CCC for suspended ceiling

[officeguy]

Hey Everyone,

Regarding CCC table for cable rating, would placing a cable over a suspended ceiling, covered in insulation be the same rating as placing a cable over a plasterboard ceiling covered in insulation?

I understand that the materials are different (suspended ceiling being mineral fibre), but I don't know how this relates to the rating.

I know it's bad practice to place wires over a suspended ceiling, however I'm mainly concerned about cable rating here, rather than neatness/ease of access.

Thanks

 

[RF Lighting]

No as the cable will not be in constant contact with the suspended ceiling. It'll sit on the tee bars and maybe the middle of the tiles. Also the tiles will have a different heat conduction pattern to plasterboard.

Personally I'd class them as fully surrounded in thermal insulation, and they would need derating by 50%


Can you not attach the cables up to the top of the void above all the insulation etc?

 

[ericmark]

With a suspended ceiling normally you have tray work for the cables. Often with plug in ceiling roses for lighting attached to it so single lights can be unplugged for maintenance so whole office does not have lights off.

The steel tray will help dissipate heat.

The bunching of cables is duel edged I have seen many a harness where a cable has only burnt out where it enters and leaves the harness as the other cables have keep it cool, but also the reverse can be true where other cables warm it up.

In the main I would not want to run near the edge but one has to consider with for example a 100A consumer unit with a bunch of twenty cables coming out the whole current spread over 20 cables can't exceed 100A so there will both cool and heat each other so although bunched it makes little difference to current carrying capacity. However two cables one to oven and one to hob bunched then clearly both could be on max together so allowance must be made.

To my mind if I have to work out the CCC of a cable to such close limits I ask myself if really worth it or would next size up be better?

 

[JohnW2]

Regarding CCC table for cable rating, would placing a cable over a suspended ceiling, covered in insulation be the same rating as placing a cable over a plasterboard ceiling covered in insulation? ... I understand that the materials are different (suspended ceiling being mineral fibre), but I don't know how this relates to the rating.

I agree largely with what everyone else has said. However, what sort of circuits are we talking about? If lighting circuits, there probably isn't really an issue, even with maximum (50%) de-rating, even 1mm² cable will have a CCC of 8A, hence fine for a 6A lighting circuit.

At the other extreme, if one were talking of sockets circuits (or 'worse'), a 50% derating could be a very major problem - you would need 10mm² cable for a 32A radial or 6mm² for a 32A ring (since regs require a minimum CCC of 20A for ring finals) - which is pretty ridiculous (and totally impractical) In those situations, it would clearly be desirable to find an alternative route of the cable (e.g. as suggested by RF) - or, perhaps ('not nice' thought it may be!), increase the cable size for just the bits which ran over the ceiling.

Kind Regards, John

 

[bernardgreen]

The framework of a suspended ceiling can have sharp edges and if yours has sharp edges then the cables need to be kept clear of those sharp edges.

Vibration can occur in suspended ceiling when doors opening into the room cause air movement. It is only a small amount of movement but over time this will help sharp edges to cut into cables pressed onto the frame by the weight of insulation on the cable.

 

[officeguy]

Hey Everyone,

It's the 1.5mm cable protected by a 10A MCB as in my other thread (however it's locked now so can't update it).

A 50% de-rating of 1.5mm is 10A, so I'm probably still ok?

Thanks

 

[JohnW2]

It's the 1.5mm cable protected by a 10A MCB as in my other thread (however it's locked now so can't update it).

Ah, I see - I hadn't twigged! If you wish to continue that other discussion, hopefully without distractions, you could always start another thread.

A 50% de-rating of 1.5mm is 10A, so I'm probably still ok?

Yes - "just" (although, as EFLI would probably point out, "just OK" means "OK"!).

Kind Regards, John

 

[ban-all-sheds]

I really cannot urge too strongly that you need to replace your electrician.

He is supposedly a qualified one, member of NICEIC etc, and yet you have been pursuing these details of circuit design to a remarkable extent. Not not because you simply have an interest, it's because, as you said in the other topic, that you "want my own understanding to be on the safe side".

Why do you need your own understanding in order to be safe? Why do you worry that you would, or might, be unsafe if you couldn't check what your electrician was doing?

Don't want anyone pulling the wool over my eyes.

So you don't trust your electrician's competence, or his integrity. IHNI if that level of suspicion is justified or not, but even if it is completely not, even if you do not have a single genuine reason to distrust him, you clearly do.

That level of distrust is not conducive to good relationships, and you'll never stop worrying that he might have some something wrong, or pulled the wool over your eyes, in every single thing he does for you.

Can't believe you would want to feel that way about someone you'd engaged to do work for you. Time to cut him loose and get someone in who you do trust.

 

[OwainDIYer]

The framework of a suspended ceiling can have sharp edges and if yours has sharp edges then the cables need to be kept clear of those sharp edges.

Also cables should not be relying on the ceiling grid for support but should use cable tray etc. When the ceiling disintegrates in a fire the cables hanging down are an entrapment hazard. This was a contributing factor to the deaths of two firefighters in Shirley Tower Southampton

 

[sparkwright]

The framework of a suspended ceiling can have sharp edges and if yours has sharp edges then the cables need to be kept clear of those sharp edges.

Also cables should not be relying on the ceiling grid for support but should use cable tray etc. When the ceiling disintegrates in a fire the cables hanging down are an entrapment hazard. This was a contributing factor to the deaths of two firefighters in Shirley Tower Southampton

This is quite right of course, and it looks like soon we will be using metal fixings for cables etc more often than we perhaps already do.

I don't think much of laying cables across a ceiling grid. I understand though in some circumstances it's practical to do so, such as if the space above is tight and removing tiles is virtually impossible, but there you go...

Very bad to lay many cables across a ceiling grid. It's surprising how heavy several long lengths of cable are. I gather an amusement arcade somewhere was wired in this fashion, and it brought the ceiling down.