0.75mm Flex into 6A MCB

[uksparks]

Hi,

As per title, how would you feel about connecting a few garage lights onto some 0.75mm flex straight into the 6A MCB, ignore switching, just that its 0.75mm.

I have to say I normally go via a FCU but in this occasion I don't feel I need to as its in the garage with a 6A MCB I can come off.

referring to table 4F3A on page 378 it states 0.75 can take 6A.

id say I'm good to go, I k ow I'm probably stating the obvious but it's not something I usually do, hooking such a small co doctor up to an MCB.

reason for it being small is due to the fact I'm using 3 core and earth flex so I can use the switched output on the lights so they Interconnect on PIR.

 

[UKMeterman]

Would suggest a ferrule at the very least, did the conductor get clamped correctly?

 

[uksparks]

Yes I'll ferrule it, I always do with fine stranded.

its just a job I'm doing tomorrow, not badly thought out but more a case of that's the easiest option!

 

[uksparks]

its hot today isn't it.

 

[davesparks]

Why not use 1mm flex? Then you will be complying with the minimum size of 1mm for cables in lighting circuits.

 

[Richard Burns]

The minimum size conductors that can be used for lighting circuits is 1.0mm² according to table 52.3.

 

[Dave OCD]

1mm would be the better option really, if using flex it should still be easy to interconnect.

 

[Andy78]

As much as I'd like to disagree with this, table 52.3 says flex is OK at 0.75mm. News to me though, and I certainly wouldn't do it.
Where In = It I'd always rather step it up a size.

 

[uksparks]

let me look at that...

 

[HappyHippyDad]

Table 52.3 is very clear Tom, 1mm for lighting circuits.

I cant understand your reason for using 0.75mm, why not 1mm as Dave said?

 

[uksparks]

I must admit I often use 0.75 for flood lights etc, but always through a FCU to 3A with appropriate label on FCU.

Ill just do it in 1.00mm three core and earth grey instead then, its all inside the garage anyway, just thinking about it I can, I was thinking it was going on the outer wall.

 

[uksparks]

Table 52.3 is very clear Tom, 1mm for lighting circuits.

I cant understand your reason for using 0.75mm, why not 1mm as Dave said?

Its just that I have a lot of 0.75mm 4 core flex that I use for "specific applications" such as a couple of outside lights strung together off a FCU from a ring maybe... Im not trying to bend the rules, but its quite adequately protected and only has a very small load on it usually.

 

[uksparks]

The minimum size conductors that can be used for lighting circuits is 1.0mm² according to table 52.3.

Yes you are quite right, the only time I would ever use 0.75 is on a single light if I have no 1mm, I have had this conversation with the NICEIC and they have always said so long as its protected adequately its fine, I appreciate they do not make the rules up, well not all of them, only the ones you cant find in the book!

 

[uksparks]

I think ill buy myself some 1mm 4 core flex for next time.

 

[HappyHippyDad]

Its just that I have a lot of 0.75mm 4 core flex that I use for "specific applications" such as a couple of outside lights strung together off a FCU from a ring maybe... Im not trying to bend the rules, but its quite adequately protected and only has a very small load on it usually.

I thought that was more likely to be the reason :smile5:

 

[davesparks]

Its just that I have a lot of 0.75mm 4 core flex that I use for "specific applications" such as a couple of outside lights strung together off a FCU from a ring maybe... Im not trying to bend the rules, but its quite adequately protected and only has a very small load on it usually.

Is it of the heavy duty type or were you planning to install mechanical protection for it or otherwise prevent mechanical damage etc?

 

[HappyHippyDad]

Yes you are quite right, the only time I would ever use 0.75 is on a single light if I have no 1mm, I have had this conversation with the NICEIC and they have always said so long as its protected adequately its fine, I appreciate they do not make the rules up, well not all of them, only the ones you cant find in the book!

I suppose you could argue that it may not be protected adequately. The 0.75 can take 6A, the 6A MCB will take 30 mins to trip with 9A running through it and wont trip at all with 8A going through it, so it's not great to design a circuit this way. In reality it's probably going to be fine, at least for now, and then someone may start adding to your circuit at a later date. I cant see anything wrong with using it for a single appliance, which will have a fixed load, but not straight from the MCB.

