10mm T&E for Consumer unit supply

[Sav]

Morning everyone,

Hope your all well.

Just a a quick question. We have been asked to quote for a HMO conversion, in a domestic property. So basically, the developer buys these houses and makes rooms out if them.
Leach room is basic, small ensuite shower ( not electric ), lights and a few sockets. There is a communal kitchen and each room is supplied by its own consumer unit.
There are 2 circuits, lights and sockets.
What I'm asking is, the consumer units are fed via 10mm T&E that go down to 60A isolators, which in turn go into a big Henley block.
There are about 5 rooms, so five isolators.
I would have thought that the consumer units for each room , from the isolators had have 16mm earth ?
Obviously things like communal smokes and fire alarm system etc are fed from landlord board.
The supply is a TN-S single phase

regards,
sav

 

[sparksburnout]

Can I just ask, why are some guys speccing SWA when it is within the confines of a domestic dwelling??? Are we anticipating the occupants to be trying to dig the place up?

 

[D Skelton]

Re: 10mm T&E for Consumer unit supply

Oops:lipsrsealed2:

Hey thats why Im banned from texting especially when trying to get into some women's ........

So just out of interest, and I am by no means fluent in this aspect, but, Im led to believe that discrimination or even partial discrimination is not possible between MCB and MCB.

It depends entirely on the prospective fault current. Discrimination for the purpose of overload protection is entirely affordable though.

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Can I just ask, why are some guys speccing SWA when it is within the confines of a domestic dwelling??? Are we anticipating the occupants to be trying to dig the place up?

Because that's what the OP said the customer asked for.

Are you happy to bury a non-RCD protected T+E submain in the wall then?

 

[sparksburnout]

Re: 10mm T&E for Consumer unit supply

It depends entirely on the prospective fault current. Discrimination for the purpose of overload protection is entirely affordable though.

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Because that's what the OP said the customer asked for.

Are you happy to bury a non-RCD protected T+E submain in the wall then?

No of course not, there would be an RCD protecting each one, as someone has already suggested. No one is going to price for SWA and expect to get the job???

 

[Zebra Stripe]

Yes overload I understand, but with short circuit its more complicated. I thought cascading/discrimination was usually in the ratio 2:1 with fuses or perhaps a mixture of fuses and MCBs. Although I've read that you need the downstream device to have a total I2t less than the pre arcing I2t of the upstream device, I've yet to get round to fully grasping this aspect and to be honest have never been in a situation where I've need the knowledge. Perhaps I need to read up on this and maybe start a new thread.:smile5:

 

[D Skelton]

Re: 10mm T&E for Consumer unit supply

No of course not, there would be an RCD protecting each one, as someone has already suggested. No one is going to price for SWA and expect to get the job???

An RCD up front and then another in the board?!

Dunno where you get your SWA from but I get it cheaper that T+E.

 

[Zebra Stripe]

Re: 10mm T&E for Consumer unit supply

No of course not, there would be an RCD protecting each one, as someone has already suggested. No one is going to price for SWA and expect to get the job???

Can you explain what you actually mean? So are you saying put a front end 30mA RCD on each supply to each flat with T&E submain burried?

 

[nickblake]

Loads of switch fuses and henly block dont like the sound of that,, one main switch to isolate the Complete supply then one decent TP CU with single phase conversion kit , with a circuit supplying each Flat, if its a new install the i would use a 3 core swa and use 16mm reason for this is if they decide to up grade the flats in the future then the supplies are there , always plan for the future

 

[D Skelton]

Yes overload I understand, but with short circuit its more complicated. I thought cascading/discrimination was usually in the ratio 2:1 with fuses or perhaps a mixture of fuses and MCBs. Although I've read that you need the downstream device to have a total I2t less than the pre arcing I2t of the upstream device, I've yet to get round to fully grasping this aspect and to be honest have never been in a situation where I've need the knowledge. Perhaps I need to read up on this and maybe start a new thread.:smile5:

That only applies to fuses. What you don't want to have is a situation where a circuit tripping a downstream device causes the upstream fuse to have to 'prepare to trip' to use a phrase. Pre arcing is a state where the fuse is subjected to the fault current but has not yet tripped. The more this happens the more damaged the fuse becomes.

Say for example you have a DB where the highest rated MCB is 50A and you plan to supply this via a sub main. If for example you choose a 63A fuse and it has a prearcing current of 45A (it won't have, this is purely an example) then this is bad design. You would need to uprate the supply fuse or, if possible, downgrade the downstream device.

A very simplistic explanation I know.

 

[D Skelton]

Loads of switch fuses and henly block dont like the sound of that,, one main switch to isolate the Complete supply then one decent TP CU with single phase conversion kit , with a circuit supplying each Flat, if its a new install the i would use a 3 core swa and use 16mm reason for this is if they decide to up grade the flats in the future then the supplies are there , always plan for the future

And you've just priced yourself out of the work!

 

[Guest55]

jesus , dunno wtf's going on in this thread anymore so im out.

 

[sparksburnout]

Re: 10mm T&E for Consumer unit supply

An RCD up front and then another in the board?! Ah yes good point, each room would have to have it's own RCD or it would take out the whole lot. Could you not use a 100mA time delay RCD in a CU after the meter, and then 30mA in each individual CU, that way protecting the T&E?

Dunno where you get your SWA from but I get it cheaper that T+E.

I must check my supplier then, that seems mad!!

 

[Zebra Stripe]

Well, evidently thats far too expensive and you'll never in a month of sundays win the job, cos I'm priceing to have one shower unit RCD CU in the basement and just split off this 40amp MCB with 4mm flex for each flat clipped direct to the stairwell walls using metal clips with a 13amp FCU in each flat for 1 socket and, further fused down with an 3amp FCU for lights :wacko:

 

[D Skelton]

Re: 10mm T&E for Consumer unit supply

Could you not use a 100mA time delay RCD in a CU after the meter, and then 30mA in each individual CU, that way protecting the T&E?

Additional protection for a T+E cable buried less than 50mm in a wall can only be afforded by a 30mA RCD

I must check my supplier then, that seems mad!!

Not really, SWA is far cheaper to produce than T+E

 

[nickblake]

And you've just priced yourself out of the work!

Probably ,lol

 

[sparksburnout]

Re: 10mm T&E for Consumer unit supply

Can you explain what you actually mean? So are you saying put a front end 30mA RCD on each supply to each flat with T&E submain burried?

No, I didn't make myself very clear I admit. I was initially suggesting only one RCD in a CU that would then feed each individual CU, but I now realise after giving it more than 10 secs thought that this would be crap. However, could you not use the method i suggested in my reply to D Skelton? I was not advocating having a buried T&E with no RCD protection!