Kitchen Appliance circuit

[Snapester]

Designing a kitchen appliance circuit, i plan to feed a bank of grid switches with a 6mm T&E fed via a 32A MCB, then each switch will have a 2.5mm cable to a fcu supplying an under counter appliance.
My main question is, is it ok to change down to 2.5mm and then feeding the FCU? As the cable cannot be overloaded as the main load is connected to the FCU at position?

Regards

 

[DaveyD]

DaveyD just read your other post to which i have just also stated the same opinion as yourself!

... ah, so now do YOU have a hat on ?

 

[Snapester]

So in essence the radial basically has many 2.5mm spurs! And i wouldn't need FCU i could just use unswitched socket outlets.
Wow learn something everyday on here!

 

[D Skelton]

OK - I'm not tring to be contentious here - I'm sure you are more experienced than me; but are you saying that my reply is incorrect? I was thinking say for example on a 32a radial in 4mm T&E. You are permitted to spur off to one double socket in just 2.5mm T&E. We are not suggesting that 2.5mm will be ok at 32a.
Happy to be corrected so long as it's constructive.

I read what you wrote in post #19 to mean that if an appliance is only pulling 13A then it's ok to use a 2.5mm radial and protect it using a 32A breaker. If that is indeed what you meant then yes, your reply is incorrect, however I'm not so sure now that is what you meant.

Could you eleaborate for me so that I understand fully the solution you propose?

 

[DaveyD]

I read what you wrote in post #19 to mean that if an appliance is only pulling 13A then it's ok to use a 2.5mm radial and protect it using a 32A breaker. If that is indeed what you meant then yes, your reply is incorrect, however I'm not so sure now that is what you meant.

Could you eleaborate for me so that I understand fully the solution you propose?

OK, cool. I was just repying to IPU post #15 asking what would be protecting the 2.5 from the switch to the FCU. I think he was asking what was stopping the full 32a from going down that spur. The OP was designing a radial circuit with a 32a CB in 6mm and then suggesting drops from each switch to FCU in 2.5. I was telling IPU in my opinion this was ok as each drop was a single spur off the radial via a DP switch. The radial itself would need to be =>4mm on 32a but a single spur to a FCU would not.

 

[D Skelton]

Ah, ok, well, yes, unfortunately, in my opinion, you are incorrect as there is no such thing as a spur on a radial circuit. The breaker has to be selected so that it protects all cables within the circuit.

If you have a 32A breaker and 4mm feeding the grid switch, you would need 4mm feeding each 13A SFCU also. Only from the SFCU to the appliance could you then use 2.5mm cable.

Lets say for example you wanted to take a feed off a 40A cooker circuit to feed a single socket outlet (not that anyone would do it this way) then you wouldn't be able to take a 2.5mm feed from the cooker outlet plate to a SFCU, then on to the socket. The feed to the SFCU would have to be 6mm, the same as the cooker circuit. If you took a 2.5mm feed from the cooker outlet plate to a SFCU, even if it was only a few inches, then that length of cable between the cooker outlet plate and the SFCU would not be adequately protected.

My solution to the OP's problem seems the simplest:

Use two seperate A3 radial circuits, each supplying half of the 20A DP switches within the grid, then use 2.5mm cable straight from each individual grid switch to each individual socket outlet designed to supply an appliance. There is also no need for SFCU's whatsoever.

I myself have done this a couple of times but only using one A3 radial to supply the grid switches as there was only 4/5 appliances in each scenario.

 

[Engineer54]

I'd like to see someone get 2 X 6mm cables into a grid switch terminal, even with 4mm the terminal screws are going to be sticking out a mile!! Haven knows what the cable fill is going to be like in the back box!! Strange what some people's idea's can come up with for circuit designs!!

 

[DaveyD]

So why do you feel the brb permits a 2.5 to a double socket from a 4mm radial?

 

[D Skelton]

I get where you're coming from, however if you were feeding a grid switch with 4mm, and then coming off that to feed every 'spur' with 2.5mm then it wouldn't be feeding one socket outlet, it would be feeding every grid switch and every circuit.

The only way to get around this would be to feed each single grid switch with 4mm and then go from each switch with 2.5mm cable. Bear in mind, things are then going to be very tight behind that faceplate!

Although the regulations would technically permit doing it this way instead, I still would never do it.

