Derating only part of circuit

[kingeri]

Hi all, I have a question about derating a radial circuit. Is it acceptable to only derate a small section (a branch or at the end of the circuit) using an FCU? For example, if only a short length of cable goes through insulation, is it necessary to derate the whole circuit from the CU, or just that part of the circuit?

 

[Guitarist]

You need to work on "the weakest link" scenario. If your cable is derated by going through insulation then your circuit protective device must be suitable. If you have fitted an FCU to compensate for this then you are protecting the cable.

 

[Deleted member 26818]

The Regulations require that where the Current Carrying Capacity of a conductor reduces, either because of installation method, or a reduction in CSA, then both overload and fault protection must be provided within 3m of where the reduction occurs.
This can be achieved by derating the entire conductor, or by derating just the part which has reduced current capacity.
In some instances, overload protection can be omitted, if there is fault protection, and the likelyhood of an overload occuring is negligable.
An RCD protecting the whole length of the conductor would be deemed as providing fault protection.

 

[kingeri]

Many thanks. My kitchen sockets are in the form of a 4mm radial on a 32a MCB. The circuit is RCD protected. The last leg of the circuit supplies the combi boiler. At present, the cable goes on the outside of the boxed-in pipes under the boiler, clipped direct to the wall. This was only ever meant to be a temporary measure. Two years later, I am planning on re-boxing the pipes in a better looking box, and obviously it's a good time to relocate the cable. I want the 4mm T&E to be inside the box. The temperature inside the box will get quite high....obviously the central heating flow and return and hot water pipe will be enclosed in a small area. The cable will be around 10cm from the pipes inside the box (including the gas supply). This section of the circuit will be carrying only the supply for the boiler (0.6a). Is derating even necessary? The switched FCU with 3a fuse is at present located above the boxed pipes, just below the boiler. Should the fuse be located before the cable enters the box? If so, it's not practical to have the switch at that location, so would it be ok to use a seperate FCU (un-switched) before the box and just an un-fused DP isolator switch at the top of the box (to avoid having two fuses)? Am I worrying about it too much?
Also, would you put the cable in trunking inside the boxed area?

 

[Deleted member 26818]

The overcurrent device can be placed along the run of the conductor, within 3 meters of where the reduction in CCC occurs.
So if from where the cable enters the boxing in, to where you wish to place the FCU, the cable is less than 3m, then everything is fine and dandy.
This 3m rule is what allows us to spur off of a RFC in 2.5mm², and why often DNOs will only allow 3m between the head and the CU unless a switch fuse is installed.

 

[kingeri]

That makes sense. Thanks for the very concise reply. I think I was overthinking it and getting nowhere, just for a change.

 

[malcolmsanford]

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.

 

[telectrix]

if a new boxing is being fitted, would it not make sense to fit some YT2 neatly up the outside, then no need to derate.

 

[Deleted member 26818]

I haven' got my copy of BS7671 available at present, so I can't look up the particular Regulation you are referring to Malcolm.
However, I can't imagine a scenario, where they would have a Regulation that prohibits the use of unused spurs, yet retain Appendix 15?

 

[kingeri]

if a new boxing is being fitted, would it not make sense to fit some YT2 neatly up the outside, then no need to derate.

The wife insists that is not an option.

 

[kingeri]

I haven't got a copy of the big green book, but I do have the green on site guide. I am struggling to get to the bottom of this one.

Would my idea of having the fuse before the point of derating and the switch local to the boiler be viable? I know the fuse protects the boiler flex as well, so does the fuse need to be at the point where the T&E joins the flex? If the fuse is before the point of derating, then everything after it, including the flex, will be capped at 3a, so I can't see an issue.

Then again, in the case of an unfused spur from a RFC, the load itself limits the maximum current that the cable will be subjected to...what's the difference in my case if the boiler is fused at 3a then the 2m of cable passing through the boxed area just before it can't be subjected to more than 3a....except under fault conditions I suppose.

 

[HandySparks]

Would my idea of having the fuse before the point of derating and the switch local to the boiler be viable?

Yes.

I know the fuse protects the boiler flex as well, so does the fuse need to be at the point where the T&E joins the flex?

No. Providing that the fuse protecting the flex is upstream, it doesn't matter where.

If the fuse is before the point of derating, then everything after it, including the flex, will be capped at 3a, so I can't see an issue.

Indeed.

Then again, in the case of an unfused spur from a RFC, the load itself limits the maximum current that the cable will be subjected to...what's the difference in my case if the boiler is fused at 3a then the 2m of cable passing through the boxed area just before it can't be subjected to more than 3a....except under fault conditions I suppose.

4mm² T&E supplying your boiler, fused at the load end at 3A will be just fine, even if the OPD at the origin is 32A.

The actual current rating of 4mm² T&E only starts to go below 32A once the ambient goes above 40C, if clipped direct and no other factors. So with a little ventilation at top and bottom of the boxing in to give some convection cooling, I'd be happy to use it on a normal 32A circuit, provided that the cable wasn't actually touching the hot pipework.

It might make sense, for several reasons, to fit some insulation over the heating pipework before boxing in.

 

[kingeri]

Thanks HandySparks. I will insulate the pipework as advised. I just wanted to get it right first time, I don't fancy having to undo my joinery in the future!

Would you put the cable in trunking within the boxing, or not?

 

[HandySparks]

Would you put the cable in trunking within the boxing, or not?

Only if hammering in clips was proving too difficult (wall too hard or too soft).

Just make sure that the cable can't move and rest against the hot pipes.

 

[Deleted member 26818]

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.

Malcolm, 433.2.1 has not changed with the amendment.
433.2.2 still allows a device for overload protection to be positioned along the run of the conductor, within 3m.
The part about branch circuits or outlets refers to there being none between the point of reduction and the position of the protective device.
This is why we are only allowed one socket-outlet on an unfused spur.

 

[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.