Bunched cables, derating.

[JohnW2]

What do you think?

N = M?

Sure, in Bernard's case we would have N=M. Per that Note, that means that the 'other cables' (of which there are none, N-M=0) can be de-rated on the basis of their being N-M (i.e. zero) of them. However, as I asked, the note appears to be silent about the derating of cables which are not 'others' (i.e. all M of them in Bernard's case) - so, if you take the wording of the note at face value, that silence would seem to imply that no 'relaxation of derating requirements' can be applied to them. However, that's so daft that it surely can't be what was intended?

Are we perhaps all missing some other note or text which is saying that if none of the cables are carrying more than 30% of what would be their derated CCC, then no derating is required?

Kind Regards, John

 

[bernardgreen]

I forgot to ask - with the current single FCU proposal, why on earth is there to be a 5A FCU after a 6A MCB?

Probably the electrician cannot ( will not ) terminate the twin and earth cable from the CU into a non BS7671 appliance ( the switch panel )

The installation method is the bunch will be drawn though several metres of duct that will take ten 1mm² at a squeeze.

 

[JohnW2]

I forgot to ask - with the current single FCU proposal, why on earth is there to be a 5A FCU after a 6A MCB?

Probably the electrician cannot ( will not ) terminate the twin and earth cable from the CU into a non BS7671 appliance ( the switch panel )

I'm not sure what you mean by a "non BS7671 appliance" - do you mean an appliance which, for some reason is not BS7671-compliant? If so, then I would have thought there would be a problem in including it in fixed wiring, with or without an intervening FCU!

Kind Regards, John

 

[ban-all-sheds]

Are we perhaps all missing some other note or text which is saying that if none of the cables are carrying more than 30% of what would be their derated CCC, then no derating is required?

That's how I read it. You don't need to get as far as the explanation with M and N.

The first sentence leads you to ignoring all of them, i.e. you don't have a group at all.

 

[ban-all-sheds]

Probably the electrician cannot ( will not ) terminate the twin and earth cable from the CU into a non BS7671 appliance ( the switch panel )

As Jonh sort of said - if he won't do that why would he be happy to take T/E from the FCU?

In what way does the switch panel contravene BS 7671?

The installation method is the bunch will be drawn though several metres of duct that will take ten 1mm² at a squeeze.

Sounds like a flagrant disregard of the guidance in GN3, and a fair chance of contravening 522.8.1 & 522.8.6

 

[JohnW2]

Are we perhaps all missing some other note or text which is saying that if none of the cables are carrying more than 30% of what would be their derated CCC, then no derating is required?

That's how I read it. You don't need to get as far as the explanation with M and N. The first sentence leads you to ignoring all of them, i.e. you don't have a group at all.

Yes, I have to agree - but I they could have found clearer words. One has to do a little thinking to work out that, as you say, that first sentence 'leads you to ignoring them all.

However, that interpretation only works when 'M=N' (i.e. all cables are carrying low current). I think my uncertainty about what it means remains if N is greater than M (i.e. not all cables are carrying low current). In that situation, the words say that one ignores the M 'low current cables' when working out the de-rating of the other N-M cables, but it does not appear to tell one what, if any, derating should be applied to that first M cables, does it?

One point is that this 'relaxation' only applies to a cable carrying <30% of its 'grouped rating'. I take that to mean that one has to first work out what its derated CCC would be of no cables were 'ignored', and that could be quite low for large groups - which might sometimes push loads over the 30% mark (e.g. with N=20, it would only be about 10% of the tabulated CCC).

Kind Regards, True

 

[ban-all-sheds]

Bernards N=10, so his Cg will be about 0.48.

If we assume his It for 1mm² is 11.5, his group rating is 5.52, and 30% of that is 1.66.

Isn't the load on each cable 100W, max?


But it all turns out to be pretty academic - he has a switch panel so (odd? flaky?) that something about it contravenes the Wiring Regulations and he has a proposal to damage the cables by pulling them into conduit which is far to small.

 

[JohnW2]

Probably the electrician cannot ( will not ) terminate the twin and earth cable from the CU into a non BS7671 appliance ( the switch panel )

As Jonh sort of said - if he won't do that why would he be happy to take T/E from the FCU?

Maybe he wouldn't be happy to do that either - perhaps he plans, bizarrely, to run flex from the FCU to the switch panel, whence cabling reverts to being countless T+E cables to go to the lamps In any event, as we've both said, if the switch panel is, for some reason, non-compliant with BS7671, then I think there's probably an over-riding problem!

