Downsizing a cable mid-circuit

[spark 68]

How will the adiabatic help?

Sorry Chris, mis-typed on my part, I was thinking of Darkwoods earlier post whilst typing. I will edit my earlier post to maintain clarity

 

[Chr!s]

Sorry Chris, mis-typed on my part, I was thinking of Darkwoods earlier post whilst typing. I will edit my earlier post to maintain clarity

No Probs mate, easily done at this time of night, especially after a few:54:. I got muddled up on the reg numbers, didn't realise till Darkwood posted, had to get the BGB out, time for more:54:then bed.

 

[Darkwood]

I can see the regs you are looking at, and agree the regs should not be taken in isolation.

Reg 433, says shall not be applied to installations situated in locations presenting a fire risk etc...

Iam sorry to have to disagree with both of you here, as a kitchen or other domestic location provides a very real risk of fire in my opinion.
You may be able to use that reg in a different environment and wiring system/containment system, other than domestic.

Iam not saying either of you are wrong per se, but I would not be happy signing that off, as I could not guarantee the conditions required to satisfy parts of 433.

I would much prefer to cover my own backside in this instance.

A very interesting debate none the less.

You seem to be giving the 433.3 more scope than its intention, this is clearly not a location where risk of explosion exists (natural flammable gases expected to build up) nor is it a situation where it presents a fire risk.... you could argue what is meant by fire risk as is very ambiguous in its printing but having applied these regs many times i conclude the fire risk is related to areas where flammable chemical storage etc where the risk of fire is higher than normal, these areas tend not to exist in the domestic domain. The reg states its not to be applied to installations situated in locations PRESENTING a risk of fire or explosion,,,, the key word been 'presenting' which wouldnt normally be associated domestically unless your storing large quantities of chemicals or got a distillery set up in your cellar.
Sorry to repeat chris's post but felt a more indepth answer might either fuel a healthy debate or let you see it from a different perspective... i do conclude i myself get it wrong too (Rarely lol ) so im always open for feedback too.

 

[spark 68]

Hi Darkwood,

Yes typical regs, double speak and cryptic lol.

The reg states its not to be applied to installations situated in locations PRESENTING a risk of fire or explosion

I deliberately left the word explosion out of my answer, as we all agree that does not apply to domestic in the normal sense of things.

As to risk of fire, well that is open to interpretation, I agree the wording of the reg. is not very clear, but I could debate the point that the wiring and containment (or lack of containment) in a domestic situation is not specifically protected against this either.

 

[Darkwood]

Hi Darkwood,

Yes typical regs, double speak and cryptic lol.

I realise why you left explosion out but i think its key to the meaning too as it gives a kind of category to the term fire risk taking it from low level risk to a higher level as the words are presented together within the sentence, it would be a strange set up to relate and follow a low level fire risk of a kitchen to a explosive risk of a car paint booth.



I deliberately left the word explosion out of my answer, as we all agree that does not apply to domestic in the normal sense of things.

As to risk of fire, well that is open to interpretation, I agree the wording of the reg. is not very clear, but I could debate the point that the wiring and containment (or lack of containment) in a domestic situation is not specifically protected against this either.

I realise why you left explosion out but i think its key to the meaning too as it gives a kind of category to the term fire risk taking it from low level risk to a higher level as the words are presented together within the sentence, it would be a strange set up to relate and follow a low level fire risk of a kitchen to a explosive risk of a car paint booth.
On the scope of domestic only i agree the kitchen would be presented as a high fire risk but this regulation isnt domestic biased, it is industry wide so with respect domestic has a low fire risk rating when put against the examples ive given, someone with a background of domestic only may read this reg differently as he may relate it solely to domestic and make his conclusion as to its meaning but it covers all circumstances including industrial chemical storage etc and read on this scope it would then put general domestic houses in the low fire risk cat'

 

[spark 68]

Hi darkwood,

I understand what Chris and yourself are referring to re: reg 433 wrt to fire risk categories (agreed btw), but I cannot agree on any of the other sections of 433.3.1 in this situation (we know what kitchen fitters are like).

I can find at least two regs which states the 3m rule should apply generally in most cases, and a couple of regs which are debatable in this instance, and depend on certain other conditions been met, which are outside the scope of domestic, on why it need not be.
The other regs which say it need not be provided are for specialised applications/situations.

I said earlier if the OCPD at the origin was suitable for the reduced cable csa, then this need not be a problem, other than the 2m isolation point.

 

[Chr!s]

Hi darkwood,

I understand what Chris and yourself are referring to re: reg 433 wrt to fire risk categories (agreed btw), but I cannot agree on any of the other sections of 433.3.1 in this situation (we know what kitchen fitters are like).

I can find at least two regs which states the 3m rule should apply generally in most cases, and a couple of regs which are debatable in this instance, and depend on certain other conditions been met, which are outside the scope of domestic, on why it need not be.
The other regs which say it need not be provided are for specialised applications/situations.

I said earlier if the OCPD at the origin was suitable for the reduced cable csa, then this need not be a problem, other than the 2m isolation point.

Spark how do we then consider a light pendent within a kitchen? How would you code it, a departure?

Regards Chris

 

[Chr!s]

We use 433.3.1 Part (ii) for a light fitting say for example

I see no reason why we couldn't utilize 433.3.1 parts (i) and (ii) in this instance.

6mm t/e RM C 47 amps feeding 1 hob and 1 oven.

If we are happy that a 10mm is sufficient after diversity then surly 6mm feeding one hob and 1 6mm feeding the oven will be fine.

And 433.3.1 allows us

REgards Chris

 

[spark 68]

Hi Chris,

Are you referring to the CCC of the pendant in general ? or are you being specific to suitability for the environment and being appropriate for the location ?

If you are referring to the CCC part of the pendant only, then this would acceptable, as luminaires of this type are covered by 559.6.1.1 (iii) to BS EN 60598

I suspect you are also referring to 433.3.1 (ii) characteristics of the load.

If you are referring to 433.3.1 (ii), then I fall back on the premise that I cannot guarantee that the load will limit the fault current over run of the cable, especially when disobeying two other regs that specifically mention the length of the cable shall not exceed 3m etc..

 

[spark 68]

Chris,

I do see what you are saying re: 433.3.1 (i), if the OCPD on the supply side is suitable (I have already said this), but it is possible, probable even, especially in domestic where someone won't apply diversity and 'upgrade' the MCB to fit a larger wattage appliance.

There are other factors at play here too, darkwood mentioned one, where 10mm supply cable may have been fitted to satisfy VD calcs for a long run, or because the 10mm supply cable is running through thermal insulation.

To categorically say something is right or wrong is a pretty large sweeping statement, in fact I did not comment on the 'other thread' I just gave the OP of this thread the general reason why it was considered non-compliant, and pointed to the regs of this aspect.

As with all things to do with BS7671, there are few black and white answers, and plenty of grey areas, and no 'one size fit's all reg.', I always find one reg says you can do 'this' and another two that says you can't.

It says it all, when Sparks can have a debate on something with differing opinions and outcomes, you just have to look at the ongoing debates about exported PME.
It is good to talk:8:

 

[jamie-spark]

It is acceptable to reduce the cable size as long as it is suitable for the ocpd given all the relevent calcs

 

[i=p/u]

id say stick to good practice and stick to same size cable...

what they were discussing is a rule not normally associated with domestic, as it doesnt have to be done that way.. where if industrial you could tap of busbar chamber in 2.5mm as long as you ocp is within 3m.