Cable size... conflicting results

[bernardgreen]

What is the maximum safe continuous current for 2 of 10mm CSA copper twin and earth in the following submains ?. Different sources give different results.

Two cables leave the incoming CU from 32 amp MCBs. Route is 1.5 clipped to plaster on stone wall. then through a 600 mm thick stone wall in a shared conduit, then a further 2 metres clipped to a timber cable tray to the two CUs they are feeding. One CU per cable. Largest MCB in CUs will be 16 amp for radials.

Design load maximum is under 30 amps per cable, no fixed electric heating, no electric shower and no immersion heater on these circuits. Maybe a one/two Kw fan heater.

The incoming CU supplies 5 sub mains from MCBs. Sub mains 1 & 2 to these two CUs in main 5 room cottage, Sub main 3 feeds the CU for the kitchen ( room 6 ) and the other two feed remote CUs in outbuildings.

I consider the cables being fused at 32 amps make this a safe design but one electrician doubts it is acceptable under BS7671.

 

[bernardgreen]

I think the confusion is in the wording of the de-rating factor for bunched cables.

10mm CSA in conduit in insulated wall (Refence method A) gives 44 amps. ( probably the safe current is higher as the stone is not thetmally insulating )
and the derating for 2 cables bunched is 80%

Derating BY 80% takes the current down to 8.8 amps ( which is ridiculous )

Derating TO 80% takes the current down to 35.2 amps ( my interpretation
hence the 32 amp MCBs for the submains. )

Maybe the electrician has a different derating factor for bunched cables that derates BY enough to bring the calculated safe current below 32 amps.

 

[ricicle]

10mm² t+e ref. method A = 44 A . 2 circuits = factor of 0.8

44 x 0.8 = 35.2 amps

Your circuit does comply with regard to protection.

But the discrimination is questionable Bernard ?

 

[JohnW2]

I 10mm CSA in conduit in insulated wall (Refence method A) gives 44 amps. ( probably the safe current is higher as the stone is not thetmally insulating ) and the derating for 2 cables bunched is 80%

Conduit within a masonry wall is surely Method B (41 or 42, depending on the properties of the stone), which gives you nearly 10A more CCC to play with.

Derating BY 80% takes the current down to 8.8 amps ( which is ridiculous )
Derating TO 80% takes the current down to 35.2 amps ( my interpretation
hence the 32 amp MCBs for the submains. )

Your interpretation is obviously correct.

Maybe the electrician has a different derating factor for bunched cables that derates BY enough to bring the calculated safe current below 32 amps.

I can't see how - you would need to challenge him/her. It is clear that the de-rating for 2 grouped cables is 20% (i.e. TO 80%) - see all the equaltions in 5.1 of Appendix 4 of the regs. One would question the 'electrical common sense' of this 'electrician' - would it not be 'intuitively surprising' to be implying (as he is) that one would need 16mm² cable to have a CCC ≥32A, with any installation method?

Kind Regards, John.

 

[JohnW2]

But the discrimination is questionable Bernard ?

Very questionable, but I do wonder about this discrimination business. Whilst having two MCBs with their In at least 3:1 different should give discrimination in relation to overload currents, I doubt that one can ever have any confidence as regards discrimination in relation in the presence of fault currents (more likely than overloads if circuits have been designed properly), no matter what the relative In of the two devices.

Kind Regards, John.

 

[EFLImpudence]

you would need to challenge him/her. It is clear that the de-rating for 2 grouped cables is 20% (i.e. TO 80%) - see all the equaltions in 5.1 of Appendix 4 of the regs.

That is so obviously correct that it is almost unnecessary to comment but -

point out to the electrician that the factor number goes down the more cables that are bunched.

So by his reasoning, twenty cables grouped would only be derated BY 38%.
Thus, according to him, the more cables that are grouped together - the higher the CCC would be.

 

[JohnW2]

point out to the electrician that the factor number goes down the more cables that are bunched.
So by his reasoning, twenty cables grouped would only be derated BY 38%. Thus, according to him, the more cables that are grouped together - the higher the CCC would be.

Quite so. However, to be fair to the electrician, Bernard has not said that this is what the electrician believed - he was merely mentioning it as a possible source of confusion as regards grouping factors, and hence a possible reason why the electrician was questionning whther 32A protection was adequate for the 10mm² cable.