Kind Regards, John

 

[JohnW2]

Bernards N=10, so his Cg will be about 0.48. If we assume his It for 1mm² is 11.5, his group rating is 5.52, and 30% of that is 1.66. Isn't the load on each cable 100W, max?

Yep. Once we (you!) found that note at the bottom of the page (and I worked outwhat it meant!), all Bernard's derating concerns appear to have gone out of the window.

But it all turns out to be pretty academic - he has a switch panel so (odd? flaky?) that something about it contravenes the Wiring Regulations and he has a proposal to damage the cables by pulling them into conduit which is far to small.

Indeed. However, as for the panel, we're guessing what Bernard meant by "non BS7671 appliance" - maybe he meant something other than non-compliant with BS7671? Maybe, even, he meant "non BS1363 accessory"? Hopefully, he will be able to clarify.

Kind Regards, John

 

[bernardgreen]

Maybe, even, he meant "non BS1363 accessory"? Hopefully, he will be able to clarify.

That is probably what the electrician said, I am getting it second hand from the client.

But it all turns out to be pretty academic - he has a switch panel so (odd? flaky?) that something about it contravenes the Wiring Regulations

Would the wiring reqs apply to the construction of a hard wired range cooker ?

and he has a proposal to damage the cables by pulling them into conduit which is far to small.

The reason it is 8 possibly 10 is to ensure the cable bundle in not too large for the available duct route. It is not a bodging DIYer doing the install. It is a qualified electrician who appears to blindly follow rules.

 

[ban-all-sheds]

That is probably what the electrician said, I am getting it second hand from the client.

What services are you providing to the client?

Would the wiring reqs apply to the construction of a hard wired range cooker ?

No.

Are you suggesting that this electrician won't connect those either?

The reason it is 8 possibly 10 is to ensure the cable bundle in not too large for the available duct route. It is not a bodging DIYer doing the install. It is a qualified electrician who appears to blindly follow rules.

There are no explicit requirements in the regulations about how full conduits or trunking can be.

There is guidance in GN1, and I can assure you that "several metres of duct that will take ten 1mm² at a squeeze" would be a flagrant disregard of them even with 8 cables.

 

[bernardgreen]

Ban,

I do support you in your efforts to protect the ignorant ( un-informed and un-aware ) DIYer from causing harm to themselves and other people.

Sometimes your way of doing that necessary service is a bit rude and sometimes because it appears rude it is ignored or discounted and therefor does not have any effect in reducing the risk to the DIYer.

It really is none of your business what service(s) if any I am providing to the client.

I expect the electrician would connect a range cooker to a cooker connection point as that is "normal" but would not connect the twin and earth from the CU directly to the terminals inside the range cooker as the range cooker is not a BS xxxx recognised wiring accessary.

 

[ban-all-sheds]

It really is none of your business what service(s) if any I am providing to the client.

I wasn't trying to pry, or be nosey - I just wondered what your involvement was in electrical design and construction, that's all.

I expect the electrician would connect a range cooker to a cooker connection point as that is "normal" but would not connect the twin and earth from the CU directly to the terminals inside the range cooker as the range cooker is not a BS xxxx recognised wiring accessary.

Why on earth does it have to be?

What kind of electrician won't install a cooker, or a shower, or a heater, or a light, or a doorbell transformer, or a fan, or..., or..., or... ?

 

[JohnW2]

I expect the electrician would connect a range cooker to a cooker connection point as that is "normal" but would not connect the twin and earth from the CU directly to the terminals inside the range cooker as the range cooker is not a BS xxxx recognised wiring accessary.

That sounds rather like an 'electrician' who has been taught a set of rules and hasn't much of a clue about electrical principles and/or common sense. Do I take it that, by the same logic, he will not connect the T+E of a domestic installation to, say, light fittings?

Kind Regards, John

 

[bernardgreen]

I just wondered what your involvement was in electrical design and construction, that's all.

None in the case in discussion. My "claim to fame" was in 1980 when I installed two RCDs in a new installation ( the house we were self building ) and the immediate reaction from the meter man was that it could not be connected. A brief word with the office and he was sent back to connect us. The logic of having RCD 1 for building site and RCD 2 for the caravan ensuring caravan lights for first aid after a RCD tripping accident was accepted.

What kind of electrician won't install a cooker, or a shower, or a heater, or a light, or a doorbell transformer, or a fan, or..., or..., or... ?

I think the answer is "there is a cooker connection point on sale, there are no shower connection points on sale "