 

[uksparks]

Yes it's good chunky stuff, it's not think cheapo stuff.

this is why I queried it really, I'll just use grey TC&E, I can get 1mm 4 core flex quite cheap, think I'll get myself a drum.

 

[uksparks]

I suppose you could argue that it may not be protected adequately. The 0.75 can take 6A, the 6A MCB will take 30 mins to trip with 9A running through it and wont trip at all with 8A going through it, so it's not great to design a circuit this way. In reality it's probably going to be fine, at least for now, and then someone may start adding to your circuit at a later date. I cant see anything wrong with using it for a single appliance, which will have a fixed load, but not straight from the MCB.

Yes, totally agree, it's a bit sailing close to the wind!

 

[davesparks]

I suppose you could argue that it may not be protected adequately. The 0.75 can take 6A, the 6A MCB will take 30 mins to trip with 9A running through it and wont trip at all with 8A going through it, so it's not great to design a circuit this way. In reality it's probably going to be fine, at least for now, and then someone may start adding to your circuit at a later date. I cant see anything wrong with using it for a single appliance, which will have a fixed load, but not straight from the MCB.

Wtf? The problem is not with the flex's current rating, it's tabulated at 6A and after factors being applied that may well be adequate. The tabulated current ratings have been arrived at after taking in to account of short duration overloads etc, they aren't the maximum steady current the cable could handle safely or anywhere near the current required to harm the cable.

The problem is that the minimum size cable for a lighting circuit is 1mm, and there are specific requirements about the type of flex or other protection to be used if flex is to be used as an installed cable.

 

[7029 dave]

I am un easy with flex, as hard wiring anyway. Just MO

 

[davesparks]

Yes, totally agree, it's a bit sailing close to the wind!

It's not sailing close to the wind, all of that stuff is allowed for in the tabulated ccc, it's the other regulations pertaining to the installation of flex that are the concern

 

[davesparks]

I am un easy with flex, as hard wiring anyway. Just MO

I assume that has a lot to do with the earlier versions of the regs which strictly prohibited it?

 

[Darkwood]

In reality it could be classed as supplying a fixed load if you only have a certain number of fittings with fixed wattage lamps.. in this scenario your cable doesn't actually require O/L protection and only short circuit protection.

In the domestic realm though this is not practiced as it would be subject to abuse and mis-understanding hence you are not taught this, but in design especially in the Industrial sector, utilising this exemption for O/L protect can save large amounts of money on material costs, the negative side been having O/L higher than you cable rating has the effect of reducing the permitted ELI - I've wired plenty a motor/machine supply where a cable CCC is lointo wer than the OCPD.

I personally wouldn't introduce this into the domestic realm as it just confuses the hell out of other Sparks that follow you.

 

[hotfingers]

I wonder why flex on a crp is only 0.75 if the minimum is 1mm on a lighting circuit,bit silly init..

 

[davesparks]

I wonder why flex on a crp is only 0.75 if the minimum is 1mm on a lighting circuit,bit silly init..

No,
1- there is a regulation which deals with this specifically
2- it's not part of the circuit, it's part of the light fitting

 

[uksparks]

I am un easy with flex, as hard wiring anyway. Just MO

The only time I would use it is if it's going on the outside of a building to a light where twin and earth is just wrong so I would use a pond flex.

 

[uksparks]

The problem is that the minimum size cable for a lighting circuit is 1mm, and there are specific requirements about the type of flex or other protection to be used if flex is to be used as an installed cable.

could you just expand on what you mean by "other protection to be used if flex is to be used as an installed cable", surely if it's got a PVC outer like twin and earth has, and has insulated conductors inside, just like twin and earth, why would it require "other protection" when it's the same thing but a different shape?

im not looking to pick your comment apart, but more looking for clarity as I can't quite understand where you are going...

