C&G 8202 past exam paper question. Cable calc.

[richardindorset]

Just run through a past exam paper June 2022.

The last question is, as usual, a cable calc.

I was pretty close to the right answers on most of it. However two things threw me, when I checked the anwer sheet.

We've been taught that for a basic radial power circuit, You find the Ib (design current), select a protective device to get In. Then apply C factors for temp, thermal insulation, grouping etc etc. to get It. Then you look up the next size cable in the tables, taking into account installation reference method, etc. to get Iz.

In the answers sheet, their calcs take It and apply the Ca factor to it. This gives a different answer to mine.

I'm fairly sure I'm right and they are wrong but I'm only going on what we've been taught....

For more info, here's the question:

Their answer says to use a 32A breaker, then look up table 4D2A to find a suitable cable, they say 36A. Then they apply a Ca of 0.94 to get Iz of 33.84A

I think the correct answer is 32A breaker, apply 0.94 Ca factor to get 34.04A. Table F5(i) OSG shows Ref method C 2 core 36A capacity 4mm2.

The final result is not really much different but the calcs are not right.

Maybe the fact that the cable size is already identified in the question is the reason?

 

[davelx]

You're applying the ambient temperature correction factor to the wrong thing - you apply it to the CCC of the CABLE not the OPD. You just need to check that the corrected CCC of the cable is still >= the OPD rating.

The CCC of the cable is 36A method C @30C. You apply the ambient Ca correction of 0.94 to the 36A which gives you 33.84. This is > than the OPD rating so job's a goodun.

 

[richardindorset]

You're applying the ambient temperature correction factor to the wrong thing - you apply it to the CCC of the CABLE not the OPD. You just need to check that the corrected CCC of the cable is still >= the OPD rating.

The CCC of the cable is 36A method C @30C. You apply the ambient Ca correction of 0.94 to the 36A which gives you 33.84. This is > than the OPD rating so job's a goodun.

Hi Dave, the OSG says In is the overcurrent device rating and the factors, Ca Cg etc are applied ( In is divided by the factors ) and this gives a number It, the tabulated current capacity of the cable. You find the next value up from this to get Iz in the cable tables.

This is what the book says and what we were taught, but yes it does seem odd to apply factors to the rating of the breaker and not the cable!

The Regs book says same: "Divide the rated current of the protective device by any applicable rating factors" Appendix 4 section 5.1.1

Actually, reading on in the Regs Book, it also says that rating factors can be applied to the cable as multiplers instead but that requires some trial and error so the other method is more desirable.

So in the exam question answer, this is what they do. Multiply the cable capacity by the factor instead of dividing the protective device capacity by the factors!

Problem solved.

It threw me as we've not been told about this alternative method. It does make sense though.

 

[davelx]

IMO that's a bit backwards. It's the CCC of the cable that is reduced by a higher Ta, not the current rating of the OPD. Their reasoning is that you have to apply the correction for 3036 fuses that way so you might as well do them all that way. Who uses 3036 fuses these days?

As you suggest, I think they have done it the way they did because the OPD rating and cable size is given.

Note also that you are not being asked to design the circuit, but to check compliance of the given design.

I concede that the method in the appendix has the advantage if there are multiple correction factors in effect.

 

[securespark]

Problem solved.

Phew!

I thought for a second I was going to have to get in touch with City and Guilds....

 

[richardindorset]

Phew!

I thought for a second I was going to have to get in touch with City and Guilds....

 

[securespark]

Did you sort out the other question issue?

 

[richardindorset]

Did you sort out the other question issue?

I think so, I need to dig back through my notes. I think that they apply cable factors to a ring final circuit, to ensure that the cable is able to carry 20A, so in some cases 4mm2 is used.

 

[richardindorset]

Did you sort out the other question issue?

I've put an update on that other question thread.

 

[EFLImpudence]

I think so, I need to dig back through my notes. I think that they apply cable factors to a ring final circuit, to ensure that the cable is able to carry 20A, so in some cases 4mm2 is used.

Well - yes of course but that applies to any circuit with a design current of 20A.

 

[richardindorset]

Well - yes of course but that applies to any circuit with a design current of 20A.

Well they equate Ib with In at 32A in their answer, so in their view the design current is 32A. In their answer they say "Minimum current carrying capacity of 20A used from regulation 433.1.204"

 

[EFLImpudence]

Well they equate Ib with In at 32A in their answer, so in their view the design current is 32A.

Yes, it is but there are two conductors sharing it so each must have a CCC of at least 20A because of the conditions for the UK ring circuit.

They say a minimum of 20A but specifically prohibit 1.5mm² cable because the regulation is still written for a BS3036 30A fuse.
With a 32A MCB it could use 1,5mm² - or use a 40A MCB.

I think you are not allowing for the fact that the UK ring circuit requirements are a bodge and so applying the same parameters as a radial circuit, or logic, is futile.

In their answer they say "Minimum current carrying capacity of 20A used from regulation 433.1.204"

Yes but that is not the case, is it?

It is the same as having a 32A MCB on a radial circuit and stating the minimum CCC required is 32A but you must use 6mm².

 

[EFLImpudence]

Sorry - edited - I should have written 6 in the last line.

 

[richardindorset]

Yes, it is but there are two conductors sharing it so each must have a CCC of at least 20A because of the conditions for the UK ring circuit.

They say a minimum of 20A but specifically prohibit 1.5mm² cable because the regulation is still written for a BS3036 30A fuse.
With a 32A MCB it could use 1,5mm² - or use a 40A MCB.

I think you are not allowing for the fact that the UK ring circuit requirements are a bodge and so applying the same parameters as a radial circuit, or logic, is futile.


Yes but that is not the case, is it?

It is the same as having a 32A MCB on a radial circuit and stating the minimum CCC required is 32A but you must use 6mm².

I'm just trying to follow what C&G are saying in their exam question answers. I want to make sure I answer the question in the same way, no matter how valid it is IRL.

I have no idea who marks the exams but I assume they follow the C&G answer sheets.

 

[EFLImpudence]

I'm just trying to follow what C&G are saying in their exam question answers. I want to make sure I answer the question in the same way, no matter how valid it is IRL.

Yes, I realise that but I think the only way is to learn what C&G want.

I have no idea who marks the exams but I assume they follow the C&G answer sheets.

Nor I but never assume those in charge know what they are doing.