Derating only part of circuit

[malcolmsanford]

Spin it as changed mate. In the amendment they have reworded 434.2.1 and included this part

"shall be no branch circuits or socket outlets and that part of the conductor shall"

The OP is talking about a radial circuit where there will be a reduction of CCC and then utilizing a socket and the way I now read 434.2.1 you can no longer do that.

 

[Deleted member 26818]

As I understand it, the OP is just connecting an FCU for the boiler, and that there are no branch circuits or outlets (socket or otherwise) between where the point of reduction in CCC occurs and the position the OP intends placing the FCU.
Yes there has been a change in 434.2.2, but not in 433.2.1.
The change is to include wording similar to that used in 433.2.2 regarding branch circuits and socket-outlets.
In any event, that is not really applicable in the OP's case, as fault protection is provided by means of an RCD at the origin of the circuit.

 

[Guitarist]

I have to agree with spin here. As long as you don't branch off the reduced cable, then the 3m rule still applies.

 

[malcolmsanford]

I have to agree with spin here. As long as you don't branch off the reduced cable, then the 3m rule still applies.

Or fit a socket G.

Not sure what Spin is reading but the change in the regs is to 434.2.1 and not 434.2.2.

I know that 6 sparks can read the regs and all interpreted them differently, but the addition to reg 434.2.1 is quite clear that as Spin says it was introduced to align the reg with the overload one 433.2.2.

RFC have always been a bone of contention concerning reg 433.2.2 and 434.2.1 and the 3 metre rule. By these regulations you should not fit an unfused spur off a ring over the 3 metre distance, but how often is that exceeded.

RFC/ and spurs off of ring finals are invariably feed to BS 1363 accessories and so the 13amp fuse on the load side and will limit the the overload on the cable, and the fault protection is provided by your 32amp MCB/RCBO etc at the CU. This is why the 3 metre distance is not observed generally on a unfused spur on a RFC

To take this on, this is why I do not like the practice of electricians asking if they can spur of a RFC socket to an outside garage, and then fit a small garage CU, with a 6amp and 16amp MCB. The unit is not a BS 1363 and therefore can not limit the overload, as the protection devices in the garage can be changed, whereas you can not get a fuse above 13amp to fit into a BS 1363 accessory.

 

[Deleted member 26818]

Yes, it is to 434.2.1, not 434.2.2, sorry.

 

[kingeri]

Malcolm, are you saying that I should put the fuse before the derated cable then? The derated section of cable is less than 2m and at present the fuse sits immediately after this section of cable. I'm pretty sure there are many installations out there like this!

 

[malcolmsanford]

What i'm saying is your the designer and your the one that must make the decision.

By the letter of the regulations what you want to do is not according to the regs regarding overload read 433.2.2

"The device protecting the conductor against overload may be installed along a run of that conductor if the part of the run between the point where a change occurs ( in cross sectional area, method of installation, type of cable or conductor, or in the environmental conditions) and the position of the protective device as neither branch circuits nor outlets for the connection of current- using equipment and fulfills at least one of the following conditions"

Your protection device is at the CU and your change of condition is in the kitchen, between those 2 points have you branches or current using equipment ? .........................I think you have.

An example in domestic could be you run a 10mm for a cooker point with a 45amp MCB at the CU

Someone buys a unit that is rated 32amps. Instead of trying to connect 2 X 10mms in an accessory you can derate the cable to 6mm at the CCU and still have your 45amp breaker protecting it.

Remember these are the regs and as I said your the designer, is that cable going to be under stress where you derate it, not a chance and I know what I would do, but your not me.

 

[Deleted member 26818]

Malcolm, this getting a bit tedious.
Please read the OP and subsequent posts.
The protective device is not at the CU, if it was, then the OP would not have asked the question.
The OP intends placing a protective device along the run of a conductor within 3m of a point where a reduction in the CCC of that conductor occurs.
As per Regulation 433.2.2 which you have quoted.
What is the problem?

 

[malcolmsanford]

Spin no one is making you answer or making you read, so if it is tedious for you then I'm so sorry ...................

Perhaps I have gone off topic slightly but your trying to tell me that regulations have been altered that haven't and ones that have have not. Seeing as i'm wasting your time spin I think this will be the close for me.

 

[kingeri]

Hey guys......thanks for your input....I didn't want to cause an arguement. I think you're both right really, and Malcolm you're certainly right that I'm the designer and it's my decision. I just wanted a second and third opinion, and it seems I certainly got them! Thanks to all who contributed....I'm gonna go and intall some wiring and get my Megger out! Then the part I'm really dreading.....the joinery!

 

[Deleted member 26818]

No Malcolm you are correct, I don't have to answer or read.
However, I would consider not responding to you as being rude.
You obviously addressed your first post on this thread to me : "Afraid to say that the regs have scuppered that now spin, as the point of reduction in CCC can not be applied for branch circuits or socket outlets as in reg 434.2.1. This was newly introduced in the amendment."
I'm not trying to tell you that Regulations have been altered that haven't and ones that have been altered haven't.
You told me that there had been an alteration
I'm trying to tell you that the alterations make no difference.
The fact that the Regulations now require that there is no branch circuits or socket-outlets between a point of reduction in CCC and the position of an earth fault protective device is not really relevant, as all it does is bring it into line with the requirements for positioning the overload protective device, which the OP has already stated will be complied with.
The device the OP intends using provides both earth fault and overload protection, but apart from that, earth fault protection is already provided by an RCD at the origin of the circuit. As such 434.2.1 is not applicable.
What difference does the change in 434.2.1 make in the OP's situation? None whatsoever.
There are instances where overload protection can be omitted alltogether, as long as certain criteria are met see 433.3.1.
The OP's situation appears to be one of those instances.

 

[kingeri]

"The device protecting the conductor against overload may be installed along a run of that conductor if the part of the run between the point where a change occurs ( in cross sectional area, method of installation, type of cable or conductor, or in the environmental conditions) and the position of the protective device as neither branch circuits nor outlets for the connection of current- using equipment and fulfills at least one of the following conditions"

I am taking the protective device as being the FCU and not the MCB.....there are no branches or sockets after it, so I don't see the relevance. Or am I totally wrong?

 

[Deleted member 26818]

No you're not totally wrong.