If an 'electrician' did believe that, then I might put the quote marks around 'electrician' in bold, and keep him well away from my electrical installation - since it would indicate a basic lack of 'electrical common sense'!

Kind Regards, John.

 

[DetlefSchmitz]

But the discrimination is questionable Bernard ?

Very questionable, but I do wonder about this discrimination business. Whilst having two MCBs with their In at least 3:1 different should give discrimination in relation to overload currents, I doubt that one can ever have any confidence as regards discrimination in relation in the presence of fault currents (more likely than overloads if circuits have been designed properly), no matter what the relative In of the two devices.

This is an important point. I have a submain to an extnsion fed by a 63A MCB, and when a lamp (LED type) exploded on the 6A lighting circuit, it took out both breakers, and therefore of course the rest of the extension. In this case a 60A switchfuse might have been a better choice.

But now I am faced with a shed supply sub-main fed using SWA which will comfortably take 30A but not 60A, so I can't choose a switch fuse because they aren't available below 60A, which leaves me with either an MCB (and the fault current discrimination problem), or a DIN rail HRC fuse holder, at which the distributor sucks air through his teeth and says 'Well, I suppose I could order one in..."

 

[JohnW2]

This is an important point. I have a submain to an extnsion fed by a 63A MCB, and when a lamp (LED type) exploded on the 6A lighting circuit, it took out both breakers, and therefore of course the rest of the extension. In this case a 60A switchfuse might have been a better choice.

Indeed, but I'm not sure how much guarantee one has of discrimination even with a 10:1 ratio (and even with the higher rated device being a fuse) in the presence of a full-flown (is that a pun? ) fault current. However, I imagine that discrimnation would be very likely with such a setup.

But now I am faced with a shed supply sub-main fed using SWA which will comfortably take 30A but not 60A, so I can't choose a switch fuse because they aren't available below 60A ....

The Wylex WY108/WY108M switch fuses certainly have a 45A fuse/carrier available, perhaps even smaller.

Kind Regards, John.

 

[DetlefSchmitz]

Thanks, I'll check that out...

 

[plugwash]

But now I am faced with a shed supply sub-main fed using SWA which will comfortably take 30A but not 60A, so I can't choose a switch fuse because they aren't available below 60A

As johnw2 says there is a 45A carrier available for the WY108/WY108M. TLC direct carry it as a standard item. Seems pretty pricey though.

http://www.tlc-direct.co.uk/Products/WY108.html
http://www.tlc-direct.co.uk/Products/WY108M.html
http://www.tlc-direct.co.uk/Products/WYL45.html

You can also buy a 1 way wylex standard unit from RS and your choice of cartridge fuse holder (45A, 35A, 30A, 20A, 15A or 5A) from TLC. This is a much cheaper option but obviously there will be less wiring room than with a 60A unit.

http://uk.rs-online.com/web/p/fuse-switches/1993925/
http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/Wylex_Standard_Carriers/index.html

 

[DetlefSchmitz]

Thanks, Plugwash. Food for thought!

 

[ban-all-sheds]

I have a submain to an extnsion fed by a 63A MCB using SWA which will comfortably take 30A but not 60A, so I can't choose a switch fuse because they aren't available below 60A

 

[DetlefSchmitz]

Not sure why you've taken two completely different sentences and spliced them together so the result makes no logical sense. Is this merry xmas japes time or something?

 

[bernardgreen]

Discrimation, I would have prefered wire fuses in the incoming CU, that is the arrangement for the supplies to this house and it works well. The few times in 30 years that an MCB in a sub CU has tripped the wire fuse protecting the sub main has remained intact. Maybe later if there is a recurring problem then I will replace the MCBs in the incomer with slow acting types.

The electrician with a mathmatical problem is little more than a cable installer working under instruction from a genuine electrician. He does a really good job running cables, neat tidy and very little waste. He always has the cable right way round in back boxes so the live and neutral are the same way round as the terminal. But anything that requires calculation and he is out of his depth.

The stone wall is described as limestone rubble and lime mortar and can be thermally very conductive when moist and good insulation when dry,