 

[davesparks]

could you just expand on what you mean by "other protection to be used if flex is to be used as an installed cable", surely if it's got a PVC outer like twin and earth has, and has insulated conductors inside, just like twin and earth, why would it require "other protection" when it's the same thing but a different shape?

im not looking to pick your comment apart, but more looking for clarity as I can't quite understand where you are going...

I don't know exactly what, it's as ambiguous as the rest of 7671. go and look the regulation up and make your own decision, it's a couple of pages before the minimum sizes table. It has effectively replaced the regulation which used to set the maximum length of any installed flex to 2 metres.

 

[uksparks]

I'll just go and get the book...

 

[uksparks]

It's under 521.9

 

[uksparks]

It points you to BSEN 50565-1, for grades of cable, then to 422.3.201 and 521.9.1

 

[Andy78]

So with reference to my post #8, Am I wrong in my interpretation of table 52.3 ?

I understand that 521.9 applies which is what Davesparks is referring to, and I would never personally consider flex for fixed wiring, or less than 1mm live conductors for any fixed circuit, but the table does seem to support the OP's ideas.

 

[uksparks]

Well it's not "heavy duty" as its not good for 450v i wouldn't think...

 

[uksparks]

It's a confusing grey area I say, I suppose, the bottom line to this is if the cable is suitably protected or not, I will reevaluate my usage of 0.75mm cable and use 1mm from now on.

as said, this is only to feed an outside light, I am not a silly billy wiring an entire light circuit up with it.

 

[hotfingers]

I'll just go and get the book...

521.9.1

 

[uksparks]

It's a strange one actually, Reg 422.3.201 says suitably protected against mechanical damage, but it's strange how it doesn't say a word about suitable protection if it's twin and earth which is just a different shape...

why should flex need any more protection than any other cable irrespective of the CSA of the conductors.

 

[Andy78]

It's a strange one actually, Reg 422.3.201 says suitably protected against mechanical damage, but it's strange how it doesn't say a word about suitable protection if it's twin and earth which is just a different shape...

And different conductor construction, and insulation construction, and sheath construction.......

 

[davesparks]

It's a strange one actually, Reg 422.3.201 says suitably protected against mechanical damage, but it's strange how it doesn't say a word about suitable protection if it's twin and earth which is just a different shape...

It's a fairly recent change in the regs which allowed flex to be installed at all, it was previously limited to 2 metres max length for appliance tails.
Every cable has to be suitably selected with regards to protection suitable for the environment it is in, regardless of what type it is.

 

[uksparks]

Andy, you have a point.

Dave, well I shod think myself lucky that I live in such a modern world with updated regs!

 

[davesparks]

Have you considered the max conductor operating temperature of the flex? A lot of it is rated to only 60degrees when fixed wiring and accessories are all rated for 70degrees. Will this make any difference? I don't know, but you need to consider it

 

[uksparks]

This is quite an interesting thread which I don't recall being touched upon before!

 

[davesparks]

This is quite an interesting thread which I don't recall being touched upon before!

It certainly beats a thread about stiff cutters!

 

[Andy78]

Any takers about my post #8 and the regs stating a minimum csa of 0.75mm for the use of flex ?

 

[davesparks]

Here's a bit of relevant history
14th edition

13th edition

 

[davesparks]

Any takers about my post #8 and the regs stating a minimum csa of 0.75mm for the use of flex ?

I think it's open to interpretation.
I read it as the minimum size for a lighting circuit is 1.0mm and a power circuit is 1.5mm then flexes which are not part of a such general circuits (ie appliance tails etc) are restricted to 0.75mm minimum

 

[uksparks]

I would say you are quite possibly right

 

[NDG Elecs]

Is there any reason this can't be used: https://www.tlc-direct.co.uk/Images/Products/size_3/MK5903.JPG

Though I have never seen one, never mind fitted one.

 

[uksparks]

I didn't know one existed, it's s shame you can't get 3A breakers across the different brands, o don't know if I'd have much need for one to be honest, but then there's always that once!

 

[davesparks]

If I remember correctly the smallest mcb I've fitted was a 0.5A but may have been 1A, it was just doing a contactor coil