Lighting circuit taken off ring main somewhere

[Ross1]

evening guys, ive come across lighting in one room of domestic premises that has been supplied from the ring main at some point. the cable coming from the ring is 1,5mm which goes through the switch then on to the 2 x 12V down lights. This is on an EICR I`m doing. There is no chance at all of accessing under the floor boards as there is very expensive oak flooring down every where. I believe that from memory you can use a switched fused spur for functional switching. I could then put in a 5A fuse and the load side of the cable will be protected. My concern though is the piece of 1,5mm from ring to switch? your thoughts on this would be welcome. I see this as no different to taking a spur off the ring for an extra socket etc, or am I forgetting some thing?

 

[7029 dave]

yes nothing wrong with spurring down lighting circuit of ring main. ( bad circuit design). But that 1.5mm cable needs to be replaced to 2.5mm to feed side of spur

 

[GLENNSPARK]

yes nothing wrong with spurring down lighting circuit of ring main. ( bad circuit design). But that 1.5mm cable needs to be replaced to 2.5mm to feed side of spur

does it?

are you sure

 

[Ross1]

I agree with what you are saying Dave, its for sure bad practice but from a technical point of view that 1,5mm supply to fused spur is inaccessible so no chance of to many amps being pulled through it.

 

[Newbyspark]

Wouldn't the fuse blow if more than 5 amps was being drawn, then the 1.5 cable before the fuse wouldn't be drawing any amps either.

 

[7029 dave]

understand but that cable is underrated for that circuit.

 

[Ross1]

understand but that cable is underrated for that circuit.

what about when you spur of the ring to an additional point as allowed. that 2.5mm is also under rated for the 32A MCB.

 

[GLENNSPARK]

understand but that cable is underrated for that circuit.

David...
whats the minimum CSA for FCU feeds?
and given that the largest fuse to BS1362 (13A) is under the max CCC of a 1.5/1.0 flat twin....
the fuse will dictate Max demand here so...

 

[7029 dave]

what about when you spur of the ring to an additional point as allowed. that 2.5mm is also under rated for the 32A MCB.

Take your point but its being feed by 2cable's ie lets say half the load between 2 cables.

 

[GLENNSPARK]

Take your point but its being feed by 2cable's ie lets say half the load between 2 cables.

eh?
dont get your drift here David...

 

[7029 dave]

David...
whats the minimum CSA for FCU feeds?
and given that the largest fuse to BS1362 (13A) is under the max CCC of a 1.5/1.0 flat twin....
the fuse will dictate Max demand here so...

Yes Glen I take your point that FCU aint gonnar pull more than 13amp tops (max) or min 1amp depending what size fuse you put in so yes it will dictate max demand.But its just bad practice to have that size cable.

 

[D Skelton]

But its just bad practice to have that size cable.

By that rule then every tail I take from a 200A busbar chamber would have to be 95mm??? Have you tried fitting 95mm cable into a 100A switch fuse?

It's not bad practice at all.

 

[GLENNSPARK]

By that rule then every tail I take from a 200A busbar chamber would have to be 95mm??? Have you tried fitting 95mm cable into a 100A switch fuse?

It's not bad practice at all.

about time.
Evening Damian...

 

[GLENNSPARK]

Yes Glen I take your point that FCU aint gonnar pull more than 13amp tops (max) or min 1amp depending what size fuse you put in so yes it will dictate max demand.But its just bad practice to have that size cable.

David...
you need to rethink statements like this ...as they are misleading to the pups in here...

 

[D Skelton]

Evening Damian...

It is Glenn, it is

Goooooood Evening!

 

[LittleHelper]

I take the same view as 7029 Dave when approaching similar situations as a good practice measure.

 

[GLENNSPARK]

to the O/P:

the cable feeding the FCU is adequate & compliant (CSA)..
do the usual (IR, loop)..to verify....

 

[AndyL]

Because its not the way we usually do it does not make it bad practice.
As stated above the FCU protects the 1.5 against overload. The MCB will still give adequate fault protection (Zs being correct of course!)

 

[michaelk]

Could you not fit a fcu with 5a fuse in it next to the socket the 1.5mm is coming from and put the 1.5mm on the load side and a bit of 2.5mm between the socket and fcu next to it.

 

[telectrix]

Could you not fit a fcu with 5a fuse in it next to the socket the 1.5mm is coming from and put the 1.5mm on the load side and a bit of 2.5mm between the socket and fcu next to it.

because it's probably joined into an inaccessible Jb under the oak floor. i'd just bung in an unswitched FCU with a 3A (or 5A) fuse before the light switch.