1.5mm clipped to ceiling supplied on 20A breaker

[DaveS]

Morning fellas

On job at moment. Am just looking at lighting circuits. They are supplied off 20A type C breakers run in 2.5mm to MDB's then from there each lighting circuit goes down to 1.5mm flex 5 core with 2 cores being used for dali. From what I see cable rating is 20A is a 20A breaker to high for this circuit? Thanks

 

[Pete999]

Morning fellas

On job at moment. Am just looking at lighting circuits. They are supplied off 20A type C breakers run in 2.5mm to MDB's then from there each lighting circuit goes down to 1.5mm flex 5 core with 2 cores being used for dali. From what I see cable rating is 20A is a 20A breaker to high for this circuit? Thanks

Hi Dave, with respect what are MDBs and dali?

 

[telectrix]

here he is:

View attachment 35385

 

[kingeri]

Need a bit more info really TBH.

 

[HandySparks]

Hi Dave, with respect what are MDBs and dali?

Discover DALI - http://www.dali-ag.org/discover-dali.html

 

[Andy78]

Table 4F3A suggests a CCC of 16A for 1.5mm flex

 

[Wilko]

If I've understood correctly its right on the limit. Max is 20A for 1.5mm clipped direct (4D5) and on a ceiling we might expect higher than usual temp so this might need to be derated. I would pick 16A to be sure, but that's just me.
Edit : oops, it's flex ... sorry.

 

[DaveS]

Thanks for replies guys MDB is a Modular Distribution Box. A glorified Junction box as far as I understand them. They don't contain any built in Fuses or circuit breakers. This is supplied from board circuits in 2.5mm and then the modular wiring plugs out of this in their corresponding circuit number ports and this is in 1.5 5 core flex. Dali is the low voltage lighting controls cable which are which is why a 5 core cable is being used. I personally would be inclined to swap over for 16A breakers but just want to be sure am not missing something as if I do recommend downrating would mean 4 boards being swapped over with new ones

 

[Andy78]

Each plug from the MDB, what does that supply, single lights or entire circuits ?
If you do downgrade each circuit you won't necesarilly have to change the DBs. A 13A fuse close to the MDB may be an option if the circuit loading agrees.

 

[Lucien Nunes]

Are the flexes supplying fixed loads? Is there any way any flex could be subjected to a sustained load between 16A and 20A? Does the C20 provide adequate short-circuit protection at the prevailing Zs?

 

[DaveS]

Are the flexes supplying fixed loads? Is there any way any flex could be subjected to a sustained load between 16A and 20A? Does the C20 provide adequate short-circuit protection at the prevailing Zs?

Thanks lucien the circuits are quite large not sure wattage of fittings though. If you asking is Zs reading adequate for MCB rating that is correct.

 

[DaveS]

Just to let all know I was speaking to engineer yesterday and mentioned concerns he said he had spotted this himself and that 16A Mcb's are probably going to be fitted

 

[hightower]

But if all the 1.5mm flex does is feed individual lights (if I'm reading this right), what is the issue? Surely it's no different a scenario to the 0.75mm flex in a standard pendant, provided the short circuit protection is adequate?

 

[Wilko]

But if all the 1.5mm flex does is feed individual lights (if I'm reading this right), what is the issue? Surely it's no different a scenario to the 0.75mm flex in a standard pendant, provided the short circuit protection is adequate?

Good point HT !
The difference here is perhaps the 20A C type, rather than 6A B type mcb? Looking in OSG Table B7 the C type has min 1.5mm cpc recommended. The complication (for me at least) is that it's already flex and so may not apply?
Edit : likely flex is 3 x 1.5mm so ok anyway

 

[hightower]

Good point HT !
The difference here is perhaps the 20A C type, rather than 6A B type mcb? Looking in OSG Table B7 the C type has min 1.5mm cpc recommended. The complication (for me at least) is that it's already flex and so may not apply?
Edit : likely flex is 3 x 1.5mm so ok anyway

Haven't got regs available, but there's a regulation that permits the omission of overload protection and Alan Lynch calls it "the pendant flex regulation" or some such because that's about the only time you'd use it.