Which cable derating applies ?

[bernardgreen]

Cables for sockets close to floor level will have to be bought down from ceiling in white 20mm PVC conduit fixed with saddles to a stone wall which in some cases will be lime plastered. Little if any contact between conduit and wall. Some conduit will located behind heavy draped floor to ceiling curtains to reduce visual impact of conduit.

Ciruits may be ring finals or a radials serving maximum of 4 double sockets in a room so two 2.5mm cables in each conduit. One circuit per room.

Which derating factor applies for this installion method. ? Conflicting sources give different factors to apply in the calculations that I will probably have to present to the LABC.

Conduit will turn to horizontal to locate sockets clear of the curtains and area under window and each circuit will be protected by MCB at 16 amp.

PS I know the cable is more than adequate for a 16 amp circuit but will need to "prove" it.

 

[davelx]

Hmm....my thinking is....

If you treat the curtain as insulation:

0.7 for installation method A and 0.8 for grouping, both factors applied together,

so 27A*0.7*0.8 = 15.1A

But that may be a harsh choice of installation method!

If you ignore the curtain and just consider the conduit and grouping:

27A*0.85*0.8 = 18.36A

I suspect the reality is somewhere between the two and that a 16A MCB would be fine - but will the LABC agree?

 

[JohnW2]

0.7 for installation method A...

That's an unusual way of looking at it - is that a harp back to past practices?

These days, most people would simply look to the Tables in the regs to get the CCC for Method A - and that gives 18.5A if you look at Table 4D2A or 20A if you look at Table 4D5 (T&E specific - seemingly added to legitimise 32A ring finals!). You seem to have taken the Method C ('clipped direct') figure of 27A and multiplied it by the factor of 0.7 (where does that come from?) to get an answer of 18.9A.

As for Bernard's question, I'd be inclined to ignore the curtain. However, if one uses the Table 4D5 Method A figure (which seems totally justified), the CCC is still 16A (just) after one applies the 0.8 grouping factor.

Kind Regards, John.

 

[davelx]

0.7 for installation method A...

That's an unusual way of looking at it - is that a harp back to past practices?

No. it's cos I don't have a copy of 17th Ed. to hand, only 16th. and the ECA guide to the 17th which doesn't have 2.5mm cable in the sample tables!

So I just worked out the approx. derate factor from the figures for 1.5 and 4.0 mm cable that are there which is about 0.7 for each compared to the clipped direct figure, which is the highest CCC of all the methods.

 

[JohnW2]

That's an unusual way of looking at it - is that a harp back to past practices?

No. it's cos I don't have a copy of 17th Ed. to hand, only 16th. and the ECA guide to the 17th which doesn't have 2.5mm cable in the sample tables! So I just worked out the approx. derate factor from the figures for 1.5 and 4.0 mm cable that are there which is 0.7 compared to the clipped direct figure, which is the highest CCC of all the methods.

Well, I still count that as an 'ususual' way, although it does appear to give roughly vthe same answers for Methods A & B (B with your 0.85 factor) as does reference to the 17th ed. Tables!

BTW, clipped direct (Method C) is not the method with the highest CCC (at least, not per 17th ed.!) - that would be reference Method E (in free air on on tray), which gives a CCC of 30A for 2.5mm². I guess your method would give that a 'factor' of 1.11 !

Kind Regards, John.

 

[davelx]

BTW, clipped direct (Method C) is not the method with the highest CCC (at least, not per 17th ed.!) - that would be reference Method E (in free air on on tray), which gives a CCC of 30A for 2.5mm². I guess your method would give that a 'factor' of 1.11 !

Kind Regards, John.

I'd guessed that might be the case. Clipped direct is the highest CCC in the 16th and the ECA guide doesn't give the figures for Method E.

If I'd had that, I'd have used that as the reference level, so 2.5mm² Method A would be 0.63 to give the 18.9A. I think the 20A in the other table is an anomaly introduced, as you suggest, to make standard RFCs compliant!

Perhaps we should use dBE (0dBE being the CCC for Ref Method E) to describe de-rating factors. Method A would then be -4dBE.That could be amusing.

I really must get myself a copy of 17th.