 'banjo/teardrop' brass plate.
regards SPARX
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Postby moggy1968 » Fri Apr 24, 2009 1:50 pm

save yourself the cost of elecsa and getting in another spark, just call up building control, tell them what you are doing and ask them to certify it.
I would think a job of this magnitude will satisfy elecsa (I'm with them as well) but it depends a bit on your local inspector.
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