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.

 

[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...