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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> mcb supplying more than one circuit https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1094299699 Message started by essox_lucius on Sep 4^th, 2004, 1:08pm

Title: mcb supplying more than one circuit
Post by essox_lucius on Sep 4^th, 2004, 1:08pm

does one mcb or fuse supplying more than one circuit comply with bs7671,i cannot seem to find any reference to it in the onsite guide ,anyone know?

Title: Re: mcb supplying more than one circuit
Post by ChubbyPhaseWire on Sep 4^th, 2004, 1:38pm

Take a look at 314-01 ;)

Title: Re: mcb supplying more than one circuit
Post by essox_lucius on Sep 4^th, 2004, 1:55pm

some flats i went to had bathroom heater, kitchen fan heater,and bedroom convector heater all one seperate radials fed from one 30a c/b. comply?

Title: Re: mcb supplying more than one circuit
Post by plugwash on Sep 4^th, 2004, 3:14pm

i know a sparky over on screwfix who failed two runs of 2.5mm from a 32A mcb each running to a fcu and then to a hand drier

his failing of the cuircuit was overruled by niceic who said it was fine as the breaker gave adequate short cuircuit protection and the fuses in the fcus gave adequate overcurrent protection

btw what does that reg chubby posted actually say?

Title: Re: mcb supplying more than one circuit
Post by L.Spark on Sep 4^th, 2004, 3:27pm

well NIC do what they want plug, they think they own all, IEE is very clear as far as im concerned in that every circuit should be controlled seperatly, and running two cables unless it's a ring doesnt stick with this.

short circuit capacity they most likely would have been right, but maby they are being too technical and not logical enough.

Title: Re: mcb supplying more than one circuit
Post by L.Spark on Sep 4^th, 2004, 3:32pm

whoops, for the benifit of Plug, BS 7671 reads for regulation Part 3, chapter 14, section 1 as follows

Every Installation Shall Be divided into circuits as necessary to:

1. avoid danger and minimize inconvenience in the event of a fault, and
2. to facilitate safe operation, inspection, testing and mantenance.

Title: Re: mcb supplying more than one circuit
Post by plugwash on Sep 4^th, 2004, 3:58pm

wrote on Sep 4^th, 2004, 3:32pm:

now if you took that totally literally you would have to put every socket on its own cuircuit

i really can't see how putting a few fixed appliances each with a very low probability of failure on the same cuircuit is a problem

i agree that if it is trivial to split the cuircuits i would do so but if for some reason splitting them is inpractical (lack of space in the DB etc) then i can't see any reason not to have multiple fixed appliance cuircuits off one breaker

furthermore how is taking multiple runs of 2.5mm from a 32A breaker any different from taking spurs of 2.5mm off a 32A ring?

Title: Re: mcb supplying more than one circuit
Post by supersparky on Sep 4^th, 2004, 4:17pm

the reason, as ive always seen it, that they allow single runs of 2.5mm from a ring on a 32amp breaker is because they assume some load to be on the circuit already, if you assumed 10 amp which isnt exactly going to go far then the mcb will trip before the 2.5mm goes bang.

ss

Title: Re: mcb supplying more than one circuit
Post by The_Trician on Sep 5^th, 2004, 2:22am

Yeah, this is a great scenario until you get unprotected Radial extensions in 1 or 1.5mm off a 2.5 cable without an FCU.

And just what is a single run of 2.5 T&E rated at?

NOT 32A....

I hate the concept of running Radials for sockets because they tend to be extended infinitum without any regard to reduction in cable sizes along the way.

Go under the floor boards to add a socket and 'Oooh is this part of the Ring or am I spurring off a spur?
Out comes the Test kit and a five minute job suddenly gobbles up a whole day.

Radials for sockets? I hate em.

TT

Title: Re: mcb supplying more than one circuit
Post by essox_lucius on Sep 5^th, 2004, 5:03pm

thank you for your replies, very interesting.every flat in this block was the same so it must have been specified that way.i was just interested in other peoples opinions on that circuit layout.thanks again

Title: Re: mcb supplying more than one circuit
Post by L.Spark on Sep 6^th, 2004, 10:21pm

The idea is to try and not be totaly literal with the regs PG �:)

Title: Re: mcb supplying more than one circuit
Post by sparky_si-fi on Sep 11^th, 2004, 7:28pm

Heres another scenario to tack on to this,

Standard lighting arrangement for say,a 3 bed semi.

6A breaker controlling up stairs lights,
6A breaker controling dowstairs lights,

A ground floor extension gets built.........

What would be the difference in putting the downstairs new extension lights on the already exisiting downstairs lighting?

Either you take the supply off of ,say an exisiting ceiling rose or joint box in the loop-in or you take the supply off of the circuit breaker in the board,thus revealing two circuits off of one breaker,but the breaker is designated downstairs lights.

*Chicken and the egg?*

SF

Title: Re: mcb supplying more than one circuit
Post by plugwash on Sep 11^th, 2004, 9:03pm

my take on this is

if you are running from the CU and there is a space fuse/breaker space that you can get a fuse/breaker for then its best to use that as the extra cost is negligable and it should allow for better protection better expandibility or both

if there are no usable spare spaces then i really can't see how haveing a cuircuit that involves 2 cables from a breaker is any different from having one that leaves the CU and splits later

Title: Re: mcb supplying more than one circuit
Post by L.Spark on Sep 11^th, 2004, 10:01pm

the thing is, 2x1.5mm coming froma 6A/10A breaker, is not the same as having 2x2.5mm in a ring, and another 2.5mm direct from a 32A breaker.