 

[DaveyD]

Oh yes I would not either. I usually only do 2.5 rfc in a kitchen and I don't really like grid switches as depending on loads it could put to much load at one point AND I am so slow at wiring them up. Its not so much whether one should but whether one could. Thanks very much for your assistance. I know it was not my posting but you really helped me out.

 

[Jimmyb]

Ah, ok, well, yes, unfortunately, in my opinion, you are incorrect as there is no such thing as a spur on a radial circuit. The breaker has to be selected so that it protects all cables within the circuit.

You are allowed as per bgb show on diagram.

If you have a 32A breaker and 4mm feeding the grid switch, you would need 4mm feeding each 13A SFCU also. Only from the SFCU to the appliance could you then use 2.5mm cable.

Why would you need to as the cable cant be overloaded so from grid switch to SFCU 2.5mm is more than enough but the rules are the wire must be less than 3meters long, I have always done it this way and also on my annual assessment he said that it is ok and the way he would do it.

Lets say for example you wanted to take a feed off a 40A cooker circuit to feed a single socket outlet (not that anyone would do it this way) then you wouldn't be able to take a 2.5mm feed from the cooker outlet plate to a SFCU, then on to the socket. The feed to the SFCU would have to be 6mm, the same as the cooker circuit. If you took a 2.5mm feed from the cooker outlet plate to a SFCU, even if it was only a few inches, then that length of cable between the cooker outlet plate and the SFCU would not be adequately protected.

2.5mm is fine again because it cant be overloaded and also i usually do this and use the SFCU for ex hood

My solution to the OP's problem seems the simplest:

Use two seperate A3 radial circuits, each supplying half of the 20A DP switches within the grid, then use 2.5mm cable straight from each individual grid switch to each individual socket outlet designed to supply an appliance. There is also no need for SFCU's whatsoever.

I do agree this way is ok but i dont like having two mcb's covering one grid switch.

I myself have done this a couple of times but only using one A3 radial to supply the grid switches as there was only 4/5 appliances in each scenario.

Just to say tho I only install grid switches if the client/builder says i have to, I dont like them really prefer SFCU above app and flex outlet behide so if a fuse blows then no need to pull out app

 

[Engineer54]

There was an article in NICEIC mag about putting grid switches on ring and how it needs to be balanced at the middle of the ring, i will see if i can dig it out! Also the grid switch is one point so i wouldn't want to run a ring to one point?

Ever heard of a ''Staggered Final Ring Circuit'' ?? To my mind All FRC's should be constructed in this way!!

 

[D Skelton]

@ Jimmyb

I guess you didn't read my last post before you posted your reply?

however:

You are allowed as per bgb show on diagram.

I'm guessing by reading my last post you now understand that I'm not disputing this, merely pointing out that the 2.5mm cable can only be used to supply one socket outlet only from a 32A radial incorporating 4mm cable.

And I stand by what I say. In my opinion, there is no such thing as a spur on a radial circuit, regardless of what Part 2 of the BGB tells you. A radial circuit is just that, a vector, that can branch off as many times as you like, and each 'branch' is still just part of the radial circuit.

Why would you need to as the cable cant be overloaded so from grid switch to SFCU

I disagree, under fault conditions, it could. Although permitted in the regs, still not a good idea IMHO.

2.5mm is fine again because it cant be overloaded and also i usually do this and use the SFCU for ex hood

Dangerous IMHO, and as far as I can tell, nothing in the regs permits this.

I do agree this way is ok but i dont like having two mcb's covering one grid switch.

Nothing wrong with this at all. You have two MCB's feeding landing and hallway switches for two way lighting in most domestic situations. Although technically, in both these instances they should be labelled accordingly.

Just to say tho I only install grid switches if the client/builder says i have to, I dont like them really prefer SFCU above app and flex outlet behide so if a fuse blows then no need to pull out app

I agree.

 

[Snapester]

Ever heard of a ''Staggered Final Ring Circuit'' ?? To my mind All FRC's should be constructed in this way!!

Like a breath of fresh air......... You should go work for NICEIC, they must be wasting paper writing these articles when you have the answer right here...

 

[Engineer54]

Well my friend, at least i know how NOT to design a simple circuit!!

 

[Snapester]

So my radial circuit is an absolute mistake I forgot that we only should install a STAGGERED RFC ever, damn I'll go supply everything like this now!!!